classloader.java

gcc的组建

   * <code>java.system.class.loader</code>, 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. The parent
   * class loader passed in the constructor is the default system class
   * loader.
   *
   * <p>Note that this is different from the bootstrap classloader that
   * actually loads all the real "system" classes.
   *
   * <p>A security check will be performed for
   * <code>RuntimePermission("getClassLoader")</code> 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 <code>java.system.class.loader</code> fails to load
   * @since 1.2
   */
  public static ClassLoader getSystemClassLoader()
  {
    // Check if we may return the system classloader
    SecurityManager sm = SecurityManager.current;
    if (sm != null)
      {
	ClassLoader cl = VMStackWalker.getCallingClassLoader();
	if (cl != null && cl != StaticData.systemClassLoader)
	  sm.checkPermission(new RuntimePermission("getClassLoader"));
      }

    return StaticData.systemClassLoader;
  }

  /**
   * Defines a new package and creates a Package object. The package should
   * be defined before any class in the package is defined with
   * <code>defineClass()</code>. The package should not yet be defined
   * before in this classloader or in one of its parents (which means that
   * <code>getPackage()</code> should return <code>null</code>). All
   * parameters except the <code>name</code> of the package may be
   * <code>null</code>.
   *
   * <p>Subclasses should call this method from their <code>findClass()</code>
   * implementation before calling <code>defineClass()</code> on a Class
   * in a not yet defined Package (which can be checked by calling
   * <code>getPackage()</code>).
   *
   * @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 <code>Runtime.loadLibrary()</code> to get an absolute path
   * to a (system specific) library that was requested by a class loaded
   * by this classloader. The default implementation returns
   * <code>null</code>. 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 <code>java.library.path</code> 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(String)
   * @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(boolean)
   * @see #setClassAssertionStatus(String, boolean)
   * @see #clearAssertionStatus()
   * @since 1.4
   */
  public synchronized void setPackageAssertionStatus(String name,
                                                     boolean enabled)
  {
    if (packageAssertionStatus == null)
      packageAssertionStatus
        = new HashMap(StaticData.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(boolean)
   * @see #setPackageAssertionStatus(String, boolean)
   * @see #clearAssertionStatus()
   * @since 1.4
   */
  public synchronized void setClassAssertionStatus(String name,
                                                   boolean enabled)
  {
    if (classAssertionStatus == null)
      classAssertionStatus = 
        new HashMap(StaticData.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;
  }

  private static URL[] getExtClassLoaderUrls()
  {
    String classpath = SystemProperties.getProperty("java.ext.dirs", "");
    StringTokenizer tok = new StringTokenizer(classpath, File.pathSeparator);
    ArrayList list = new ArrayList();
    while (tok.hasMoreTokens())
      {
	try
	  {
	    File f = new File(tok.nextToken());
	    File[] files = f.listFiles();
	    if (files != null)
	      for (int i = 0; i < files.length; i++)
		list.add(files[i].toURL());
	  }
	catch(Exception x)
	  {
	  }
      }
    URL[] urls = new URL[list.size()];
    list.toArray(urls);
    return urls;
  }

  private static void addFileURL(ArrayList list, String file)
  {
    try
      {
	list.add(new File(file).toURL());
      }
    catch(java.net.MalformedURLException x)
      {
      }
  }

  private static URL[] getSystemClassLoaderUrls()
  {
    String classpath = SystemProperties.getProperty("java.class.path", ".");
    StringTokenizer tok = new StringTokenizer(classpath, File.pathSeparator, true);
    ArrayList list = new ArrayList();
    while (tok.hasMoreTokens())
      {
	String s = tok.nextToken();
	if (s.equals(File.pathSeparator))
	    addFileURL(list, ".");
	else
	  {
	    addFileURL(list, s);
	    if (tok.hasMoreTokens())
	      {
		// Skip the separator.
		tok.nextToken();
		// If the classpath ended with a separator,
		// append the current directory.
		if (!tok.hasMoreTokens())
		    addFileURL(list, ".");
	      }
	  }
      }
    URL[] urls = new URL[list.size()];
    list.toArray(urls);
    return urls;
  }

  static ClassLoader defaultGetSystemClassLoader()
  {
    return createAuxiliarySystemClassLoader(
        createSystemClassLoader(getSystemClassLoaderUrls(),
            createExtClassLoader(getExtClassLoaderUrls(), null)));
  }

  static ClassLoader createExtClassLoader(URL[] urls, ClassLoader parent)
  {
    if (urls.length > 0)
      return new URLClassLoader(urls, parent);
    else
      return parent;
  }

  static ClassLoader createSystemClassLoader(URL[] urls, ClassLoader parent)
  {
    return
	new URLClassLoader(urls, parent)
	{
	    protected synchronized Class loadClass(String name,
		boolean resolve)
		throws ClassNotFoundException
	    {
		SecurityManager sm = SecurityManager.current;
		if (sm != null)
		{
		    int lastDot = name.lastIndexOf('.');
		    if (lastDot != -1)
			sm.checkPackageAccess(name.substring(0, lastDot));
		}
		return super.loadClass(name, resolve);
	    }
	};
  }

  static ClassLoader createAuxiliarySystemClassLoader(ClassLoader parent)
  {
    String loader = SystemProperties.getProperty("java.system.class.loader", null);
    if (loader == null)
      {
	return parent;
      }
    try
      {
	Constructor c = Class.forName(loader, false, parent)
	    .getConstructor(new Class[] { ClassLoader.class });
	return (ClassLoader)c.newInstance(new Object[] { parent });
      }
    catch (Exception e)
      {
	System.err.println("Requested system classloader " + loader + " failed.");
	throw (Error)
	    new Error("Requested system classloader " + loader + " failed.")
		.initCause(e);
      }
  }

  /**
   * Before doing anything "dangerous" please call this method to make sure
   * this class loader instance was properly constructed (and not obtained
   * by exploiting the finalizer attack)
   * @see #initialized
   */
  private void checkInitialized()
  {
    if (! initialized)
      throw new SecurityException("attempt to use uninitialized class loader");
  }
}