分析AppClassLoader,ExtClassLoader 和URLClassLoader 的关系

分析AppClassLoader,ExtClassLoader 和URLClassLoader 的关系

分类： JAVA_WEB 2011-12-06 16:43  133人阅读  评论(0)  收藏  举报

分析AppClassLoader,ExtClassLoader 和URLClassLoader 的关系

测试代码: 

 class Hello
 {
     public String str = "Hello World";
     public void fun()
     {
         System.out.println(str);
     }
 }

 public class Test
 {
     public static void main(String[] args)
     {
         Hello hello = new Hello();
         hello.fun();

         System.out.println("----------------------");

         //Hello类的类加载器
         ClassLoader classLoaderOfHello = Hello.class.getClassLoader();

         System.out.println("Hello is Loaded by : "+classLoaderOfHello);

         System.out.println("----------------------");

         //Hello类的类加载器的Class对象
         Class AppClazz = classLoaderOfHello.getClass();

         //分析Hello类的类加载器的Class对象的类继承关系
         while(AppClazz != null)
         {
             System.out.println(AppClazz);

             AppClazz = AppClazz.getSuperclass();
         }

         System.out.println("----------------------");

         //取得扩展器加载器的类对象Class
         Class ExtClazz = classLoaderOfHello.getParent().getClass();

         while(ExtClazz != null)
         {
             System.out.println(ExtClazz);

             ExtClazz = ExtClazz.getSuperclass();
         }
     }
 }

结论:

1.  用户自定义的类是由 应用(系统)类加载器AppClassLoader加载

2.  在”父亲委托机制”中,扩展类加载器ExtClassLoader是AppClassLoader的父亲.

3.  AppClassLoader 和 ExtClassLoader 都扩展于 URLClassLoader加载器.

4.  也同时说明AppClassLoader而非继承ExtClassLoader.

继承关系:

java.lang.Object

       --- java.lang.ClassLoader
              --- java.security.SecureClassLoader
                      ---  java.net.URLClassLoader
                            --- sun.misc.Launcher$ExtClassLoader

java.lang.Object

       --- java.lang.ClassLoader
              --- java.security.SecureClassLoader
                      ---  java.net.URLClassLoader
                            --- sun.misc.Launcher$AppClassLoader

其实很简单嘛,直接看AppClassLoader的源代码就可以了嘛,哈哈,终于找到了好东东,上

JDK7: http://download.java.net/openjdk/jdk7/

JDK6: http://download.java.net/openjdk/jdk6/

下载其源代码就可以了

现在直接来看其源代码:

 /**
      * The class loader used for loading from java.class.path.
      * runs in a restricted security context.
      */
     static class AppClassLoader extends URLClassLoader {

         static {
             ClassLoader.registerAsParallelCapable();
         }

         public static ClassLoader getAppClassLoader(final ClassLoader extcl)
             throws IOException
         {
             final String s = System.getProperty("java.class.path");
             final File[] path = (s == null) ? new File[0] : getClassPath(s);


             return AccessController.doPrivileged(
                 new PrivilegedAction<AppClassLoader>() {
                     public AppClassLoader run() {
                     URL[] urls =
                         (s == null) ? new URL[0] : pathToURLs(path);
                     return new AppClassLoader(urls, extcl);
                 }
             });
         }

         /*
          * Creates a new AppClassLoader
          */
         AppClassLoader(URL[] urls, ClassLoader parent) {
             super(urls, parent, factory);
         }

         /**
          * Override loadClass so we can checkPackageAccess.
          */
         public Class loadClass(String name, boolean resolve)
             throws ClassNotFoundException
         {
             int i = name.lastIndexOf('.');
             if (i != -1) {
                 SecurityManager sm = System.getSecurityManager();
                 if (sm != null) {
                     sm.checkPackageAccess(name.substring(0, i));
                 }
             }
             return (super.loadClass(name, resolve));
         }

         /**
          * allow any classes loaded from classpath to exit the VM.
          */
         protected PermissionCollection getPermissions(CodeSource codesource)
         {
             PermissionCollection perms = super.getPermissions(codesource);
             perms.add(new RuntimePermission("exitVM"));
             return perms;
         }

         /**
          * This class loader supports dynamic additions to the class path
          * at runtime.
          *
          * @see java.lang.instrument.Instrumentation#appendToSystemClassPathSearch
          */
         private void appendToClassPathForInstrumentation(String path) {
             assert(Thread.holdsLock(this));

             // addURL is a no-op if path already contains the URL
             super.addURL( getFileURL(new File(path)) );
         }

         /**
          * create a context that can read any directories (recursively)
          * mentioned in the class path. In the case of a jar, it has to
          * be the directory containing the jar, not just the jar, as jar
          * files might refer to other jar files.
          */

         private static AccessControlContext getContext(File[] cp)
             throws java.net.MalformedURLException
         {
             PathPermissions perms =
                 new PathPermissions(cp);

             ProtectionDomain domain =
                 new ProtectionDomain(new CodeSource(perms.getCodeBase(),
                     (java.security.cert.Certificate[]) null),
                 perms);

             AccessControlContext acc =
                 new AccessControlContext(new ProtectionDomain[] { domain });

             return acc;
         }
     }

哈,看了AppClassLoader的源代码后,大家明白了吧,AppClassLoader 继承了URLClassLoader,而且构造函数是直接调用URLClassLoader的构造

函数,loadClass(String name, boolean resolve)方法只是简单做了包的安全检查,然后就调用ClassLoader的 loadClass(String name, boolean resolve)方法了,其它的话,也是差不多..所以其功能和URLClassLoader差不多...

在ExtClassLoader也差不多,大家看看源代码就明了的:

 /*
        * Creates a new ExtClassLoader for the specified directories.
        */
       public ExtClassLoader(File[] dirs) throws IOException {
           super(getExtURLs(dirs), null, factory);
       }

       private static File[] getExtDirs() {
           String s = System.getProperty("java.ext.dirs");
           File[] dirs;
           if (s != null) {
               StringTokenizer st =
                   new StringTokenizer(s, File.pathSeparator);
               int count = st.countTokens();
               dirs = new File[count];
               for (int i = 0; i < count; i++) {
                   dirs[i] = new File(st.nextToken());
               }
           } else {
               dirs = new File[0];
           }
           return dirs;
       }

       private static URL[] getExtURLs(File[] dirs) throws IOException {
           Vector<URL> urls = new Vector<URL>();
           for (int i = 0; i < dirs.length; i++) {
               String[] files = dirs[i].list();
               if (files != null) {
                   for (int j = 0; j < files.length; j++) {
                       if (!files[j].equals("meta-index")) {
                           File f = new File(dirs[i], files[j]);
                           urls.add(getFileURL(f));
                       }
                   }
               }
           }
           URL[] ua = new URL[urls.size()];
           urls.copyInto(ua);
           return ua;
       }

       /*
        * Searches the installed extension directories for the specified
        * library name. For each extension directory, we first look for
        * the native library in the subdirectory whose name is the value
        * of the system property <code>os.arch</code>. Failing that, we
        * look in the extension directory itself.
        */
       public String findLibrary(String name) {
           name = System.mapLibraryName(name);
           URL[] urls = super.getURLs();
           File prevDir = null;
           for (int i = 0; i < urls.length; i++) {
               // Get the ext directory from the URL
               File dir = new File(urls[i].getPath()).getParentFile();
               if (dir != null && !dir.equals(prevDir)) {
                   // Look in architecture-specific subdirectory first
                   // Read from the saved system properties to avoid deadlock
                   String arch = VM.getSavedProperty("os.arch");
                   if (arch != null) {
                       File file = new File(new File(dir, arch), name);
                       if (file.exists()) {
                           return file.getAbsolutePath();
                       }
                   }
                   // Then check the extension directory
                   File file = new File(dir, name);
                   if (file.exists()) {
                       return file.getAbsolutePath();
                   }
               }
               prevDir = dir;
           }
           return null;
       }

       private static AccessControlContext getContext(File[] dirs)
           throws IOException
       {
           PathPermissions perms =
               new PathPermissions(dirs);

           ProtectionDomain domain = new ProtectionDomain(
               new CodeSource(perms.getCodeBase(),
                   (java.security.cert.Certificate[]) null),
               perms);

           AccessControlContext acc =
               new AccessControlContext(new ProtectionDomain[] { domain });

           return acc;
       }
   }