Java 守护线程概述

Java的线程分为两种:User Thread(用户线程)、DaemonThread(守护线程)。

只要当前JVM实例中尚存任何一个非守护线程没有结束,守护线程就全部工作;只有当最后一个非守护线程结束是,守护线程随着JVM一同结束工作,Daemon作用是为其他线程提供便利服务,守护线程最典型的应用就是GC(垃圾回收器),他就是一个很称职的守护者。

User和Daemon两者几乎没有区别,唯一的不同之处就在于虚拟机的离开:如果 User Thread已经全部退出运行了,只剩下Daemon Thread存在了,虚拟机也就退出了。 因为没有了被守护者,Daemon也就没有工作可做了,也就没有继续运行程序的必要了。

首先看一个例子,主线程中建立一个守护线程,当主线程结束时,守护线程也跟着结束。

package com.daemon;  
  
import java.util.concurrent.TimeUnit;  
  
public class DaemonThreadTest  
{  
    public static void main(String[] args)  
    {  
        Thread mainThread = new Thread(new Runnable(){  
            @Override  
            public void run()  
            {  
                Thread childThread = new Thread(new ClildThread());  
                childThread.setDaemon(true);  
                childThread.start();  
                System.out.println("I'm main thread...");  
            }  
        });  
        mainThread.start();  
    }  
}  
  
class ClildThread implements Runnable  
{  
    @Override  
    public void run()  
    {  
        while(true)  
        {  
            System.out.println("I'm child thread..");  
            try  
            {  
                TimeUnit.MILLISECONDS.sleep(1000);  
            }  
            catch (InterruptedException e)  
            {  
                e.printStackTrace();  
            }  
        }  
    }  
}

运行结果:

I'm child thread..  
I'm main thread...

如果不何止childThread为守护线程,当主线程结束时,childThread还在继续运行,如下:

package com.daemon;  
  
import java.util.concurrent.TimeUnit;  
  
public class DaemonThreadTest  
{  
    public static void main(String[] args)  
    {  
        Thread mainThread = new Thread(new Runnable(){  
            @Override  
            public void run()  
            {  
                Thread childThread = new Thread(new ClildThread());  
                childThread.setDaemon(false);  
                childThread.start();  
                System.out.println("I'm main thread...");  
            }  
        });  
        mainThread.start();  
    }  
}  
  
class ClildThread implements Runnable  
{  
    @Override  
    public void run()  
    {  
        while(true)  
        {  
            System.out.println("I'm child thread..");  
            try  
            {  
                TimeUnit.MILLISECONDS.sleep(1000);  
            }  
            catch (InterruptedException e)  
            {  
                e.printStackTrace();  
            }  
        }  
    }  
}

运行结果:

I'm main thread...  
I'm child thread..  
I'm child thread..  
I'm child thread..  
I'm child thread..  
I'm child thread..(无限输出)

可以看到,当主线程结束时,childThread是非守护线程,就会无限的执行。

守护线程有一个应用场景,就是当主线程结束时,结束其余的子线程(守护线程)自动关闭,就免去了还要继续关闭子线程的麻烦。不过博主推荐,如果真有这种场景,还是用中断的方式实现比较合理。

还有补充一点,不是说当子线程是守护线程,主线程结束,子线程就跟着结束,这里的前提条件是:当前jvm应用实例中没有用户线程继续执行,如果有其他用户线程继续执行,那么后台线程不会中断,如下:

package com.daemon;  
  
import java.util.concurrent.TimeUnit;  
  
public class DaemonThreadTest  
{  
    public static void main(String[] args)  
    {  
        Thread mainThread = new Thread(new Runnable(){  
            @Override  
            public void run()  
            {  
                Thread childThread = new Thread(new ClildThread());  
                childThread.setDaemon(true);  
                childThread.start();  
                System.out.println("I'm main thread...");  
            }  
        });  
        mainThread.start();  
          
        Thread otherThread = new Thread(new Runnable(){  
            @Override  
            public void run()  
            {  
                while(true)  
                {  
                    System.out.println("I'm other user thread...");  
                    try  
                    {  
                        TimeUnit.MILLISECONDS.sleep(1000);  
                    }  
                    catch (InterruptedException e)  
                    {  
                        e.printStackTrace();  
                    }  
                }  
            }  
        });  
        otherThread.start();  
    }  
}  
  
class ClildThread implements Runnable  
{  
    @Override  
    public void run()  
    {  
        while(true)  
        {  
            System.out.println("I'm child thread..");  
            try  
            {  
                TimeUnit.MILLISECONDS.sleep(1000);  
            }  
            catch (InterruptedException e)  
            {  
                e.printStackTrace();  
            }  
        }  
    }  
}

运行结果:

I'm other user thread...  
I'm child thread..  
I'm main thread...  
I'm other user thread...  
I'm child thread..  
I'm other user thread...  
I'm child thread..  
I'm child thread..  
I'm other user thread...  
I'm other user thread...  
I'm child thread..  
I'm child thread..  
I'm other user thread...  
I'm other user thread...  
I'm child thread..  
I'm other user thread...  
I'm child thread..  
I'm other user thread...  
I'm child thread..  
I'm other user thread...  
I'm child thread..  
I'm other user thread...  
I'm child thread..  
I'm other user thread...  
I'm child thread..  
I'm other user thread...  
I'm child thread..(无限输出)

如果需要在主线程结束时,将子线程结束掉,可以采用如下的中断方式:

package com.self;  
  
import java.util.concurrent.ExecutorService;  
import java.util.concurrent.Executors;  
import java.util.concurrent.TimeUnit;  
  
public class ThreadTest  
{  
  
    public static void main(String[] args)  
    {  
        Thread mainThread = new Thread(new Runnable(){  
            public void run()  
            {  
                System.out.println("主线程开始...");  
                Thread sonThread = new Thread(new Thread1(Thread.currentThread()));  
                sonThread.setDaemon(false);  
                sonThread.start();  
                  
                try  
                {  
                    TimeUnit.MILLISECONDS.sleep(10000);  
                }  
                catch (InterruptedException e)  
                {  
                    e.printStackTrace();  
                }  
                System.out.println("主线程结束");  
            }  
        });  
        mainThread.start();  
    }  
      
}  
  
class Thread1 implements Runnable  
{  
    private Thread mainThread;  
      
    public Thread1(Thread mainThread)  
    {  
        this.mainThread = mainThread;  
    }  
      
    @Override  
    public void run()  
    {  
        while(mainThread.isAlive())  
        {  
            System.out.println("子线程运行中....");  
            try  
            {  
                TimeUnit.MILLISECONDS.sleep(1000);  
            }  
            catch (InterruptedException e)  
            {  
                e.printStackTrace();  
            }  
        }  
    }  
      
}

运行结果:

主线程开始...  
子线程运行中....  
子线程运行中....  
子线程运行中....  
子线程运行中....  
子线程运行中....  
子线程运行中....  
子线程运行中....  
子线程运行中....  
子线程运行中....  
子线程运行中....  
子线程运行中....  
主线程结束

主线程结束

回归正题,这里有几点需要注意:
(1) thread.setDaemon(true)必须在thread.start()之前设置,否则会跑出一个IllegalThreadStateException异常。你不能把正在运行的常规线程设置为守护线程。
(2) 在Daemon线程中产生的新线程也是Daemon的。
(3) 不要认为所有的应用都可以分配给Daemon来进行服务,比如读写操作或者计算逻辑。
写java多线程程序时,一般比较喜欢用java自带的多线程框架,比如ExecutorService,但是java的线程池会将守护线程转换为用户线程,所以如果要使用后台线程就不能用java的线程池。
如下,线程池中将daemon线程转换为用户线程的程序片段:

static class DefaultThreadFactory implements ThreadFactory {  
    private static final AtomicInteger poolNumber = new AtomicInteger(1);  
    private final ThreadGroup group;  
    private final AtomicInteger threadNumber = new AtomicInteger(1);  
    private final String namePrefix;  
  
    DefaultThreadFactory() {  
        SecurityManager s = System.getSecurityManager();  
        group = (s != null) ? s.getThreadGroup() :  
                              Thread.currentThread().getThreadGroup();  
        namePrefix = "pool-" +  
                      poolNumber.getAndIncrement() +  
                     "-thread-";  
    }  
  
    public Thread newThread(Runnable r) {  
        Thread t = new Thread(group, r,  
                              namePrefix + threadNumber.getAndIncrement(),  
                              0);  
        if (t.isDaemon())  
            t.setDaemon(false);  
        if (t.getPriority() != Thread.NORM_PRIORITY)  
            t.setPriority(Thread.NORM_PRIORITY);  
        return t;  
    }  
}

注意到,这里不仅会将守护线程转变为用户线程,而且会把优先级转变为Thread.NORM_PRIORITY。
如下所示,将守护线程采用线程池的方式开启:

package com.daemon;  
  
import java.util.concurrent.ExecutorService;  
import java.util.concurrent.Executors;  
import java.util.concurrent.TimeUnit;  
  
public class DaemonThreadTest  
{  
    public static void main(String[] args)  
    {  
        Thread mainThread = new Thread(new Runnable(){  
            @Override  
            public void run()  
            {  
                ExecutorService exec = Executors.newCachedThreadPool();  
                Thread childThread = new Thread(new ClildThread());  
                childThread.setDaemon(true);  
                exec.execute(childThread);  
                exec.shutdown();  
                System.out.println("I'm main thread...");  
            }  
        });  
        mainThread.start();  
    }  
}  
  
class ClildThread implements Runnable  
{  
    @Override  
    public void run()  
    {  
        while(true)  
        {  
            System.out.println("I'm child thread..");  
            try  
            {  
                TimeUnit.MILLISECONDS.sleep(1000);  
            }  
            catch (InterruptedException e)  
            {  
                e.printStackTrace();  
            }  
        }  
    }  
}

运行结果:

I'm main thread...  
I'm child thread..  
I'm child thread..  
I'm child thread..  
I'm child thread..  
I'm child thread..  
I'm child thread..  
I'm child thread..  
I'm child thread..  
I'm child thread..(无限输出)

上面代码证实了线程池会将守护线程转变为用户线程。



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