1292 lines
57 KiB
Plaintext
1292 lines
57 KiB
Plaintext
\input texinfo @c -*-texinfo-*-
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@c %**start of header
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@setfilename vmintegration.info
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@settitle GNU Classpath VM Integration Guide
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@c %**end of header
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@setchapternewpage off
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@ifinfo
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This file contains important information you will need to know if you
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are going to write an interface between GNU Classpath and a Virtual
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Machine.
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Copyright (C) 1998-2002, 2004, 2005 Free Software Foundation, Inc.
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@ifnotplaintext
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@dircategory GNU Libraries
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@direntry
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* VM Integration: (vmintegration). GNU Classpath VM Integration Guide
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@end direntry
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@end ifnotplaintext
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@end ifinfo
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@titlepage
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@title GNU Classpath VM Integration Guide
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@author John Keiser
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@author C. Brian Jones
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@author Mark Wielaard
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@page
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@vskip 0pt plus 1filll
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Copyright @copyright{} 1998-2002 Free Software Foundation, Inc.
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@sp 2
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Permission is granted to make and distribute verbatim copies of
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this document provided the copyright notice and this permission notice
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are preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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document under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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into another language, under the above conditions for modified versions,
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except that this permission notice may be stated in a translation
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approved by the Free Software Foundation.
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@end titlepage
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@ifinfo
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@node Top, Introduction, (dir), (dir)
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@top GNU Classpath Hacker's Guide
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This file contains important information you will need to know if you
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are going to write an interface between GNU Classpath and a Virtual
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Machine.
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This document is incomplete, as we are still in alpha with the interface.
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@end ifinfo
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@menu
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* Introduction:: An introduction to the Classpath project
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* Initialization:: Initializing the classes
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* Classpath Hooks:: Hooks from Classpath to the VM
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* VM Hooks:: Hooks from the underlying VM to Classpath
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* JNI Implementation:: Hooking the VM to jni.h
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* Miscellaneous VM Requirements::
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@end menu
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@node Introduction, Initialization, Top, Top
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@comment node-name, next, previous, up
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@chapter Introduction
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The Classpath Project's ambition to be a 100% clean room implementation
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of the standard Java class libraries cannot be fulfilled without some
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level of integration with the Virtual Machine, the underlying machinery
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that actually runs Java.
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There are several VMs out there, here is a small list.
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@itemize @bullet
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@item @uref{http://www.hungry.com/old-hungry/products/japhar/,Japhar}
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Japhar was the first VM to use GNU Classpath. Today you can see that
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sort of relationship in the source tree which denotes several Japhar
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specific files as a reference implementation of those pieces. This VM
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has been primarily tested against Linux and lacks garbage collections, a
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JIT, and suffers recently from slow development.
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@item @uref{http://www.intel.com/research/mrl/orp/,Intel's Open Runtime Platform}
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Intel surprised us not long ago with the release of this rather advanced
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VM that uses GNU Classpath for a set of class libraries and works on
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Linux and Windows 2000. As of June, 2004, it does not appear that ORP
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is under active development.
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@item @uref{http://www.sablevm.org/,SableVM}
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SableVM is a robust, extremely portable, efficient, and
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specifications-compliant Java Virtual Machine that aims to be easy to
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maintain and to extend. It features a state-of-the-art, efficient
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interpreter engine. Its source code is very accessible and easy to
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understand, and has many robustness features that have been the object
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of careful design.
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@item @uref{http://www.kaffe.org,Kaffe}
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Kaffe is an advanced VM and together with its own class libraries
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provides a Java 1.1 compatible environment.
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@item @uref{http://www.mozilla.org/projects/ef,Electrical Fire}
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The Electrical File VM continues to be listed as a Mozilla project
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though development has been somewhat quiet. A number of concepts from
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EF were expected at one point to be rolled into Japhar, but that
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development has not occurred as of yet.
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@item @uref{http://latte.snu.ac.kr/,LaTTe}
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This VM project so far supports only Sun UltraSparc processors using the
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proprietary Solaris 2.5.1 or higher operating system. LaTTe was derived
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from Kaffe but claims a number of improvements.
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@item @uref{http://gcc.gnu.org/java/,GNU Compiler for Java (GCJ)}
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This is a portable, optimizing, ahead-of-time compiler for the Java
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Programming Language. It can compile Java source code directly to native
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machine code, Java source code to Java bytecode (class files), and Java
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bytecode to native machine code. Compiled applications are linked with the
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GCJ runtime, libgcj which is based on the GNU Classpath code, which provides
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the core class libraries, a garbage collector, and a bytecode interpreter.
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libgcj can dynamically load and interpret class files, resulting in mixed
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compiled/interpreted applications.
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GCJ is part of the GNU Compiler Collection (@uref{http://gcc.gnu.org/,GCC}).
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On March 6 2000 the libgcj and GNU Classpath projects were officially merged
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and there is active work on merging all the classes between the projects.
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Licensed under GPL+exception, just as GNU Classpath is.
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@item @uref{http://kissme.sourceforge.net/,Kissme}
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This is a free Java Virtual Machine that is being developed on GNU/Linux
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and can run console Java applications. Kissme also provides support for
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orthogonally persistent Java.
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@c I don't know what ``orthogonally persistent Java'' is, and I bet
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@c there are other people don't know either. -- Steve Augart, 4 June 2004
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@item @uref{http://jamvm.sourceforge.net/,JamVM}
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A simple, small bytecode interpreter that works out-of-the-box with
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pure GNU Classpath; it is emerging as the preferred platform for
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quickly testing a new build of GNU Classpath. Licensed under the GPL.
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@item @uref{http://oss.software.ibm.com/jikesrvm,Jikes RVM}
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A free runtime environment for Java, written in Java. Works
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out-of-the-box with pure GNU Classpath. Features an optimizing JIT.
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Runs on the x86 and PowerPC architectures, on the AIX, Linux, and Mac
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OS/X operating systems. Licensed under the CPL (Common Public
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License). Extensively documented. Actively developed as of June,
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2004.
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@end itemize
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In the past integration efforts were focused mainly on Japhar with an eye
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towards getting Electrical Fire to work. Most information contained in
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this document is gleaned from these efforts. Recently more work has been
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done on getting gcj, orp and kissme to work out of the box with GNU Classpath
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but there is much to do before that becomes a reality.
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@node Initialization, Classpath Hooks, Introduction, Top
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@comment node-name, next, previous, up
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@chapter Initialization
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The order of initialization, as far as I can tell, doesn't matter just
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yet. However, when we move to 1.2 support, it probably will matter, so
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we'll have a note in here at that time.
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The initialization order is currently documented in the
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@file{Runtime.java} source file.
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@node Classpath Hooks, VM Hooks, Initialization, Top
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@comment node-name, next, previous, up
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@chapter Classpath Hooks
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The primary method of interaction between Classpath and the VM is via
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the helper classes, which are named after the relevant core library
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class, but include an additional `VM' prefix. The library classes from
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Classpath call out to these to get certain VM-specific dirty work done.
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A reference copy of each VM class exists. The majority consist of a
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series of static methods, some of which are simply declared
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@code{native}, and some which provide a default implementation. VMs may
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either use these as is, or create their own local variations. When
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using the default implementations, the VM is responsible for
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implementing any of the code marked as @code{native} which corresponds
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to functionality they wish their VM to provide. When using their own
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versions of the classes, VM implementors may choose to change the mix of
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native and non-native methods from that below, so as to best suit their
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implementation.
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@menu
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* java.lang::
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* gnu.classpath::
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* java.util::
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* java.io::
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* java.security::
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* java.net::
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* java.nio::
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* java.nio.channels::
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* gnu.java.nio::
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* Classpath Callbacks::
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@end menu
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@node java.lang, gnu.classpath, Classpath Hooks, Classpath Hooks
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@comment node-name, next, previous, up
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@section @code{java.lang}
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@code{java.lang} is the core Java package, being imported automatically by all
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classes. It includes basic classes as @code{Object} and @code{String}.
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A VM must implement at least some parts of this package in order to
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become operable.
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@menu
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* java.lang.VMClass::
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* java.lang.VMObject::
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* java.lang.VMClassLoader::
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* java.lang.VMSystem::
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* java.lang.VMThrowable::
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* java.lang.VMCompiler::
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* java.lang.VMDouble::
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* java.lang.VMFloat::
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* java.lang.VMProcess::
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* java.lang.VMRuntime::
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* java.lang.VMString::
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* java.lang.VMThread::
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@end menu
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@node java.lang.VMClass, java.lang.VMObject ,java.lang,java.lang
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@subsection @code{java.lang.VMClass}
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The core class, @code{java.lang.Class}, and the corresponding VM class,
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@code{java.lang.VMClass}, provide two main functions within GNU Classpath.
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@enumerate
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@item For basic VM operation, @code{java.lang.Class} provides the link between
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the Java-based representation of a class it embodies and the VM's own
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internal structure for a class. @xref{VM Hooks}.
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@item As far as the user is concerned, the main function of
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@code{java.lang.Class} is as an entry point to the reflection
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facilities, and so it also provides this functionality, backed by the
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VM class.
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@end enumerate
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This VM class lists the following methods, organized by the version of the
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Java specification in which they occur. All are @code{native}, unless
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otherwise specified, and pertain to reflection. As a result, the VM only
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needs to implement these methods in order to provide reflection support,
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and then only to the degree required.
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@itemize @bullet
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@item 1.0
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@itemize @bullet
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@item @code{isInterface(Class)} -- This is simply a property test, and matches
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the presence of an appropriate flag within the class file.
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@item @code{getName(Class)} -- Returns the fully-qualified name of the class.
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@item @code{getSuperclass(Class)} -- Returns a @code{Class} instance which
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represents the superclass. Again, the class file contains an element directly
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relating to this. @code{null} is returned for primitives, interfaces and
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@code{Object}.
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@item @code{getInterfaces(Class)} -- Same as the above, but the implemented
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or extended interfaces rather than the superclass. An empty array should
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be returned, rather than @code{null}.
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@item @code{getDeclaredClasses(Class,boolean)} -- Returns the internal classes
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this instance declares directly. The flag determines whether or not the
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VM should filter out non-public classes.
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@item @code{getDeclaredFields(Class,boolean)} -- The same for fields.
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@item @code{getDeclaredMethods(Class,boolean)} -- And for methods.
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@item @code{getDeclaredConstructors(Class,boolean)} -- And constructors.
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@item @code{getClassLoader(Class)} -- Returns the @code{ClassLoader} instance
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which is responsible for the specified class.
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@item @code{forName(String)} -- The VM should create a @code{Class} instance
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corresponding to the named class. As noted in @ref{VM Hooks}, the internal
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content of the instance is the responsibility of the VM.
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@item @code{isArray(Class)} -- Another property test, corresponding to a
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class file flag.
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@item @code{initialize(Class)} -- The VM should initialize the class fully,
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if it has not already done so.
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@item @code{loadArrayClass(String,ClassLoader)} -- This is called if
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@code{forName} returns @code{null} and the string specifies an array class.
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The specified array class should be loaded with the supplied class loader.
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@item @code{throwException(Throwable)} -- The VM should throw the supplied
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checked exception, without declaring it.
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@end itemize
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@item 1.1
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@itemize @bullet
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@item @code{isInstance(Class,Object)} -- This is a reflection-based equivalent
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of the @code{instanceof} operator.
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@item @code{isAssignableFrom(Class,Class)} -- Mainly a shorthand for the above,
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removing the need to create an instance to test assignability.
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@item @code{isPrimitive(Class)} -- Returns true if this class is simply
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a representation of one of the primitive types: @code{boolean}, @code{byte},
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@code{char}, @code{short}, @code{int}, @code{long}, @code{float},
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@code{double} and @code{void}.
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@item @code{getComponentType(Class)} -- Produces a @code{Class} instance which
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represents the type of the members of the array the class instance represents.
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Classes which don't represent an array type return @code{null}.
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@item @code{getModifiers(Class,boolean)} -- Returns an integer which encodes
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the class' modifiers, such as @code{public}. Again, this relates to
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information stored in the class file.
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@item @code{getDeclaringClass(Class)} -- Returns the class that declared
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an inner or member class, or @code{null} if the instance refers to a top-level
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class.
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@end itemize
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@end itemize
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@node java.lang.VMObject, java.lang.VMClassLoader, java.lang.VMClass, java.lang
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@subsection @code{java.lang.VMObject}
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@code{VMObject} is the bridge between the low level @code{Object} facilities
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such as making a clone, getting the class of the object and the wait/notify
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semantics. This is accomplished using the following @code{native}
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methods.
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@itemize @bullet
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@item @code{getClass(Object)} -- Returns the @code{Class} instance for the
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object. @code{Class} objects are produced by the VM, as described in
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@ref{VM Hooks}.
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@item @code{clone(Cloneable)} -- The VM should produce a low-level clone of the
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specified object, creating a field-by-field shallow copy of the original.
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The only difference between the two is that the new object should still be
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@code{finalizable}, even if the original is not.
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@item @code{notify(Object)} -- The VM should choose one of the threads waiting
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for a lock on the specified object arbitrarily, and wake it. If the current
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thread does not currently hold the lock on the object, then an
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@code{IllegalMonitorStateException} should be thrown.
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@item @code{notifyAll(Object)} -- Same as the above, but all threads are
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awakened.
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@item @code{wait(Object,long,int)} -- The VM should set the current thread
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into a waiting state, which persists until it receives a notify signal or the
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specified time (in milliseconds and nanoseconds) is exceeded. The nanoseconds
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restriction may be ignored if such granularity is not available, and a
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@code{IllegalMonitorStateException} should be thrown if the current thread
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doesn't own the object.
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@end itemize
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@node java.lang.VMClassLoader, java.lang.VMSystem, java.lang.VMObject, java.lang
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@subsection @code{java.lang.VMClassLoader}
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@code{VMClassLoader} provides methods for defining and resolving core and
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primitive classes, as well as handling resources, packages and assertions.
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The class is a mixture of @code{native} methods and Java-based
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implementations, with some of the latter being @emph{stubs}.
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@itemize @bullet
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@item Native Methods
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@itemize @bullet
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@item @code{defineClass(ClassLoader,String,byte[],int,int,ProtectionDomain)}
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-- The VM should create a @code{Class} instance from the supplied byte array.
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@item @code{resolveClass(Class)} -- Resolve references to other classes in the
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supplied class.
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@item @code{loadClass(name,boolean)} -- Load a class using the bootstrap
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loader.
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@item @code{getPrimitiveClass(char)} -- The VM should provide a @code{Class}
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implementation for one of the primitive classes. The supplied character
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matches the JNI code for the primitive class e.g. `B' for byte and
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`Z' for boolean.
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@end itemize
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@item Java Methods
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@itemize @bullet
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@item @code{getResource(String)} -- The default implementation calls
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@code{getResources} and returns the first element in the returned enumeration,
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or @code{null} if there are no elements.
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@item @code{getResources(String)} -- By default, this compiles a list of
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URLs via the boot class path. Any matching files within a zip file are added,
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and directories on the boot class path are automatically converted to file
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URLs that refer to join the directory with the resource name (whether or not
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it actually exists).
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@item @code{getPackage(String)} -- Always returns null, which may be suitable
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if the VM does not wish to return a @code{Package} implementation. Otherwise,
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it may be necessary to make this a @code{native} method.
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@item @code{getPackages()} -- As with the last, a default stub implementation
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exists (returning an empty array) which may be replaced if support is
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required.
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@item @code{defaultAssertionStatus()} -- A stub which can be implemented
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by VMs providing assertion support. At present, it always returns @code{true}.
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@item @code{packageAssertionStatus()} -- Much the same status as the above.
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The method should return a map converting package names to boolean status
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values. The stub implementation provides an empty map.
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@item @code{classAssertionStatus()} -- Same as the last, but for classes.
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@item @code{getSystemClassLoader()} -- The default calls @code{ClassLoader}
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to create a new auxillary class loader with a system and extension class
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loader. The VM may wish to replace it if it wishes to supply its own custom
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system class loader.
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@end itemize
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@end itemize
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@node java.lang.VMSystem, java.lang.VMThrowable, java.lang.VMClassLoader, java.lang
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@subsection @code{java.lang.VMSystem}
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@code{VMSystem} handles the default I/O streams, provides access to the
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system clock and environment variables and provides methods for
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@code{System.arraycopy} and the @code{identityHashCode} of an
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@code{Object}. It consists of @code{native} methods, but the default
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implementation also provides some helper methods to simplify stream
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creation.
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@itemize @bullet
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@item Native Methods
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@itemize @bullet
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@item @code{arraycopy(Object,int,Object,int,int)} -- The VM should copy
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a specified number of array objects from one array to another, with
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appropriate checks for compatible typing, available elements and space.
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The VM should be able to perform this more efficiently using native code
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and direct memory manipulation than would have been achieved by using Java.
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@item @code{identityHashCode(Object)} -- This is the hashcode for
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@code{Object}, which relates to the actual location of the object in memory.
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@item @code{setIn(InputStream)} -- Set the system input stream.
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@item @code{setOut(PrintStream)} -- Set the system output stream.
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@item @code{setErr(PrintStream)} -- Set the system error stream.
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@item @code{currentTimeMillis()} -- Gets the system time in milliseconds.
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@item @code{getenv(String)} -- Returns the value of the specified environment
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variable.
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@end itemize
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@item Java Methods
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@itemize @bullet
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@item @code{makeStandardInputStream()} -- Helps provide the functionality of
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@code{System.in} by wrapping the appropriate file descriptor in a buffered
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file input stream. VMs may choose to create the stream from the descriptor
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differently rather than using this method.
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@item @code{makeStandardOutputStream()} -- Helps provide the functionality of
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@code{System.out} by wrapping the appropriate file descriptor in a buffered
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file output stream. VMs may choose to create the stream from the descriptor
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differently rather than using this method.
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@item @code{makeStandardErrorStream()} -- Helps provide the functionality of
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@code{System.err} by wrapping the appropriate file descriptor in a buffered
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file output stream. VMs may choose to create the stream from the descriptor
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differently rather than using this method.
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@end itemize
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@end itemize
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Classpath also provides native implementations of
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@itemize @bullet
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@item @code{setIn(InputStream)}
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@item @code{setOut(PrintStream)}
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@item @code{setErr(PrintStream)}
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@item @code{currentTimeMillis()}
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@item @code{getenv(String)}
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@end itemize
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|
|
making a VM implementation optional.
|
|
|
|
@node java.lang.VMThrowable, java.lang.VMCompiler, java.lang.VMSystem, java.lang
|
|
@subsection @code{java.lang.VMThrowable}
|
|
@code{VMThrowable} is used to hold the VM state of a throwable, created either
|
|
when a @code{Throwable} is created or the @code{fillInStackTrace()} method is
|
|
called (i.e. when the actual stack trace is needed, as a lot of exceptions are
|
|
never actually used). The actual class has two @code{native} methods,
|
|
one (@code{fillInStackTrace()}) being a method of the class used to obtain
|
|
instances, and the other an instance method, @code{getStackTrace()}.
|
|
@itemize @bullet
|
|
@item @code{fillInStackTrace(Throwable)} -- The VM should return the current
|
|
execution state of the @code{Throwable} in the form of a @code{VMThrowable}
|
|
instance. The VM may also return @code{null} if it does not support this
|
|
functionality.
|
|
@item @code{getStackTrace()} -- This is used to create a real
|
|
@code{StackTraceElement} array for the exception, using the state data
|
|
stored during creation of the instance.
|
|
@end itemize
|
|
|
|
@node java.lang.VMCompiler, java.lang.VMDouble, java.lang.VMThrowable, java.lang
|
|
@subsection @code{java.lang.VMCompiler}
|
|
|
|
@code{VMCompiler} provides an interface for VMs which wish to provide
|
|
JIT compilation support. The default implementation is simply a series
|
|
of stubs. The property, @code{java.compiler}, should point to a library
|
|
containing the function @code{java_lang_Compiler_start()} if such support
|
|
is to be provided.
|
|
|
|
@itemize @bullet
|
|
@item @code{compileClass(Class)} -- Invoke the compiler to compile the specific
|
|
class, returning @code{true} if successful.
|
|
@item @code{compileClasses(String)} -- The compiler should compile the classes
|
|
matching the specified string, again returning @code{true} on success.
|
|
@item @code{command(Object)} -- The object represents a command given to the
|
|
compiler, and is specific to the compiler implementation.
|
|
@item @code{enable} -- Enable the operation of the compiler.
|
|
@item @code{disable} -- Disable compiler operation.
|
|
@end itemize
|
|
|
|
@node java.lang.VMDouble, java.lang.VMFloat, java.lang.VMCompiler, java.lang
|
|
@subsection @code{java.lang.VMDouble}
|
|
|
|
@code{VMDouble} provides native support for the conversion and parsing
|
|
of doubles.
|
|
|
|
@itemize @bullet
|
|
@item @code{doubleToLongBits(double)} -- Converts the double to the IEEE 754
|
|
bit layout, collapsing NaNs to @code{0x7ff8000000000000L}.
|
|
@item @code{doubleToRawLongBits(double)} -- Same as the above, but preserves
|
|
NaNs.
|
|
@item @code{longBitsToDouble(long)} -- This is the inverse of the last method,
|
|
preserving NaNs so that the output of one can be fed into the other without
|
|
data loss.
|
|
@item @code{toString(double,boolean)} -- Converts the double to a string,
|
|
giving a shorter value if the flag @code{isFloat} is @code{true}, indicating
|
|
that the conversion was requested by @code{java.lang.Float} rather than
|
|
@code{java.lang.Double}.
|
|
@item @code{initIDs} -- Used by JNI-based solutions to initialize the cache
|
|
of the static field IDs. The default @code{VMDouble} implementation has a
|
|
static initializer which loads the JNI library and calls this method.
|
|
@item @code{parseDouble} -- Turn the string into a usable double value.
|
|
@end itemize
|
|
|
|
Classpath provides native implementations of all these, making VM
|
|
implementation optional.
|
|
|
|
@node java.lang.VMFloat, java.lang.VMProcess, java.lang.VMDouble, java.lang
|
|
@subsection @code{java.lang.VMFloat}
|
|
|
|
@code{VMFloat} provides native support for the conversion of floats.
|
|
|
|
@itemize @bullet
|
|
@item @code{floatToIntBits(float)} -- Converts the float to the IEEE 754
|
|
bit layout, collapsing NaNs to @code{0x7fc00000}.
|
|
@item @code{floatToRawIntBits(float)} -- Same as the above, but preserves
|
|
NaNs.
|
|
@item @code{intBitsToFloat(int)} -- This is the inverse of the last method,
|
|
preserving NaNs so that the output of one can be fed into the other without
|
|
data loss.
|
|
@end itemize
|
|
|
|
Classpath provides native implementations of all these, making VM
|
|
implementation optional.
|
|
|
|
@node java.lang.VMProcess, java.lang.VMRuntime, java.lang.VMFloat, java.lang
|
|
@subsection @code{java.lang.VMProcess}
|
|
|
|
@code{VMProcess} handles the execution of external processes. In the
|
|
default implementation, threads are spawned and reaped by @code{ProcessThread}.
|
|
A constructor creates a new @code{VMProcess}, which extends rather than
|
|
complements @code{Process}, using an array of arguments, an array of
|
|
environment variables and a working directory. The instance maintains
|
|
system input, output and error streams linked to the external process.
|
|
Three @code{native} methods are used, and implementations are provided
|
|
for all three by Classpath, making VM implementation optional. These use
|
|
the POSIX functions, @code{fork()}, @code{waitpid()} and @code{kill()}.
|
|
|
|
@itemize @bullet
|
|
@item @code{nativeSpawn(String[],String[],File)} -- The VM should create a
|
|
new process which uses the specified command-line arguments, environment
|
|
variables and working directory. Unlike the other two methods, this
|
|
method is linked to an instance, and must call @code{setProcessInfo()} with
|
|
the results before returning.
|
|
@item @code{nativeReap()} -- This is called to perform a reap of any
|
|
zombie processes, and should not block, instead returning a boolean as to
|
|
whether reaping actually took place.
|
|
@item @code{nativeKill(long)} -- The VM should terminate the specified PID.
|
|
@end itemize
|
|
|
|
@node java.lang.VMRuntime, java.lang.VMString, java.lang.VMProcess, java.lang
|
|
@subsection @code{java.lang.VMRuntime}
|
|
|
|
The @code{VMRuntime} class provides a series of native methods
|
|
which divulge information about the runtime or invoke certain
|
|
operations. This includes retrieving the amount of available memory,
|
|
and scheduling the garbage collector. There are two exceptions: the
|
|
@code{enableShutdownHooks} method, which allows the VM to put in its own
|
|
shutdown hooks when @code{Runtime.addShutdownHook()} is first invoked,
|
|
and @code{exec(String[],String[],File)} which spawns an external process.
|
|
These are Java-based static methods instead. The first is simply a stub by
|
|
default, while the second simply links to the functionality of
|
|
@code{VMProcess} (and should be changed if a different @code{Process}
|
|
implementation is used).
|
|
|
|
@itemize @bullet
|
|
@item @code{availableProcessors()} -- Returns the number of processors
|
|
available to the VM.
|
|
@item @code{freeMemory()} -- Returns the amount of memory the VM has available
|
|
on the heap for allocating.
|
|
@item @code{totalMemory()} -- Returns the size of the heap.
|
|
@item @code{maxMemory()} -- Returns the maximum memory block the VM will
|
|
attempt to allocate. May be simply @code{Long.MAX_VALUE} (8 exabytes!)
|
|
@item @code{gc()} -- Allows users to explicitly invoke the garbage collector.
|
|
This is a suggestion to the VM, rather than a command, and the garbage
|
|
collector should run anyway @emph{without} it being invoked.
|
|
@item @code{runFinalization()} -- Like the above, but related to the
|
|
finalilzation of objects rather than the garbage collector.
|
|
@item @code{runFinalizationForExit()} -- Called immediately prior to VM
|
|
shutdown in order to finalize all objects (including `live' ones)
|
|
@item @code{traceInstructions(boolean)} -- This turns on and off the optional
|
|
VM functionality of printing a trace of executed bytecode instructions.
|
|
@item @code{traceMethodCalls(boolean)} -- This turns on and off the optional
|
|
VM functionality of printing a trace of methods called.
|
|
@item @code{runFinalizersOnExit(boolean)} -- A toggleable setting for
|
|
running the finalization process at exit.
|
|
@item @code{exit(int)} -- The VM should shutdown with the specified exit code.
|
|
@item @code{nativeLoad(String,ClassLoader)} -- Attempts to load a file,
|
|
returning an integer which is non-zero for success. Nothing happens if the
|
|
file has already been loaded.
|
|
@item @code{mapLibraryName(String)} -- The VM should map the system-independent
|
|
library name supplied to the platform-dependent equivalent (e.g. a @code{.so}
|
|
or @code{.dll} file)
|
|
@end itemize
|
|
|
|
@node java.lang.VMString, java.lang.VMThread, java.lang.VMRuntime, java.lang
|
|
@subsection @code{java.lang.VMString}
|
|
@code{VMString} is responsible for handling interned strings. If two strings
|
|
are equal (using the @code{equals()} method), then the results of calling
|
|
the @code{intern()} method on each of them makes them equal
|
|
(using @code{==}). Thus, the same string object is always returned by
|
|
@code{intern} if the two strings are equal. The default implementation
|
|
is Java-based and implements @code{intern(String)} by maintaining a
|
|
@code{WeakHashMap} which links the strings to their @code{WeakReference}.
|
|
A new mapping is created for each new string being @code{intern}ed.
|
|
A VM may implement this differently by implementing this method,
|
|
which is @code{static} and the only one in @code{VMString}.
|
|
|
|
@node java.lang.VMThread,, java.lang.VMString, java.lang
|
|
@subsection @code{java.lang.VMThread}
|
|
|
|
@code{VMThread} provides the link between Java's threads and the platform
|
|
threading support. A @code{VMThread} is created via a private constructor
|
|
and linked to a @code{Thread} instance. This occurs when the @code{Thread}
|
|
instance is started by the static @code{create(Thread,long)} method (the second
|
|
argument requests a certain stack size, usually zero). The thread itself is
|
|
executed via the @code{run()} method, which handles any problems with the
|
|
running of the thread and its eventual death.
|
|
|
|
@code{VMThread} provides the following accessors and mutators for accessing
|
|
the thread state via @code{VMThread},
|
|
|
|
@itemize @bullet
|
|
@item @code{getName()}
|
|
@item @code{setName(String)}
|
|
@item @code{getPriority()}
|
|
@item @code{setPriotity(int)}
|
|
@item @code{isDaemon()}
|
|
@end itemize
|
|
|
|
all of which refer to the @code{Thread} instance. @code{setPriority(int)} also
|
|
calls the appropriate native method. @code{stop(Throwable)} similarly wraps
|
|
a native method, merely adding in a check for the state of the thread.
|
|
|
|
The default implementation also provides Java-based implementations of
|
|
@code{join(long,int)}, @code{sleep(long,int)} and
|
|
@code{holdsLock(Object)}. @code{join} and @code{sleep} simply wait for
|
|
the appropriate amount of time, with @code{join} additionally waiting
|
|
for the thread instance to become @code{null}. @code{holdsLock} simply
|
|
checks if an object is locked by the current thread by trying to invoke
|
|
the @code{notify} method, and catching the failing exception if this is
|
|
not the case.
|
|
|
|
The remainder of the class is a series of @code{native} methods, some of
|
|
which are mandatory for VM implementation and others which provide optional
|
|
or deprecated functionality.
|
|
|
|
@itemize @bullet
|
|
@item Mandatory Instance Methods
|
|
@itemize @bullet
|
|
@item @code{start(long)} -- The VM should create the native thread and start
|
|
it running using the @code{run} method of the @code{VMThread} instance on
|
|
which this method is called.
|
|
@item @code{interrupt()} -- The VM should interrupt the running thread and
|
|
throw an appropriate exception.
|
|
@item @code{isInterrupted()} -- Checks the interrupted state of the thread.
|
|
@item @code{suspend()} -- The thread should be suspended until resumed.
|
|
@item @code{resume()} -- The thread should be resumed from its suspended state.
|
|
This pair of methods are deprecated, due to the possibility of a deadlock
|
|
occuring when a thread with locks is suspended.
|
|
@item @code{nativeSetPriority(int)} -- Called by @code{setPriority}
|
|
to allow the setting to flow down to the native thread.
|
|
@item @code{nativeStop(Throwable)} -- The VM should stop the thread abnormally
|
|
and throw the specified exception. This is clearly deprecated, due to the
|
|
ambiguous state an abruptly-stopped thread may leave.
|
|
@end itemize
|
|
@item Mandatory Class Methods
|
|
@itemize @bullet
|
|
@item @code{currentThread()} -- Return a reference to the thread currently
|
|
being executed.
|
|
@item @code{yield()} -- The VM should allow some other thread to run.
|
|
The current thread maintains its locks even though it stops executing for
|
|
the time being.
|
|
@item @code{interrupted()} -- A shortcut to obtaining the interrupted state
|
|
of the current thread.
|
|
@end itemize
|
|
@item Other Methods
|
|
@itemize @bullet
|
|
@item @code{countStackFrames()} -- Returns a count of the number of stack
|
|
frames in the thread. This depends on the deprecated method @code{suspend()}
|
|
having returned true, and is thus deprecated as a result.
|
|
@end itemize
|
|
@end itemize
|
|
|
|
@node gnu.classpath, java.util, java.lang, Classpath Hooks
|
|
@section @code{gnu.classpath}
|
|
|
|
The @code{gnu.classpath} package provides Classpath-specific functionality,
|
|
primarily relating to the features in @code{java.lang}. At present, this
|
|
includes the context of a class (the stack) and the system properties.
|
|
|
|
@menu
|
|
* gnu.classpath.VMStackWalker::
|
|
* gnu.classpath.VMSystemProperties::
|
|
@end menu
|
|
|
|
@node gnu.classpath.VMStackWalker,gnu.classpath.VMSystemProperties,gnu.classpath,gnu.classpath
|
|
@subsection @code{gnu.classpath.VMStackWalker}
|
|
|
|
@code{VMStackWalker} provides access to the class context or stack. The
|
|
default implementation consists of a @code{native} @code{static} method,
|
|
@code{getClassContext()}, which obtains the class context, and two helper
|
|
methods which obtain the calling class (the 3rd element in the context array)
|
|
and its class loader, respectively.
|
|
|
|
@itemize @bullet
|
|
@item @code{getClassContext()} -- The VM should return an array of
|
|
@code{Class} objects, each of which relates to the method currently being
|
|
executed at that point on the stack. Thus, the first item (index 0) is the
|
|
class that contains this method.
|
|
@item @code{getCallingClass()} -- A Java-based helper method which returns
|
|
the @code{Class} object which contains the method that called the method
|
|
accessing @code{getCallingClass()}.
|
|
@item @code{getCallingClassLoader()} -- Like the last, but returning the class
|
|
loader of the class.
|
|
@end itemize
|
|
|
|
@node gnu.classpath.VMSystemProperties,,gnu.classpath.VMStackWalker,gnu.classpath
|
|
@subsection @code{gnu.classpath.VMSystemProperties}
|
|
|
|
@code{VMSystemProperties} allows the VM to hook into the property creation
|
|
process, both before and after the system properties are added by GNU
|
|
Classpath. The default implementation assumes that the VM will add its
|
|
properties first, by making the pre-initialisation method @code{native},
|
|
and that the Classpath properties may then be altered by a Java-based
|
|
post-initialisation method.
|
|
|
|
As these methods are called as part of the bootstrap process, caution should
|
|
be used as to what classes are used, and properties should only be set
|
|
using @code{Properties.setProperty()}. Specifically, I/O classes should be
|
|
avoided at this early stage.
|
|
|
|
@itemize @bullet
|
|
@item @code{preInit(Properties)} -- Allows the VM to add properties
|
|
@emph{before} the Classpath properties are added. The default implementation
|
|
includes a full list of properties that @emph{must} be added by the VM, but
|
|
additional VM-specific ones may also be added.
|
|
@item @code{postInit(Properties)} -- Same as the last, but called after the
|
|
Classpath properties have been added. The main purpose of this is to allow
|
|
the VM to alter the properties added by GNU Classpath to suit it.
|
|
@end itemize
|
|
|
|
@node java.util, java.io, gnu.classpath, Classpath Hooks
|
|
@section java.util
|
|
|
|
The @code{java.util} VM hooks provide links between the mix of functionality
|
|
present in that package, which includes collections, date and time handling
|
|
and parsing. At present, there is only one hook, which connects GNU Classpath
|
|
to the timezone information provided by the underlying platform.
|
|
|
|
@menu
|
|
* java.util.VMTimeZone::
|
|
@end menu
|
|
|
|
@node java.util.VMTimeZone,,java.util,java.util
|
|
@subsection @code{java.util.VMTimeZone}
|
|
|
|
@code{VMTimeZone} joins @code{TimeZone} to the platform timezone information
|
|
via the static method, @code{getDefaultTimeZoneId()}. The VM hook is
|
|
expected to return a @code{TimeZone} instance that represents the current
|
|
timezone in use by the platform. The default implementation provides
|
|
this functionality for POSIX or GNU-like systems, and VMs that want this
|
|
functionality can keep this implementation and implement the native
|
|
method, @code{getSystemTimeZoneId()}. This method is only called when
|
|
obtaining the timezone name from the @code{TZ} environment variable,
|
|
@code{/etc/timezone} and @code{/etc/localtime} all fail. This fallback
|
|
mechanism also means that a system which doesn't provide the above three
|
|
methods, but does provide a timezone in string form, can still use this
|
|
implementation.
|
|
|
|
@node java.io, java.security, java.util, Classpath Hooks
|
|
@section java.io
|
|
|
|
The @code{java.io} package is heavily reliant on access to the I/O facilities
|
|
of the underlying platform. As far as its VM hooks go, they provide two
|
|
areas of functionality to GNU Classpath, these being
|
|
|
|
@itemize @bullet
|
|
@item File and directory queries and manipulation
|
|
@item Serialization of objects
|
|
@end itemize
|
|
|
|
The first corresponds directly to most of the @code{File} class, while
|
|
the latter underlies the functionality provided by the
|
|
@code{ObjectInputStream} and @code{ObjectOutputStream}. More low-level I/O
|
|
is provided by @ref{java.nio}.
|
|
|
|
@menu
|
|
* java.io.VMFile::
|
|
* java.io.VMObjectInputStream::
|
|
* java.io.VMObjectStreamClass::
|
|
@end menu
|
|
|
|
@node java.io.VMFile,java.io.VMObjectInputStream,java.io,java.io
|
|
@subsection @code{java.io.VMFile}
|
|
|
|
@code{VMFile} allows GNU Classpath's @code{File} representations to
|
|
probe and modify the file system using the native functions of the
|
|
platform. The default implementation (which consists of both a
|
|
@code{VMFile} class and the native methods) is primarily UNIX-centric,
|
|
working with POSIX functions and assuming case-sensitive filenames,
|
|
without the restriction of the 8.3 format. It consists mainly of
|
|
@code{static} @code{native} methods, with a few Java helper methods.
|
|
The native methods represent the file as a string containing its path,
|
|
rather than using the object itself.
|
|
|
|
@itemize @bullet
|
|
@item Native Methods
|
|
@itemize @bullet
|
|
@item @code{lastModified(String)} -- The native method should return a
|
|
@code{long} value that represents the last modified date of the file.
|
|
@item @code{setReadOnly(String)} -- Sets the file's permissions to read only,
|
|
in whichever way this is realised by the platform.
|
|
@item @code{create(String)} -- Create the named file.
|
|
@item @code{list(String)} -- The native method opens the named directory,
|
|
reads the contents and returns them as a Java @code{String} array.
|
|
@item @code{renameTo(String,String)} -- Renames the first file to the second.
|
|
@item @code{length(String)} -- Returns a @code{long} value representing
|
|
the file size.
|
|
@item @code{exists(String)} -- Tests for the existence of the named file
|
|
or directory.
|
|
@item @code{delete(String)} -- Deletes the file or directory.
|
|
@item @code{setLastModified(String,long)} -- Change the last modified time.
|
|
@item @code{mkdir(String)} -- Creates the named directory.
|
|
@item @code{isFile(String)} -- Tests that the named path references a file.
|
|
@item @code{canWrite(String)} -- Tests that the file can be written to.
|
|
This method is @code{synchronized}, so the object is locked during the check.
|
|
@item @code{canRead(String)} -- Complement of the last method.
|
|
@item @code{isDirectory(String)} -- Tests that the named path references
|
|
a directory.
|
|
@end itemize
|
|
@item Java Helper Methods
|
|
@itemize @bullet
|
|
@item @code{canWriteDirectory(File)} -- Checks that the directory can be
|
|
written to, by trying to create a temporary file in it.
|
|
@item @code{listRoots()} -- Returns the root of a GNU filesystem i.e. `/'
|
|
in an array.
|
|
@item @code{isHidden(String)} -- Checks whether the file starts with `.',
|
|
which is how files are hidden on UNIX-style systems.
|
|
@item @code{getName(String)} -- Pulls the actual filename from the end of
|
|
the path, by breaking off the characters after the last occurrence of the
|
|
platform's file separator.
|
|
@item @code{getCanonicalForm(String)} -- This converts a UNIX path to
|
|
its canonical form by removing the `.' and `..' sections that occur within.
|
|
@end itemize
|
|
@end itemize
|
|
|
|
@node java.io.VMObjectInputStream,java.io.VMObjectStreamClass,java.io.VMFile,java.io
|
|
@subsection @code{java.io.VMObjectInputStream}
|
|
|
|
This class consists of two methods which provide functionality used in
|
|
deserializing an object. @code{currentClassLoader()} provides the first
|
|
user-defined class loader from the class context
|
|
(@xref{gnu.classpath.VMStackWalker},) via a @code{PrivilegedAction}.
|
|
@code{allocateObject(Class,Class,Constructor)} is a @code{native} method
|
|
(a reference implementation is provided) which creates an object but
|
|
calls the constructor of another class, which is a superclass of the
|
|
object's class.
|
|
|
|
@node java.io.VMObjectStreamClass,,java.io.VMObjectInputStream,java.io
|
|
@subsection @code{java.io.VMObjectStreamClass}
|
|
|
|
@code{VMObjectStreamClass} is a series of @code{static} @code{native}
|
|
methods that provide some of the groundwork for @code{ObjectStreamClass}
|
|
and @code{ObjectStreamField}. @code{hasClassInitializer(Class)} works
|
|
with the former, and checks for the presence of a static initializer.
|
|
The remaining methods are of the form @code{setXXXNative(Field,Object,XXX)}
|
|
and support @code{ObjectStreamField}. One exists for each of the main types
|
|
(boolean, float, double, long, int, short, char, byte and object) and is used
|
|
to set the specified field in the supplied instance to the given value.
|
|
|
|
A default implementation is provided for all of them, so a VM implementation
|
|
is optional.
|
|
|
|
@node java.security, java.net, java.io, Classpath Hooks
|
|
@section java.security
|
|
|
|
The @code{java.security} package provides support for Java's security
|
|
architecture. At present, @code{VMAccessController} represents the sole
|
|
VM hook for this.
|
|
|
|
@menu
|
|
* java.security.VMAccessController::
|
|
@end menu
|
|
|
|
@node java.security.VMAccessController,,java.security,java.security
|
|
@subsection @code{java.security.VMAccessController}
|
|
|
|
The @code{AccessController} is used to perform privileged actions. Its
|
|
hook class, @code{VMAccessController}, maintains the
|
|
@code{AccessControlContext} and the default implementation is purely
|
|
Java-based. The VM may choose to replace this with their own.
|
|
The methods in the reference version are as follows:
|
|
|
|
@itemize @bullet
|
|
@item @code{pushContext(AccessControlContext)} -- Adds a new context to the
|
|
stack for the current thread. This is called before a privileged action
|
|
takes place.
|
|
@item @code{popContext()} -- Removes the top context from the stack. This
|
|
is performed after the privileged action takes place.
|
|
@item @code{getContext()} -- Either derives a context based on the
|
|
@code{ProtectionDomain}s of the call stack (see the next method) or returns
|
|
the top of the context stack.
|
|
@item @code{getStack()} -- Provides access to the call stack as a pair of
|
|
arrays of classes and method names. The actual implementation returns
|
|
an empty array, indicating that there are no permissions.
|
|
@end itemize
|
|
|
|
@node java.net, java.nio, java.security, Classpath Hooks
|
|
@section java.net
|
|
|
|
The @code{java.net} package is heavily reliant on access to the networking
|
|
facilities of the underlying platform. The VM hooks provide information
|
|
about the available network interfaces, and access to lookup facilities
|
|
for network addresses.
|
|
|
|
@menu
|
|
* java.net.VMInetAddress::
|
|
* java.net.VMNetworkInterface::
|
|
@end menu
|
|
|
|
@node java.net.VMInetAddress,java.net.VMNetworkInterface,java.net,java.net
|
|
@subsection @code{java.net.VMInetAddress}
|
|
|
|
@code{VMInetAddress} is a series of @code{static} @code{native} methods
|
|
which provide access to the platform's lookup facilities. All the methods
|
|
are implemented by GNU Classpath, making VM implementation optional, and
|
|
are as follows:
|
|
|
|
@itemize @bullet
|
|
@item @code{getLocalHostname()} -- Wraps the @code{gethostname} function, and
|
|
falls back on `localhost'.
|
|
@item @code{lookupInaddrAny()} -- Returns the value of @code{INADDR_ANY}.
|
|
@item @code{getHostByAddr(byte[])} -- Looks up the hostname based on an IP
|
|
address.
|
|
@item @code{getHostByName(String)} -- The reverse of the last method, it
|
|
returns the IP addresses which the given host name resolves to.
|
|
@end itemize
|
|
|
|
@node java.net.VMNetworkInterface,,java.net.VMInetAddress,java.net
|
|
@subsection @code{java.net.VMNetworkInterface}
|
|
|
|
@code{VMNetworkInterface} currently consists of a single @code{static}
|
|
@code{native} method, @code{getInterfaces()}, which retrieves the
|
|
network interfaces available on the underlying platform as a @code{Vector}.
|
|
The current GNU Classpath implementation is a native stub.
|
|
|
|
@node java.nio, java.nio.channels, java.net, Classpath Hooks
|
|
@section java.nio
|
|
|
|
The @code{java.nio} package is part of the New I/O framework added in
|
|
Java 1.4. This splits I/O into the concepts of @emph{buffers},
|
|
@emph{charsets}, @emph{channels} and @emph{selectors}, and
|
|
@code{java.nio} defines the buffer classes. As far as native and VM
|
|
code is concerned, the new package needs support for low-level efficient
|
|
buffer operations.
|
|
|
|
@menu
|
|
* java.nio.VMDirectByteBuffer::
|
|
@end menu
|
|
|
|
@node java.nio.VMDirectByteBuffer,,java.nio,java.nio
|
|
@subsection @code{java.nio.VMDirectByteBuffer}
|
|
|
|
A @code{ByteBuffer} maintains a buffer of bytes, and allows it to be
|
|
manipulated using primitive operations such as @code{get}, @code{put},
|
|
@code{allocate} and @code{free}. A direct buffer avoids intermediate
|
|
copying, and uses native data which shouldn't be manipulated by a
|
|
garbage collector. The VM class consists of @code{static} @code{native}
|
|
methods, all of which are given default implementations by GNU
|
|
Classpath.
|
|
|
|
@itemize @bullet
|
|
@item @code{init()} -- Creates an instance of an appropriate
|
|
@code{gnu.classpath.RawData} class. This class is not garbage
|
|
collected, is created natively and is used in the other methods to reference
|
|
the buffered data.
|
|
@item @code{allocate(int)} -- Allocates the memory for the buffer using
|
|
@code{malloc} and returns a reference to the @code{RawData} class.
|
|
@item @code{free(RawData)} -- Frees the memory used by the buffer.
|
|
@item @code{get(RawData,int)} -- Returns the data at the specified index.
|
|
@item @code{get(RawData,int,byte[],int,int)} -- Copies a section of the
|
|
data into a byte array using @code{memcpy}.
|
|
@item @code{put(RawData,int,byte)} -- Puts the given data in the buffer
|
|
at the specified index.
|
|
@item @code{adjustAddress(RawData,int)} -- Adjusts the pointer into the buffer.
|
|
@item @code{shiftDown(RawData,int,int,int)} -- Moves the content of the buffer
|
|
at an offset down to a new offset using @code{memmove}.
|
|
@end itemize
|
|
|
|
@node java.nio.channels, gnu.java.nio, java.nio, Classpath Hooks
|
|
@section java.nio.channels
|
|
|
|
Channels provide the data for the buffers with the New I/O packages.
|
|
For example, a channel may wrap a file or a socket. The VM hooks,
|
|
at the moment, simply allow the channels to be accessed by @code{java.io}
|
|
streams.
|
|
|
|
@menu
|
|
* java.nio.channels.VMChannels::
|
|
@end menu
|
|
|
|
@node java.nio.channels.VMChannels,,java.nio.channels,java.nio.channels
|
|
@subsection @code{java.nio.channels.VMChannels}
|
|
|
|
@code{VMChannels} provides the methods that create the channels or
|
|
streams. The default implementation is in pure Java and simply wraps
|
|
the channels in standard I/O classes from @code{java.io}.
|
|
|
|
@itemize @bullet
|
|
@item @code{createStream(Class,Channel)} -- Creates a @code{FileChannel}
|
|
which wraps an instance of the specified stream class, created by reflection.
|
|
This method is private, and is used by the other two.
|
|
@item @code{newInputStream(ReadableByteChannel)} -- Wraps the channel
|
|
in a @code{FileInputStream}.
|
|
@item @code{newOutputStream(WritableByteChannel)} -- Wraps the channel
|
|
in a @code{FileOutputStream}.
|
|
@end itemize
|
|
|
|
@node gnu.java.nio, Classpath Callbacks, java.nio.channels, Classpath Hooks
|
|
@section gnu.java.nio
|
|
|
|
The @code{gnu.java.nio} class provides Classpath implementations of the
|
|
interfaces provided by @code{java.nio}. The VM classes provide the native
|
|
support necessary to implement @emph{pipes} and @emph{selectors}.
|
|
|
|
@menu
|
|
* gnu.java.nio.VMPipe::
|
|
* gnu.java.nio.VMSelector::
|
|
@end menu
|
|
|
|
@node gnu.java.nio.VMPipe,gnu.java.nio.VMSelector,gnu.java.nio,gnu.java.nio
|
|
@subsection @code{gnu.java.nio.VMPipe}
|
|
|
|
@code{VMPipe} provides the native functionality for a uni-directional pipe
|
|
between a source and a destination (sink) channel. It consists of one
|
|
@code{static} @code{native} method, @code{init(PipeImpl,SelectorProvider)},
|
|
the reference implementation of which is currently a native stub. Ideally,
|
|
this should initialise the pipe at the native level.
|
|
|
|
@node gnu.java.nio.VMSelector,,gnu.java.nio.VMPipe,gnu.java.nio
|
|
@subsection @code{gnu.java.nio.VMSelector}
|
|
|
|
A @code{Selector} selects between multiple @code{SelectableChannel}s based
|
|
on their readiness and a key set. The VM hook for the Classpath implementation
|
|
of this is @code{VMSelector}, and this allows the actual @code{select()}
|
|
operation to be performed. This is represented by the @code{static}
|
|
@code{native} method, @code{select(int[],int[],int[],long)}, and a default
|
|
implementation of this is provided.
|
|
|
|
@node Classpath Callbacks, , gnu.java.nio, Classpath Hooks
|
|
Some of the classes you implement for the VM will need to call back to
|
|
package-private methods in Classpath:
|
|
|
|
@itemize @bullet
|
|
@item @code{java.lang.ThreadGroup.addThread(Thread)}
|
|
Call this method from @code{Thread} when a new @code{Thread} is created, to add it to
|
|
the group.
|
|
|
|
@item @code{java.lang.ThreadGroup.removeThread(Thread)}
|
|
Call this method from @code{Thread} when a @code{Thread} is stopped or destroyed.
|
|
@end itemize
|
|
|
|
@node VM Hooks, JNI Implementation, Classpath Hooks, Top
|
|
@comment node-name, next, previous, up
|
|
@chapter VM Hooks
|
|
|
|
VMs need to do some dirty work; there are some things in the VM that
|
|
unfortunately are dependent on the internal structure of various
|
|
classes. This is a guide to all of the things the VM itself needs to
|
|
know about classes.
|
|
|
|
Some of the core classes, while being implemented by GNU Classpath,
|
|
provide space for state (in the form of a @code{vmdata} object) to be
|
|
stored by the VM, and can not be constructed normally.
|
|
|
|
@itemize @bullet
|
|
@item java.lang.Class
|
|
@item java.lang.ClassLoader
|
|
@end itemize
|
|
|
|
The default implementations of some VM classes also follow this methodology,
|
|
when it is intended that most VMs will keep the default.
|
|
|
|
@itemize @bullet
|
|
@item java.lang.VMThread
|
|
@item java.lang.VMThrowable
|
|
@end itemize
|
|
|
|
Several core classes must be completely implemented by the VM for Classpath to
|
|
work, although reference implementations are provided. These classes are:
|
|
|
|
@itemize @bullet
|
|
@item java.lang.reflect.Constructor
|
|
@item java.lang.reflect.Method
|
|
@item java.lang.reflect.Field
|
|
@end itemize
|
|
|
|
The following issues are of note;
|
|
|
|
@itemize @bullet
|
|
@item @code{java.lang.Class} @*
|
|
The GNU Classpath implementation of @code{java.lang.Class} provides an
|
|
object for storing the internal state of the class maintained by the VM.
|
|
This is the only known place where this matters. The class is
|
|
constructed with this data by the VM. Some VMs do not create the
|
|
@code{Class} object at the point where the class is defined; instead,
|
|
they wait until a @code{Class} object is actually used.
|
|
|
|
@item Array Classes @*
|
|
When you are creating an array class, you should set the
|
|
@code{ClassLoader} of the array class to the @code{ClassLoader} of its
|
|
component type. Whenever you add a class to a @code{ClassLoader}, you
|
|
need to notify the @code{ClassLoader} and add the new @code{Class} to
|
|
its internal cache of classes. To do this, call
|
|
@code{ClassLoader.addVMCreatedClass(Class)}. @emph{Note: this is
|
|
written in anticipation of 1.2 support and does not apply just yet.}
|
|
|
|
@item Primordial Class Loader @*
|
|
When the primordial class loader loads a class, it needs to tell
|
|
Classpath what it has done in order for security stuff to work right.
|
|
To do this, call the static method
|
|
@code{ClassLoader.newPrimordialClass(Class)}.
|
|
|
|
Even the first few core classes need to do this; in order to do it,
|
|
simply call this method @emph{after} the initial class loading has been
|
|
done. No harm will come, as long as you follow the guidelines in the
|
|
@pxref{Initialization} section.
|
|
|
|
@emph{Note: this is written in anticipation of 1.2 support and does not
|
|
apply just yet.}
|
|
|
|
@item Top-level Exception Handler @*
|
|
Exceptions take care of themselves in Classpath; all you need to do in
|
|
the top-level exception handler is call @code{Throwable.printStackTrace()}.
|
|
|
|
@item Security and Traces @*
|
|
There will eventually be a feature in the 1.2 security that keeps the
|
|
@code{AccessController} from having to evaluate @emph{all} of the
|
|
@code{ProtectionDomain}s every time a security check is made. I think a common
|
|
case is a single method doing a lot of things that require security
|
|
checks. However, I don't want to bog down the method stack too much, so
|
|
this feature of the VM will have the @code{AccessController} for a thread
|
|
calling out to the VM to tell it how high it was on the stack when it
|
|
made the last security request. Every time the stack goes lower than
|
|
that number, the VM will decrement the number. The @code{AccessController}
|
|
will remember what the accumulated protection status was at every stack
|
|
level (an @code{AccessControlContext}) and use that aggregated information to
|
|
do the check. I am not sure, however, whether the savings are
|
|
substantial enough to outweigh the integer check and set after every
|
|
method call. I will investigate.
|
|
|
|
@item Threading @*
|
|
I figured I'd put this here because a VM guy might be wondering about it.
|
|
We implement @code{ThreadGroup}, but that class is almost entirely
|
|
VM-independent. The root @code{ThreadGroup}, a static field called
|
|
@code{ThreadGroup.root}, should be initialized by Classpath, but if you wish to
|
|
reinitialize it yourself, there should be no harm.
|
|
|
|
@end itemize
|
|
|
|
@node JNI Implementation, Miscellaneous VM Requirements, VM Hooks, Top
|
|
@comment node-name, next, previous, up
|
|
@chapter JNI Implementation
|
|
|
|
Classpath comes with its own implementation of @file{jni.h}. This
|
|
file can be customized by the VM in a few ways, by defining macros
|
|
that affect the interpretation of the file. These macros are all
|
|
intended for use by a VM which uses GNU Classpath and which wants to
|
|
use a single copy of @file{jni.h} for both internal and external use.
|
|
|
|
@itemize @bullet
|
|
@item _CLASSPATH_VM_JNI_TYPES_DEFINED
|
|
Some VMs like to define JNI ``object'' types in a special way. If
|
|
this macro is defined, the Classpath @file{jni.h} will avoid defining
|
|
these types. By default, these types are defined in @file{jni.h}.
|
|
The full list of types and macros treated this way is: @samp{jobject},
|
|
@samp{jclass}, @samp{jstring}, @samp{jthrowable}, @samp{jweak},
|
|
@samp{jarray}, @samp{jobjectArray}, @samp{jbyteArray},
|
|
@samp{jshortArray}, @samp{jintArray}, @samp{jlongArray},
|
|
@samp{jbooleanArray}, @samp{jcharArray}, @samp{jfloatArray},
|
|
@samp{jdoubleArray}, @samp{JNIEnv}, @samp{JavaVM}, @samp{JNI_TRUE}
|
|
(macro), @samp{JNI_FALSE} (macro).
|
|
|
|
@item _CLASSPATH_VM_INTERNAL_TYPES_DEFINED
|
|
If the VM has its own definitions for @samp{jfieldID} and
|
|
@samp{jmethodID}, then it should define this macro. Otherwise,
|
|
@file{jni.h} will provide definitions for these types.
|
|
|
|
@item _CLASSPATH_JNIIMPEXP
|
|
Three functions -- @samp{JNI_GetDefaultJavaVMInitArgs},
|
|
@samp{JNI_CreateJavaVM}, and @samp{JNI_GetCreatedJavaVMs} -- must be
|
|
marked as @samp{JNIIMPORT} when seen by user code, but most likely
|
|
should be marked as @samp{JNIEXPORT} when defined in the VM
|
|
implementation. This macro can be defined to one or the other by the
|
|
VM as appropriate. If this macro is not defined, it defaults to
|
|
@samp{JNIIMPORT}.
|
|
|
|
@item _CLASSPATH_JNIENV_CONTENTS
|
|
A VM can add fields to the @samp{JNIEnv} structure by defining this to
|
|
be a sequence of field declarations.
|
|
|
|
@end itemize
|
|
|
|
|
|
@node Miscellaneous VM Requirements, , JNI Implementation, Top
|
|
@comment node-name, next, previous, up
|
|
@chapter Miscellaneous VM Requirements
|
|
|
|
Classpath places a few requirements on the VM that uses it.
|
|
|
|
@menu
|
|
* JNI Version::
|
|
* VM Threading Model::
|
|
* Boot Library Path Property::
|
|
@end menu
|
|
|
|
@node JNI Version, VM Threading Model, Miscellaneous VM Requirements, Miscellaneous VM Requirements
|
|
@comment node-name, next, previous, up
|
|
@section JNI Version
|
|
|
|
Classpath currently uses only JNI 1.1, except for one JNI 1.2 function
|
|
in the JNI Invocation API: GetEnv(). And GetEnv() is only used in the
|
|
``portable native sync'' code, so it's only actually used by Jikes RVM
|
|
and Kaffe.
|
|
|
|
A future direction will probably be to require that all VMs provide
|
|
JNI 1.2. If this poses problems, please raise them on the classpath
|
|
mailing list.
|
|
|
|
@node VM Threading Model, Boot Library Path Property, JNI Version, Miscellaneous VM Requirements
|
|
@comment node-name, next, previous, up
|
|
@section VM Threading Model
|
|
|
|
Classpath's AWT peers use GTK+. GTK+ uses GLIB. Normally, Classpath
|
|
will initialize GLIB's @dfn{gthreads} to use
|
|
the platform's native threading model@footnote{The native threading
|
|
model is pthreads on Linux and AIX, the two platforms Classpath
|
|
currently runs on.}
|
|
|
|
If the Java runtime doesn't use the native threading model, then you
|
|
will want Classpath to tell GLIB to use the Java threading primitives
|
|
instead. Otherwise, GLIB would use the native threading model to
|
|
perform operations such as creating thread-local data, and that just
|
|
doesn't work on systems (such as Kaffe in some configurations, and
|
|
such as Jikes RVM) that use @i{m}:@i{n} threading.
|
|
|
|
Historically, enabling the Java threading primitives had been done at
|
|
build time, by configuring classpath with the
|
|
@option{--portable-native-sync} option. This had bad consequences,
|
|
though -- it meant that the prebuild GNU Classpath package distributed
|
|
with Debian GNU/Linux would not be usable with VMs that could
|
|
otherwise have used it. Instead, we encourage
|
|
the use of the Java system property
|
|
@code{gnu.classpath.awt.gtk.portable.native.sync}. A VM that wants
|
|
GLIB to use the Java threading primitives should modify
|
|
@code{VMRuntime.insertSystemProperties()} to include code like the
|
|
following:
|
|
|
|
@example
|
|
static void insertSystemProperties(Properties @var{p})
|
|
@end example
|
|
...
|
|
@example
|
|
@var{p}.put("gnu.classpath.awt.gtk.portable.native.sync", "true");
|
|
@end example
|
|
|
|
So, the configure option
|
|
@option{--portable-native-sync} is deprecated, and should go away in a
|
|
subsequent release of GNU Classpath.
|
|
|
|
@node Boot Library Path Property, , VM Threading Model, Miscellaneous VM Requirements
|
|
@comment node-name, next, previous, up
|
|
@section Boot Library Path Property
|
|
|
|
As of GNU Classpath 0.15 a system property named @code{gnu.classpath.boot.library.path}
|
|
can be set by the VM to specify the directories which contain GNU Classpath's native
|
|
libraries. Usually this value is given at configuration time and is then hardcoded
|
|
in the VM. However for development purposes it is handy to switch to another installation
|
|
by overriding the properties' value on the command line.
|
|
|
|
A VM that does not support this feature can simply ignore the property.
|
|
|
|
For compatibility reasons we suggest to set the default value of @code{java.library.path}
|
|
to the value of the @code{LD_LIBRARY_PATH} environment if it exists on your platform.
|
|
|
|
@bye
|
|
|
|
|
|
|