测试代码:
1 class Hello 2 { 3 public String str = "Hello World"; 4 public void fun() 5 { 6 System.out.println(str); 7 } 8 } 9 10 public class Test{12 public static void main(String[] args)13 {14 Hello hello = new Hello();15 hello.fun();16 17 System.out.println("----------------------");18 19 //Hello类的类加载器20 ClassLoader classLoaderOfHello = Hello.class.getClassLoader();21 22 System.out.println("Hello is Loaded by : "+classLoaderOfHello);23 24 System.out.println("----------------------");25 26 //Hello类的类加载器的Class对象27 Class AppClazz = classLoaderOfHello.getClass();28 29 //分析Hello类的类加载器的Class对象的类继承关系30 while(AppClazz != null)31 {32 System.out.println(AppClazz);33 34 AppClazz = AppClazz.getSuperclass();35 }36 37 System.out.println("----------------------");38 39 //取得扩展器加载器的类对象Class40 Class ExtClazz = classLoaderOfHello.getParent().getClass();41 42 while(ExtClazz != null)43 {44 System.out.println(ExtClazz);45 46 ExtClazz = ExtClazz.getSuperclass();47 }48 }49 }50 51 
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，并不是继承关系，而是ExtClassLoader加载了AppClassLoader
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;
} }
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作者：irelandken 来源：CSDN 原文：https://blog.csdn.net/irelandken/article/details/7046689 版权声明：本文为博主原创文章，转载请附上博文链接！