Java中后台线程实例解析

本文研究的主要是Java中后台线程的相关问题，具体介绍如下。

以前从来没有听说过，java中有后台线程这种东西。一般来说，JVM(JAVA虚拟机）中一般会包括俩种线程，分别是用户线程和后台线程。所谓后台线程（daemon)线程指的是：在程序运行的时候在后台提供的一种通用的服务的线程，并且这种线程并不属于程序中不可或缺的部分。因此，当所有的非后台线程结束的时候，也就是用户线程都结束的时候，程序也就终止了。同时，会杀死进程中的所有的后台线程。反过来说，只要有任何非后台线程还在运行，程序就不会结束。不如执行main()的就是一个非后台线程。

基于这个特点，当虚拟机中的用户线程全部退出运行时，守护线程没有服务的对象后，JVM也就退出了。

这点JDK源码中的介绍已经说明这一点了。

* Marks this thread as either a {@linkplain #isDaemon daemon} thread
* or a user thread. The Java Virtual Machine exits when the only
* threads running are all daemon threads.

1.后台线程的启动的条件：


public class DaemonRunner implements Runnable {
	@Override
	 public void run() {
		while (true) {
			for (int i = 0; i < 3; i++) {
				System.out.println("守护线程" + i);
			}
		}
	}
	public static void main(String[] args) {
		Thread daemon = new Thread(new DaemonRunner());
		daemon.setDaemon(true);
		daemon.start();
		Scanner s = new Scanner(System.in);
		String string=s.nextLine();
		Runtime.getRuntime().addShutdownHook(new Thread(){
			@Override
			   public void run() {
				super.run();
				System.out.println("JVM退出");
				try {
					TimeUnit.MILLISECONDS.sleep(50);
				}
				catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
		}
		);
	}
}

2.在后台线程中启动的线程都属于后台线程。尽管你没有明确指明它们是后台线程，但是它们的确是后台线程。


class Daemon implements Runnable{
	private Thread[] t = new Thread[10];
	@Override
	 public void run() {
		for (int i = 0; i < t.length; i++) {
			t[i] = new Thread(new DaemonSpawn());
			t[i].start();
			System.out.println("DaemonSpawn " + i + "started");
		}
		for (int i = 0; i < t.length; i++) {
			System.out.println("t[" + i + "].isDaemon" + t[i].isDaemon());
		}
		while (true) {
			Thread.yield();
		}
	}
}
class DaemonSpawn implements Runnable{
	@Override
	 public void run() {
		while (true) {
			Thread.yield();
		}
	}
}
public class Daemons {
	public static void main(String[] args) {
		Thread d = new Thread(new Daemon());
		d.setDaemon(true);
		d.start();
		System.out.println("d.isDaemon()=" + d.isDaemon());
		try {
			TimeUnit.SECONDS.sleep(1);
			//让启动的后台的线程可以获得一定的执行的时间。
		}
		catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

最后的执行的结果如下：

d.isDaemon()=true
DaemonSpawn 0started
DaemonSpawn 1started
DaemonSpawn 2started
DaemonSpawn 3started
DaemonSpawn 4started
DaemonSpawn 5started
DaemonSpawn 6started
DaemonSpawn 7started
DaemonSpawn 8started
DaemonSpawn 9started
t[0].isDaemontrue
t[1].isDaemontrue
t[2].isDaemontrue
t[3].isDaemontrue
t[4].isDaemontrue
t[5].isDaemontrue
t[6].isDaemontrue
t[7].isDaemontrue
t[8].isDaemontrue
t[9].isDaemontrue

3.通过为Executors.newCachedThreadPool()方法指定一个ThreadFactory的对象。通过这种方法，我们也可以将
我们想要启动的线程设置为后台线程。


class DaemonThreadFactory implements ThreadFactory{
	@Override
	  public Thread newThread(Runnable r) {
		Thread t = new Thread(r);
		t.setDaemon(true);
		return t;
	}
}

public class DaemonFromFactory implements Runnable {
	@Override
	  public void run() {
		try {
			while (true) {
				TimeUnit.MILLISECONDS.sleep(100);
				System.out.println(Thread.currentThread() + " " + this);
			}
		}
		catch (InterruptedException e) {
			System.out.println("Interrupted");
		}
	}
	public static void main(String[] args) {
		ExecutorService exec = Executors.newCachedThreadPool(new DaemonThreadFactory());
		for (int i = 0; i < 10; i++) {
			exec.execute(new DaemonFromFactory());
		}
		System.out.println("All dameons started");
		try {
			TimeUnit.MILLISECONDS.sleep(500);
		}
		catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

最后的输出的结果为：

All dameons started
Thread[Thread-3,5,main] Concurrency.DaemonFromFactory@56214c1
Thread[Thread-2,5,main] Concurrency.DaemonFromFactory@5724147d
Thread[Thread-0,5,main] Concurrency.DaemonFromFactory@144fe080
Thread[Thread-1,5,main] Concurrency.DaemonFromFactory@104fa29e
Thread[Thread-8,5,main] Concurrency.DaemonFromFactory@5b069a7f
Thread[Thread-9,5,main] Concurrency.DaemonFromFactory@1a7288d1
Thread[Thread-7,5,main] Concurrency.DaemonFromFactory@25144c3e
Thread[Thread-4,5,main] Concurrency.DaemonFromFactory@288523d
Thread[Thread-6,5,main] Concurrency.DaemonFromFactory@1edae2a8
Thread[Thread-5,5,main] Concurrency.DaemonFromFactory@626007aa
Thread[Thread-3,5,main] Concurrency.DaemonFromFactory@56214c1
Thread[Thread-2,5,main] Concurrency.DaemonFromFactory@5724147d
Thread[Thread-6,5,main] Concurrency.DaemonFromFactory@1edae2a8
Thread[Thread-5,5,main] Concurrency.DaemonFromFactory@626007aa
Thread[Thread-4,5,main] Concurrency.DaemonFromFactory@288523d
Thread[Thread-9,5,main] Concurrency.DaemonFromFactory@1a7288d1
Thread[Thread-7,5,main] Concurrency.DaemonFromFactory@25144c3e
Thread[Thread-8,5,main] Concurrency.DaemonFromFactory@5b069a7f
Thread[Thread-1,5,main] Concurrency.DaemonFromFactory@104fa29e
Thread[Thread-0,5,main] Concurrency.DaemonFromFactory@144fe080
Thread[Thread-2,5,main] Concurrency.DaemonFromFactory@5724147d
Thread[Thread-3,5,main] Concurrency.DaemonFromFactory@56214c1
Thread[Thread-6,5,main] Concurrency.DaemonFromFactory@1edae2a8
Thread[Thread-1,5,main] Concurrency.DaemonFromFactory@104fa29e
Thread[Thread-0,5,main] Concurrency.DaemonFromFactory@144fe080
Thread[Thread-7,5,main] Concurrency.DaemonFromFactory@25144c3e
Thread[Thread-8,5,main] Concurrency.DaemonFromFactory@5b069a7f
Thread[Thread-5,5,main] Concurrency.DaemonFromFactory@626007aa
Thread[Thread-9,5,main] Concurrency.DaemonFromFactory@1a7288d1
Thread[Thread-4,5,main] Concurrency.DaemonFromFactory@288523d
Thread[Thread-2,5,main] Concurrency.DaemonFromFactory@5724147d
Thread[Thread-3,5,main] Concurrency.DaemonFromFactory@56214c1
Thread[Thread-8,5,main] Concurrency.DaemonFromFactory@5b069a7f
Thread[Thread-7,5,main] Concurrency.DaemonFromFactory@25144c3e
Thread[Thread-4,5,main] Concurrency.DaemonFromFactory@288523d
Thread[Thread-6,5,main] Concurrency.DaemonFromFactory@1edae2a8
Thread[Thread-1,5,main] Concurrency.DaemonFromFactory@104fa29e
Thread[Thread-0,5,main] Concurrency.DaemonFromFactory@144fe080
Thread[Thread-9,5,main] Concurrency.DaemonFromFactory@1a7288d1
Thread[Thread-5,5,main] Concurrency.DaemonFromFactory@626007aa
Thread[Thread-3,5,main] Concurrency.DaemonFromFactory@56214c1
Thread[Thread-2,5,main] Concurrency.DaemonFromFactory@5724147d
Thread[Thread-8,5,main] Concurrency.DaemonFromFactory@5b069a7f

4.首先应该意识到如果在用户线程突然退出的时候，那么后台线程在不执行finally子句的情况下就会终止其run方法。


class ADaemon implements Runnable{
  @Override
  public void run() {
    System.out.println("Starting ADaemon");
    try {
      TimeUnit.SECONDS.sleep(1);
    } catch (InterruptedException e) {
      System.out.println("Exiting via InterruptedException");
    }finally {
      System.out.println("This should always run?");
    }
  }
}
public class DaemonsDontRunFinally {
  public static void main(String[] args) {
    Thread t = new Thread(new ADaemon());
    t.setDaemon(true);
    t.start();
  }
}

最后的输出的结果如下：

Starting ADaemon

但是如果情况变为如下的情况，输出的结果又会不一样了：

class ADaemon implements Runnable{
  @Override
  public void run() {
    System.out.println("Starting ADaemon");
    try {
      TimeUnit.SECONDS.sleep(1);
    } catch (InterruptedException e) {
      System.out.println("Exiting via InterruptedException");
    }finally {
      System.out.println("This should always run?");
    }
  }
}
public class DaemonsDontRunFinally {
  public static void main(String[] args) {
    Thread t = new Thread(new ADaemon());
    t.setDaemon(true);
    t.start();
    try {
      TimeUnit.SECONDS.sleep(1);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
  }
}

由于主线程不是突然退出的，主线程在休眠期间，后台线程得到了执行的时间，所以最后的打印的结果为：

Starting ADaemon
This should always run?

总结

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Java中后台线程实例解析

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