JVM的退出

public static void main(String[] args) {
		ThreadTest t = new ThreadTest();
		t.test2();
	}

	// jvm什么时候推出
	@Test
	public void test2() {
		// 除直接kill,其它JVM终止都会调用该钩子
		Runtime.getRuntime().addShutdownHook(new Thread() {
			@Override
			public void run() {
				System.out.println("关闭JVM");
			}
		});
		Thread t = new Thread() {
			@Override
			public void run() {
				System.out.println("用户线程正在运行....");
				try {
					Thread.sleep(10 * 1000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				System.out.println("用户线程结束....");
			}
		};
		t.setDaemon(false);
		//如果是false，JVM会等到最用一个用户线程跑完才执行终止操作，JVM自动调用Shutdown,用户也可以自行调用System.exit(0)
		//如果是true，说明是守护线程，JVM任务是辅助用的，可有可无，JVM不会等待该线程
		t.start();
		System.out.println("程序执行完毕");
	}

 在eclipse中运行，如果是通过Junit的方式测试运行test2方法，是模拟不出来预想的情况的。直接就退出了，为什么呢？看下Eclipse调用Junit的入口代码就知道了

org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(String[])

try {
			RemoteTestRunner testRunServer= new RemoteTestRunner();
			testRunServer.init(args);
			testRunServer.run();
		} catch (Throwable e) {
			e.printStackTrace(); // don't allow System.exit(0) to swallow exceptions
		} finally {
			// fix for 14434
			System.exit(0);
		}

 

这里显示的调用了退出方法。所以JVM就直接退出并释放资源。

在自己的main函数中由于自己没有主动调用System.exit(0);

把处理权交给了JVM，JVM在退出前会等待所有的用户线程都关闭掉。

 

 public static void main(String[] args) {

ThreadTest t = new ThreadTest();

t.test2();

throw new RuntimeException("xxx");

}

及时main线程抛出异常，JVM一样会等待...

 

java.c  这块处理的源码

/*
 * Always detach the main thread so that it appears to have ended when
 * the application's main method exits.  This will invoke the
 * uncaught exception handler machinery if main threw an
 * exception.  An uncaught exception handler cannot change the
 * launcher's return code except by calling System.exit.
 *
 * Wait for all non-daemon threads to end, then destroy the VM.
 * This will actually create a trivial new Java waiter thread
 * named "DestroyJavaVM", but this will be seen as a different
 * thread from the one that executed main, even though they are
 * the same C thread.  This allows mainThread.join() and
 * mainThread.isAlive() to work as expected.
 */
#define LEAVE() \
    if ((*vm)->DetachCurrentThread(vm) != 0) { \
        JLI_ReportErrorMessage(JVM_ERROR2); \
        ret = 1; \
    } \
    (*vm)->DestroyJavaVM(vm); \
    return ret \