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() );
    }
}