Java中后台线程详细解答

时间:2022-10-21 18:32:28

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。

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

1.后台线程的启动的条件:
/*必须在启动线程之前调用SetDaemon()方法,才能把这个线程设置为后台线程。
* 在这个程序里面,当我们输入了字符串以后,那么Main线程就会停止运行了
* 那么程序中已经没有可以运行的用户的线程了。所以后台线程就会被停止了
* JVM也就会被停停止了,感兴趣的读者可以自己尝试一下*/
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。
/*可以通过调用isDaemon()方法来确定线程是否是一个后台线程。如果是一个后台线程,
* 那么它创建的任何线程都被自动设置成后台的线程
* 在这个实例中,Daemon线程被设置成了后台模式,然后派生出许多子线程,这些线程并没有被设置成
* 后台模式,不过它们的确是后台线程。接着,Daemon线程进入了无限循环,并且在循环里调用了yield方法
* 把控制权交给其它的线程或者进程*/

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。

/*在这个例子中,对于这个静态的构造方法:Executors.newCachedThreadPool(new DaemonThreadFactory()
* 我们可以为传入一个ThreadFactory的对象,那么我们就可以通过这种方法,将我们想要启动的线程设置为后台线程
* 这是要注意的。*/
class DaemonThreadFactory implements ThreadFactory{
@Override
public Thread newThread(Runnable r) {
Thread t = new Thread(r);
t.setDaemon(true);
return t;
}
}
/*在这个例子中,在Main方法中,会首先调用Main方法中的普通的方法,“ System.out.println("All dameons started");”
* 所以会首先打印这一条语句。然后在主线程休眠期间,相应的后台线程,就会获得执行的时间,最后在Main线程
* 结束了运行的时候,也就是当Main线程从休眠中恢复过来的时候,那么Main线性就会结束运行。接着,
* 那么所有的后台的线程都会停止。JVM也会停止执行。*/
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。
/*当你调用这个程序的时候,你将看到finally子句不会执行,但是如果你注释掉对setDaemon()的调用,你将看到
* finally 子句将会执行.
* 这种行为是正确的。即便你基于前面对finally给出的承诺,并不希望出现这种行为。但是情况就是这样
* 当最后一个非后台的线程终止的时候,后台线程就会突然的停止。因为一旦main()退出后,JVM就会立即关闭所有后台的
* 线程。因为你不能以优雅的方式来关闭后台线程,所以它们几乎不是一种好的思想。非后台的Executor通常是一种
* 更好的方式,因为Executor控制的所有的任务可以同时被关闭。*/
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?