Java核心代码（一）认识ClassLoader & Thread

1, 共5个必备线程，和1个用户线程Thread[Thread-0,5,main] //[name,priority,group.name]

       main group: 5-main; 5-Thread-0

       system group: 10-Reference Handler, 9-Signal Dispatcher, 8-Finalizer, 5-Attach Listener

2,共2个ClassLoader:

       Bootstrap classloader是C写的，故==null, 加载10-Reference Handler, 8-Finalizer

       AppClassLoader，

 

  

/** * output :  * Thread[Thread-0,5,main]====sun.misc.Launcher$AppClassLoader@5984cf08 * Thread[main,5,main]====sun.misc.Launcher$AppClassLoader@5984cf08 * Thread[Finalizer,8,system]====null [Bootstrap classloader] * Thread[Thread-0,5,main]====sun.misc.Launcher$AppClassLoader@5984cf08 * Thread[Reference Handler,10,system]====null [Bootstrap classloader] * Thread[Signal Dispatcher,9,system]====sun.misc.Launcher$AppClassLoader@5984cf08 * Thread[Attach Listener,5,system]====sun.misc.Launcher$AppClassLoader@5984cf08 * @author naijiang.wang * @date 2013-08-05 9:37:22 AM */public class T {    public static void main( String[] args )        throws Exception {        new Thread(new Runnable() {            @Override            public void run() {                Thread t0 = Thread.currentThread();                System.out.println(t0+"===="+t0.getContextClassLoader());                try {                    Thread.sleep( 1000 );                }                catch ( InterruptedException e ) {                    e.printStackTrace();                }            }        }).start();        for(Thread t : Thread.getAllStackTraces().keySet()){            System.out.println(t+"===="+t.getContextClassLoader());        }    }}

 

 2，自定义java.lang package,会导致jvm错误：

关键：可以看出加载类顺序

package java.lang;/** * output: * Exception in thread "main" java.lang.SecurityException: Prohibited package name: java.lang    at java.lang.ClassLoader.preDefineClass(ClassLoader.java:649)    at java.lang.ClassLoader.defineClass(ClassLoader.java:785)    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)    at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)    at java.net.URLClassLoader.access$100(URLClassLoader.java:71)    at java.net.URLClassLoader$1.run(URLClassLoader.java:361)    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)    at java.security.AccessController.doPrivileged(Native Method)    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)    at java.lang.ClassLoader.loadClass(ClassLoader.java:423)    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)    at java.lang.ClassLoader.loadClass(ClassLoader.java:356)    at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:476) * @author rivertime.wang * @date 2014-3-8 15:06:38 */public class TestClassLoader {public static void main(String[] args) {System.out.println("user define 'java.lang' package");}}

 

 3,bootstrap, sun.misc.Launcher$AppClassLoader, ExtClassLoader

public class Test {    public static void main( String[] args )        throws IOException {        System.out.println( System.getProperty( "java.class.path" ) );                // null，bootstrap classloader(不是ClassLoader实例)        System.out.println( System.class.getClassLoader() );                // sun.misc.Launcher$AppClassLoader@4089f3e5        System.out.println( ClassLoader.getSystemClassLoader() );        System.out.println( Test.class.getClassLoader() );                // sun.misc.Launcher$ExtClassLoader@20eb607d        System.out.println( ClassLoader.getSystemClassLoader().getParent() );        System.out.println( Test.class.getClassLoader().getParent() );    }}