classloader.java

gcc的组建

  }

  /**
   * Called for every class name that is needed but has not yet been
   * defined by this classloader or one of its parents. It is called by
   * <code>loadClass()</code> after both <code>findLoadedClass()</code> and
   * <code>parent.loadClass()</code> couldn't provide the requested class.
   *
   * <p>The default implementation throws a
   * <code>ClassNotFoundException</code>. 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:<br>
   *
   * <pre>
   * // 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);
   * </pre>
   *
   * <p><code>loadClass()</code> makes sure that the <code>Class</code>
   * returned by <code>findClass()</code> will later be returned by
   * <code>findLoadedClass()</code> 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
   * <code>findClass()</code> implementation. The name should use '.'
   * separators, and discard the trailing ".class".  The default protection
   * domain has the permissions of
   * <code>Policy.getPolicy().getPermissions(new CodeSource(null, null))</code>.
   *
   * @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 <code>findClass()</code> implementation. If the
   * domain is null, the default of
   * <code>Policy.getPolicy().getPermissions(new CodeSource(null, null))</code>
   * 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 <code>findClass(String)</code>.
   *
   * @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
   * <code>null</code>. A security check may be performed on
   * <code>RuntimePermission("getClassLoader")</code>.
   *
   * @return the parent <code>ClassLoader</code>
   * @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)[1];
        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 <code>getResource()</code>
   * on the parent classloader. If the parent classloader returns null then
   * it tries finding the resource by calling <code>findResource()</code> on
   * this classloader. The resource name should be separated by '/' for path
   * elements.
   *
   * <p>Subclasses should not override this method but should override
   * <code>findResource()</code> 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. <code>getResource()</code> 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.
   *
   * <p>The Enumeration is created by first calling <code>getResources()</code>
   * on the parent classloader and then calling <code>findResources()</code>
   * on this classloader.</p>
   *
   * @param name the resource name
   * @return an enumaration of all resources found
   * @throws IOException if I/O errors occur in the process
   * @since 1.2
   */
  public final Enumeration getResources(String name) throws IOException
  {
    Enumeration parentResources;
    if (parent == null)
      parentResources = VMClassLoader.getResources(name);
    else
      parentResources = parent.getResources(name);
    return new DoubleEnumeration(parentResources, findResources(name));
  }

  /**
   * Called whenever all locations of a named resource are needed.
   * It is called by <code>getResources()</code> after it has called
   * <code>parent.getResources()</code>. The results are combined by
   * the <code>getResources()</code> method.
   *
   * <p>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 <code>findResource</code>.
   *
   * @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
   * <code>getResource()</code> after <code>parent.getResource()</code>
   * couldn't provide the requested resource.
   *
   * <p>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);
  }