classloader1

http://www.blogjava.net/lhulcn618/archive/2006/05/25/48230.htmlClassLoader {属性：classloader Parent }public class SecureClassLoader extends ClassLoaderpublic class URLClassLoader extends SecureClassLoader implements Closeable在package sun.misc下有类public class Launcher{有内部类static class ExtClassLoader extends URLClassLoaderstatic class AppClassLoader extends URLClassLoader}AppClassLoader 的Parent 为ExtClassLoaderExtClassLoader的Parent 为null 不是 顶级的加载器（bootstrap class loader）Bootstrap启动Launcher 再启动AppClassLoader 和ExtClassLoaderthis.getClass.getClassLoader(); // 使用当前类的ClassLoader  （当前类是由什么加载器加载的）Thread.currentThread().getContextClassLoader(); // 使用当前线程的ClassLoader  （得到的Classloader是动态的，谁执行（某个线程），就是那个执行者的Classloader 但可以通过Thread.currentThread().setContextClassLoader(...);更改当前线程的contextClassLoader）注意：[this.getClass.getClassLoader()；  Thread.currentThread().getContextClassLoader()；  方 法一得到的Classloader是静态的，表明类的载入者是谁；方法二得到的Classloader是动态的，谁执行（某个线程），就是那个执行者的 Classloader。对于单例模式的类，静态类等，载入一次后，这个实例会被很多程序（线程）调用，对于这些类，载入的Classloader和执行 线程的Classloader通常都不同。]ClassLoader.getSystemClassLoader(); // 使 用系统ClassLoader，即系统的入口点所使用的ClassLoader。（注意，system ClassLoader与根 ClassLoader并不一样。JVM下system ClassLoader通常为App ClassLoader）源码：（ClassLoader中）    public static ClassLoader getSystemClassLoader() {initSystemClassLoader();if (scl == null) {    return null;}SecurityManager sm = System.getSecurityManager();if (sm != null) {    ClassLoader ccl = getCallerClassLoader();    if (ccl != null && ccl != scl && !scl.isAncestor(ccl)) {sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);    }}return scl;    }private static synchronized void initSystemClassLoader() {if (!sclSet) {    if (scl != null)throw new IllegalStateException("recursive invocation");            sun.misc.Launcher l = sun.misc.Launcher.getLauncher();    if (l != null) {Throwable oops = null;scl = l.getClassLoader();        try {    PrivilegedExceptionAction a;    a = new SystemClassLoaderAction(scl);                    scl = (ClassLoader) AccessController.doPrivileged(a);        } catch (PrivilegedActionException pae) {    oops = pae.getCause();            if (oops instanceof InvocationTargetException) {        oops = oops.getCause();    }        }if (oops != null) {    if (oops instanceof Error) {throw (Error) oops;    } else {        // wrap the exception        throw new Error(oops);    }}    }    sclSet = true;}    }（ClassLoader中内部类）class SystemClassLoaderAction implements PrivilegedExceptionAction {    private ClassLoader parent;SystemClassLoaderAction(ClassLoader parent) {this.parent = parent;    }    public Object run() throws Exception {ClassLoader sys;Constructor ctor;Class c;Class cp[] = { ClassLoader.class };Object params[] = { parent };        String cls = System.getProperty("java.system.class.loader");if (cls == null) {    return parent;}c = Class.forName(cls, true, parent);ctor = c.getDeclaredConstructor(cp);sys = (ClassLoader) ctor.newInstance(params);Thread.currentThread().setContextClassLoader(sys);return sys;    }}首先获得 父类加载器（ExtClassLoader）   sun.misc.Launcher l = sun.misc.Launcher.getLauncher();if (l != null) {scl = l.getClassLoader();//返回AppClassLoader参考sun.misc.Launcher再试图获取[用户希望的系统加载器]System.getProperty("java.system.class.loader");//用户自定义设置的加载器  String cls = System.getProperty("java.system.class.loader");if (cls == null) {    return parent;//如果不存在直接返回父类加载器（即：AppClassLoader）}如果存在则:c = Class.forName(cls, true, parent);//用父类（即：AppClassLoader）加载器加载这个classClass cp[] = { ClassLoader.class };Object params[] = { parent };ctor = c.getDeclaredConstructor(cp);sys = (ClassLoader) ctor.newInstance(params);再将生成的class对象传入[父类加载器AppClassLoader]作为参数生成实例对象sys最终返回sys。Thread.currentThread().setContextClassLoader(sys);//此时当前线程的加载器是sys