Every classloader has a parent classloader that is consulted before * the 'child' classloader when classes or resources should be loaded. * This is done to make sure that classes can be loaded from an hierarchy of * multiple classloaders and classloaders do not accidentially redefine * already loaded classes by classloaders higher in the hierarchy. * *
The grandparent of all classloaders is the bootstrap classloader, which
* loads all the standard system classes as implemented by GNU Classpath. The
* other special classloader is the system classloader (also called
* application classloader) that loads all classes from the CLASSPATH
* (java.class.path system property). The system classloader
* is responsible for finding the application classes from the classpath,
* and delegates all requests for the standard library classes to its parent
* the bootstrap classloader. Most programs will load all their classes
* through the system classloaders.
*
*
The bootstrap classloader in GNU Classpath is implemented as a couple of
* static (native) methods on the package private class
* java.lang.VMClassLoader, the system classloader is an
* instance of gnu.java.lang.SystemClassLoader
* (which is a subclass of java.net.URLClassLoader).
*
*
Users of a ClassLoader will normally just use the methods
*
loadClass() to load a class.getResource() or getResourceAsStream()
* to access a resource.getResources() to get an Enumeration of URLs to all
* the resources provided by the classloader and its parents with the
* same name.Subclasses should implement the methods *
findClass() which is called by loadClass()
* when the parent classloader cannot provide a named class.findResource() which is called by
* getResource() when the parent classloader cannot provide
* a named resource.findResources() which is called by
* getResource() to combine all the resources with the
* same name from the classloader and its parents.findLibrary() which is called by
* Runtime.loadLibrary() when a class defined by the
* classloader wants to load a native library.checkCreateClassLoader.
*
* @throws SecurityException if the security check fails
*/
protected ClassLoader() throws SecurityException
{
this(systemClassLoader);
}
/**
* Create a new ClassLoader with the specified parent. The parent will
* be consulted when a class or resource is requested through
* loadClass() or getResource(). Only when the
* parent classloader cannot provide the requested class or resource the
* findClass() or findResource() method
* of this classloader will be called. There may be a security check for
* checkCreateClassLoader.
*
* @param parent the classloader's parent, or null for the bootstrap
* classloader
* @throws SecurityException if the security check fails
* @since 1.2
*/
protected ClassLoader(ClassLoader parent)
{
// May we create a new classloader?
SecurityManager sm = System.getSecurityManager();
if (sm != null)
sm.checkCreateClassLoader();
this.parent = parent;
this.initialized = true;
}
/**
* Load a class using this ClassLoader or its parent, without resolving
* it. Calls loadClass(name, false).
*
* Subclasses should not override this method but should override
* findClass() which is called by this method.
resolveClass(). It first tries to find
* out if the class has already been loaded through this classloader by
* calling findLoadedClass(). Then it calls
* loadClass() on the parent classloader (or when there is
* no parent it uses the VM bootclassloader). If the class is still
* not loaded it tries to create a new class by calling
* findClass(). Finally when resolve is
* true it also calls resolveClass() on the
* newly loaded class.
*
* Subclasses should not override this method but should override
* findClass() which is called by this method.
loadClass() after both findLoadedClass() and
* parent.loadClass() couldn't provide the requested class.
*
* The default implementation throws a
* ClassNotFoundException. Subclasses should override this
* method. An implementation of this method in a subclass should get the
* class bytes of the class (if it can find them), if the package of the
* requested class doesn't exist it should define the package and finally
* it should call define the actual class. It does not have to resolve the
* class. It should look something like the following:
*
*
* // Get the bytes that describe the requested class
* byte[] classBytes = classLoaderSpecificWayToFindClassBytes(name);
* // Get the package name
* int lastDot = name.lastIndexOf('.');
* if (lastDot != -1)
* {
* String packageName = name.substring(0, lastDot);
* // Look if the package already exists
* if (getPackage(packageName) == null)
* {
* // define the package
* definePackage(packageName, ...);
* }
* }
* // Define and return the class
* return defineClass(name, classBytes, 0, classBytes.length);
*
*
* loadClass() makes sure that the Class
* returned by findClass() will later be returned by
* findLoadedClass() when the same class name is requested.
*
* @param name class name to find (including the package name)
* @return the requested Class
* @throws ClassNotFoundException when the class can not be found
* @since 1.2
*/
protected Class findClass(String name) throws ClassNotFoundException
{
throw new ClassNotFoundException(name);
}
/**
* Helper to define a class using a string of bytes. This version is not
* secure.
*
* @param data the data representing the classfile, in classfile format
* @param offset the offset into the data where the classfile starts
* @param len the length of the classfile data in the array
* @return the class that was defined
* @throws ClassFormatError if data is not in proper classfile format
* @throws IndexOutOfBoundsException if offset or len is negative, or
* offset + len exceeds data
* @deprecated use {@link #defineClass(String, byte[], int, int)} instead
*/
protected final Class defineClass(byte[] data, int offset, int len)
throws ClassFormatError
{
return defineClass(null, data, offset, len);
}
/**
* Helper to define a class using a string of bytes without a
* ProtectionDomain. Subclasses should call this method from their
* findClass() implementation. The name should use '.'
* separators, and discard the trailing ".class". The default protection
* domain has the permissions of
* Policy.getPolicy().getPermissions(new CodeSource(null, null)).
*
* @param name the name to give the class, or null if unknown
* @param data the data representing the classfile, in classfile format
* @param offset the offset into the data where the classfile starts
* @param len the length of the classfile data in the array
* @return the class that was defined
* @throws ClassFormatError if data is not in proper classfile format
* @throws IndexOutOfBoundsException if offset or len is negative, or
* offset + len exceeds data
* @throws SecurityException if name starts with "java."
* @since 1.1
*/
protected final Class defineClass(String name, byte[] data, int offset,
int len) throws ClassFormatError
{
return defineClass(name, data, offset, len, null);
}
/**
* Helper to define a class using a string of bytes. Subclasses should call
* this method from their findClass() implementation. If the
* domain is null, the default of
* Policy.getPolicy().getPermissions(new CodeSource(null, null))
* is used. Once a class has been defined in a package, all further classes
* in that package must have the same set of certificates or a
* SecurityException is thrown.
*
* @param name the name to give the class. null if unknown
* @param data the data representing the classfile, in classfile format
* @param offset the offset into the data where the classfile starts
* @param len the length of the classfile data in the array
* @param domain the ProtectionDomain to give to the class, null for the
* default protection domain
* @return the class that was defined
* @throws ClassFormatError if data is not in proper classfile format
* @throws IndexOutOfBoundsException if offset or len is negative, or
* offset + len exceeds data
* @throws SecurityException if name starts with "java.", or if certificates
* do not match up
* @since 1.2
*/
protected final synchronized Class defineClass(String name, byte[] data,
int offset, int len,
ProtectionDomain domain)
throws ClassFormatError
{
if (domain == null)
domain = defaultProtectionDomain;
if (! initialized)
throw new SecurityException("attempt to define class from uninitialized class loader");
Class retval = VMClassLoader.defineClass(this, name, data,
offset, len, domain);
loadedClasses.put(retval.getName(), retval);
return retval;
}
/**
* Links the class, if that has not already been done. Linking basically
* resolves all references to other classes made by this class.
*
* @param c the class to resolve
* @throws NullPointerException if c is null
* @throws LinkageError if linking fails
*/
protected final void resolveClass(Class c)
{
VMClassLoader.resolveClass(c);
}
/**
* Helper to find a Class using the system classloader, possibly loading it.
* A subclass usually does not need to call this, if it correctly
* overrides findClass(String).
*
* @param name the name of the class to find
* @return the found class
* @throws ClassNotFoundException if the class cannot be found
*/
protected final Class findSystemClass(String name)
throws ClassNotFoundException
{
return Class.forName(name, false, systemClassLoader);
}
/**
* Returns the parent of this classloader. If the parent of this
* classloader is the bootstrap classloader then this method returns
* null. A security check may be performed on
* RuntimePermission("getClassLoader").
*
* @return the parent ClassLoader
* @throws SecurityException if the security check fails
* @since 1.2
*/
public final ClassLoader getParent()
{
// Check if we may return the parent classloader.
SecurityManager sm = System.getSecurityManager();
if (sm != null)
{
Class c = VMSecurityManager.getClassContext(ClassLoader.class)[0];
ClassLoader cl = c.getClassLoader();
if (cl != null && ! cl.isAncestorOf(this))
sm.checkPermission(new RuntimePermission("getClassLoader"));
}
return parent;
}
/**
* Helper to set the signers of a class. This should be called after
* defining the class.
*
* @param c the Class to set signers of
* @param signers the signers to set
* @since 1.1
*/
protected final void setSigners(Class c, Object[] signers)
{
c.setSigners(signers);
}
/**
* Helper to find an already-loaded class in this ClassLoader.
*
* @param name the name of the class to find
* @return the found Class, or null if it is not found
* @since 1.1
*/
protected final synchronized Class findLoadedClass(String name)
{
// NOTE: If the VM is keeping its own cache, it may make sense to have
// this method be native.
return (Class) loadedClasses.get(name);
}
/**
* Get the URL to a resource using this classloader or one of its parents.
* First tries to get the resource by calling getResource()
* on the parent classloader. If the parent classloader returns null then
* it tries finding the resource by calling findResource() on
* this classloader. The resource name should be separated by '/' for path
* elements.
*
*
Subclasses should not override this method but should override
* findResource() which is called by this method.
*
* @param name the name of the resource relative to this classloader
* @return the URL to the resource or null when not found
*/
public URL getResource(String name)
{
URL result;
if (parent == null)
result = VMClassLoader.getResource(name);
else
result = parent.getResource(name);
if (result == null)
result = findResource(name);
return result;
}
/**
* Returns an Enumeration of all resources with a given name that can
* be found by this classloader and its parents. Certain classloaders
* (such as the URLClassLoader when given multiple jar files) can have
* multiple resources with the same name that come from multiple locations.
* It can also occur that a parent classloader offers a resource with a
* certain name and the child classloader also offers a resource with that
* same name. getResource() only offers the first resource (of the
* parent) with a given name. This method lists all resources with the
* same name. The name should use '/' as path separators.
*
*
The Enumeration is created by first calling getResources()
* on the parent classloader and then calling findResources()
* on this classloader.
getResources() after it has called
* parent.getResources(). The results are combined by
* the getResources() method.
*
* The default implementation always returns an empty Enumeration.
* Subclasses should override it when they can provide an Enumeration of
* URLs (possibly just one element) to the named resource.
* The first URL of the Enumeration should be the same as the one
* returned by findResource.
*
* @param name the name of the resource to be found
* @return a possibly empty Enumeration of URLs to the named resource
* @throws IOException if I/O errors occur in the process
* @since 1.2
*/
protected Enumeration findResources(String name) throws IOException
{
return EmptyEnumeration.getInstance();
}
/**
* Called whenever a resource is needed that could not be provided by
* one of the parents of this classloader. It is called by
* getResource() after parent.getResource()
* couldn't provide the requested resource.
*
*
The default implementation always returns null. Subclasses should
* override this method when they can provide a way to return a URL
* to a named resource.
*
* @param name the name of the resource to be found
* @return a URL to the named resource or null when not found
* @since 1.2
*/
protected URL findResource(String name)
{
return null;
}
/**
* Get the URL to a resource using the system classloader.
*
* @param name the name of the resource relative to the system classloader
* @return the URL to the resource
* @since 1.1
*/
public static final URL getSystemResource(String name)
{
return systemClassLoader.getResource(name);
}
/**
* Get an Enumeration of URLs to resources with a given name using the
* the system classloader. The enumeration firsts lists the resources with
* the given name that can be found by the bootstrap classloader followed
* by the resources with the given name that can be found on the classpath.
*
* @param name the name of the resource relative to the system classloader
* @return an Enumeration of URLs to the resources
* @throws IOException if I/O errors occur in the process
* @since 1.2
*/
public static Enumeration getSystemResources(String name) throws IOException
{
return systemClassLoader.getResources(name);
}
/**
* Get a resource as stream using this classloader or one of its parents.
* First calls getResource() and if that returns a URL to
* the resource then it calls and returns the InputStream given by
* URL.openStream().
*
*
Subclasses should not override this method but should override
* findResource() which is called by this method.
*
* @param name the name of the resource relative to this classloader
* @return an InputStream to the resource, or null
* @since 1.1
*/
public InputStream getResourceAsStream(String name)
{
try
{
URL url = getResource(name);
if (url == null)
return null;
return url.openStream();
}
catch (IOException e)
{
return null;
}
}
/**
* Get a resource using the system classloader.
*
* @param name the name of the resource relative to the system classloader
* @return an input stream for the resource, or null
* @since 1.1
*/
public static final InputStream getSystemResourceAsStream(String name)
{
try
{
URL url = getSystemResource(name);
if (url == null)
return null;
return url.openStream();
}
catch (IOException e)
{
return null;
}
}
/**
* Returns the system classloader. The system classloader (also called
* the application classloader) is the classloader that was used to
* load the application classes on the classpath (given by the system
* property java.class.path. This is set as the context
* class loader for a thread. The system property
* java.system.class.loader, if defined, is taken to be the
* name of the class to use as the system class loader, which must have
* a public constructor which takes a ClassLoader as a parent; otherwise this
* uses gnu.java.lang.SystemClassLoader.
*
*
Note that this is different from the bootstrap classloader that * actually loads all the real "system" classes (the bootstrap classloader * is the parent of the returned system classloader). * *
A security check will be performed for
* RuntimePermission("getClassLoader") if the calling class
* is not a parent of the system class loader.
*
* @return the system class loader
* @throws SecurityException if the security check fails
* @throws IllegalStateException if this is called recursively
* @throws Error if java.system.class.loader fails to load
* @since 1.2
*/
public static ClassLoader getSystemClassLoader()
{
// Check if we may return the system classloader
SecurityManager sm = System.getSecurityManager();
if (sm != null)
{
Class c = VMSecurityManager.getClassContext(ClassLoader.class)[0];
ClassLoader cl = c.getClassLoader();
if (cl != null && cl != systemClassLoader)
sm.checkPermission(new RuntimePermission("getClassLoader"));
}
return systemClassLoader;
}
/**
* Defines a new package and creates a Package object. The package should
* be defined before any class in the package is defined with
* defineClass(). The package should not yet be defined
* before in this classloader or in one of its parents (which means that
* getPackage() should return null). All
* parameters except the name of the package may be
* null.
*
*
Subclasses should call this method from their findClass()
* implementation before calling defineClass() on a Class
* in a not yet defined Package (which can be checked by calling
* getPackage()).
*
* @param name the name of the Package
* @param specTitle the name of the specification
* @param specVendor the name of the specification designer
* @param specVersion the version of this specification
* @param implTitle the name of the implementation
* @param implVendor the vendor that wrote this implementation
* @param implVersion the version of this implementation
* @param sealed if sealed the origin of the package classes
* @return the Package object for the specified package
* @throws IllegalArgumentException if the package name is null or it
* was already defined by this classloader or one of its parents
* @see Package
* @since 1.2
*/
protected Package definePackage(String name, String specTitle,
String specVendor, String specVersion,
String implTitle, String implVendor,
String implVersion, URL sealed)
{
if (getPackage(name) != null)
throw new IllegalArgumentException("Package " + name
+ " already defined");
Package p = new Package(name, specTitle, specVendor, specVersion,
implTitle, implVendor, implVersion, sealed);
synchronized (definedPackages)
{
definedPackages.put(name, p);
}
return p;
}
/**
* Returns the Package object for the requested package name. It returns
* null when the package is not defined by this classloader or one of its
* parents.
*
* @param name the package name to find
* @return the package, if defined
* @since 1.2
*/
protected Package getPackage(String name)
{
Package p;
if (parent == null)
p = VMClassLoader.getPackage(name);
else
p = parent.getPackage(name);
if (p == null)
{
synchronized (definedPackages)
{
p = (Package) definedPackages.get(name);
}
}
return p;
}
/**
* Returns all Package objects defined by this classloader and its parents.
*
* @return an array of all defined packages
* @since 1.2
*/
protected Package[] getPackages()
{
// Get all our packages.
Package[] packages;
synchronized(definedPackages)
{
packages = new Package[definedPackages.size()];
definedPackages.values().toArray(packages);
}
// If we have a parent get all packages defined by our parents.
Package[] parentPackages;
if (parent == null)
parentPackages = VMClassLoader.getPackages();
else
parentPackages = parent.getPackages();
Package[] allPackages = new Package[parentPackages.length
+ packages.length];
System.arraycopy(parentPackages, 0, allPackages, 0,
parentPackages.length);
System.arraycopy(packages, 0, allPackages, parentPackages.length,
packages.length);
return allPackages;
}
/**
* Called by Runtime.loadLibrary() to get an absolute path
* to a (system specific) library that was requested by a class loaded
* by this classloader. The default implementation returns
* null. It should be implemented by subclasses when they
* have a way to find the absolute path to a library. If this method
* returns null the library is searched for in the default locations
* (the directories listed in the java.library.path system
* property).
*
* @param name the (system specific) name of the requested library
* @return the full pathname to the requested library, or null
* @see Runtime#loadLibrary()
* @since 1.2
*/
protected String findLibrary(String name)
{
return null;
}
/**
* Set the default assertion status for classes loaded by this classloader,
* used unless overridden by a package or class request.
*
* @param enabled true to set the default to enabled
* @see #setClassAssertionStatus(String, boolean)
* @see #setPackageAssertionStatus(String, boolean)
* @see #clearAssertionStatus()
* @since 1.4
*/
public void setDefaultAssertionStatus(boolean enabled)
{
defaultAssertionStatus = enabled;
}
/**
* Set the default assertion status for packages, used unless overridden
* by a class request. This default also covers subpackages, unless they
* are also specified. The unnamed package should use null for the name.
*
* @param name the package (and subpackages) to affect
* @param enabled true to set the default to enabled
* @see #setDefaultAssertionStatus(String, boolean)
* @see #setClassAssertionStatus(String, boolean)
* @see #clearAssertionStatus()
* @since 1.4
*/
public synchronized void setPackageAssertionStatus(String name,
boolean enabled)
{
if (packageAssertionStatus == null)
packageAssertionStatus
= new HashMap(systemPackageAssertionStatus);
packageAssertionStatus.put(name, Boolean.valueOf(enabled));
}
/**
* Set the default assertion status for a class. This only affects the
* status of top-level classes, any other string is harmless.
*
* @param name the class to affect
* @param enabled true to set the default to enabled
* @throws NullPointerException if name is null
* @see #setDefaultAssertionStatus(String, boolean)
* @see #setPackageAssertionStatus(String, boolean)
* @see #clearAssertionStatus()
* @since 1.4
*/
public synchronized void setClassAssertionStatus(String name,
boolean enabled)
{
if (classAssertionStatus == null)
classAssertionStatus = new HashMap(systemClassAssertionStatus);
// The toString() hack catches null, as required.
classAssertionStatus.put(name.toString(), Boolean.valueOf(enabled));
}
/**
* Resets the default assertion status of this classloader, its packages
* and classes, all to false. This allows overriding defaults inherited
* from the command line.
*
* @see #setDefaultAssertionStatus(boolean)
* @see #setClassAssertionStatus(String, boolean)
* @see #setPackageAssertionStatus(String, boolean)
* @since 1.4
*/
public synchronized void clearAssertionStatus()
{
defaultAssertionStatus = false;
packageAssertionStatus = new HashMap();
classAssertionStatus = new HashMap();
}
/**
* Return true if this loader is either the specified class loader
* or an ancestor thereof.
* @param loader the class loader to check
*/
final boolean isAncestorOf(ClassLoader loader)
{
while (loader != null)
{
if (this == loader)
return true;
loader = loader.parent;
}
return false;
}
}