Java:使用Executors创建和管理线程

1. 类 Executors

此类中提供的一些方法有：
1.1 public static ExecutorService newCachedThreadPool()
创建一个可根据需要创建新线程的线程池，但是在以前构造的线程可用时将重用它们。对于执行很多短期异步任务的程序而言，这些线程池通常可提高程序性能。

1.2 public static ExecutorService newFixedThreadPool(int nThreads)
创建一个可重用固定线程数的线程池，以共享的无界队列方式来运行这些线程。

1.3 public static ExecutorService newSingleThreadExecutor()
创建一个使用单个 worker 线程的 Executor，以无界队列方式来运行该线程。

这三个方法都可以配合接口ThreadFactory的实例一起使用。并且返回一个ExecutorService接口的实例。

2. 接口 ThreadFactory

根据需要创建新线程的对象。使用线程工厂就无需再手工编写对 new Thread 的调用了，从而允许应用程序使用特殊的线程子类、属性等等。
此接口最简单的实现就是：

class SimpleThreadFactory implements ThreadFactory {
   public Thread newThread(Runnable r) {
     return new Thread(r);
   }
 }

3. 接口ExecutorService

该接口提供了管理终止的方法。

4.创建标准线程池启动线程

4.1 提供一个简单的实现Runnable接口的线程
MyThread.java

package com.zj.concurrency.executors;
 
public class MyThread implements Runnable {
    private int count = 1, number;
 
    public MyThread(int num) {
       number = num;
       System.out.println("Create Thread-" + number);
    }
 
    public void run() {
       while (true) {
           System.out.println("Thread-" + number + " run " + count+" time(s)");
           if (++count == 3)
              return;
       }
    }
}

这个线程会打印出相应的创建和执行信息。

4.2使用CachedThreadPool启动线程
CachedThreadPool.java

package com.zj.concurrency.executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
 
public class CachedThreadPool {
    public static void main(String[] args) {
       ExecutorService exec = Executors.newCachedThreadPool();
       for (int i = 0; i < 5; i++)
           exec.execute(new MyThread(i));
       exec.shutdown();
    }
}

结果：
Create Thread-0
Create Thread-1
Create Thread-2
Create Thread-3
Thread-0 run 1 time(s)
Thread-0 run 2 time(s)
Thread-1 run 1 time(s)
Thread-1 run 2 time(s)
Thread-2 run 1 time(s)
Thread-2 run 2 time(s)
Create Thread-4
Thread-4 run 1 time(s)
Thread-4 run 2 time(s)
Thread-3 run 1 time(s)
Thread-3 run 2 time(s)

4.3 使用FixedThreadPool启动线程
FixedThreadPool.java

package com.zj.concurrency.executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
 
public class FixedThreadPool {
    public static void main(String[] args) {
       ExecutorService exec = Executors.newFixedThreadPool(2);
       for (int i = 0; i < 5; i++)
           exec.execute(new MyThread(i));
       exec.shutdown();
    }
}

结果：
Create Thread-0
Create Thread-1
Create Thread-2
Create Thread-3
Create Thread-4
Thread-0 run 1 time(s)
Thread-0 run 2 time(s)
Thread-2 run 1 time(s)
Thread-2 run 2 time(s)
Thread-3 run 1 time(s)
Thread-3 run 2 time(s)
Thread-4 run 1 time(s)
Thread-4 run 2 time(s)
Thread-1 run 1 time(s)
Thread-1 run 2 time(s)

4.4 使用SingleThreadExecutor启动线程
SingleThreadExecutor.java

package com.zj.concurrency.executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
 
public class SingleThreadExecutor {
    public static void main(String[] args) {
       ExecutorService exec = Executors.newSingleThreadExecutor();
       for (int i = 0; i < 5; i++)
           exec.execute(new MyThread(i));
       exec.shutdown();
    }
}

结果：
Create Thread-0
Create Thread-1
Create Thread-2
Create Thread-3
Create Thread-4
Thread-0 run 1 time(s)
Thread-0 run 2 time(s)
Thread-1 run 1 time(s)
Thread-1 run 2 time(s)
Thread-2 run 1 time(s)
Thread-2 run 2 time(s)
Thread-3 run 1 time(s)
Thread-3 run 2 time(s)
Thread-4 run 1 time(s)
Thread-4 run 2 time(s)

5.配合ThreadFactory接口的使用
我们试图给线程加入daemon和priority的属性设置。
5.1设置后台线程属性
DaemonThreadFactory.java

package com.zj.concurrency.executors.factory;
import java.util.concurrent.ThreadFactory;
 
public class DaemonThreadFactory implements ThreadFactory {
    public Thread newThread(Runnable r) {
       Thread t = new Thread(r);
       t.setDaemon(true); //设置该进程为守候进程
       return t;
    }
}

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注：守护线程在没有用户线程可服务时自动离开，在Java中比较特殊的线程是被称为守护（Daemon）线程的低级别线程。这个线程具有最低的优先级，用于为系统中的其它对象和线程提供服务。将一个用户线程设置为守护线程的方式是在线程对象创建之前调用线程对象的setDaemon方法。典型的守护线程例子是JVM中的系统资源自动回收线程，我们所熟悉的Java垃圾回收线程就是一个典型的守护线程，当我们的程序中不再有任何运行中的Thread，程序就不会再产生垃圾，垃圾回收器也就无事可做，所以当垃圾回收线程是Java虚拟机上仅剩的线程时，Java虚拟机会自动离开。它始终在低级别的状态中运行，用于实时监控和管理系统中的可回收资源。守护进程（Daemon）是运行在后台的一种特殊进程。它独立于控制终端并且周期性地执行某种任务或等待处理某些发生的事件。也就是说守护线程不依赖于终端，但是依赖于系统，与系统“同生共死”。那Java的守护线程是什么样子的呢。当JVM中所有的线程都是守护线程的时候，JVM就可以退出了；如果还有一个或以上的非守护线程则JVM不会退出。
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5.2 设置优先级属性
最高优先级MaxPriorityThreadFactory.java

package com.zj.concurrency.executors.factory;
import java.util.concurrent.ThreadFactory;
 
public class MaxPriorityThreadFactory implements ThreadFactory {
    public Thread newThread(Runnable r) {
       Thread t = new Thread(r);
       t.setPriority(Thread.MAX_PRIORITY);
       return t;
    }
}

最低优先级MinPriorityThreadFactory.java

package com.zj.concurrency.executors.factory;
import java.util.concurrent.ThreadFactory;
 
public class MinPriorityThreadFactory implements ThreadFactory {
    public Thread newThread(Runnable r) {
       Thread t = new Thread(r);
       t.setPriority(Thread.MIN_PRIORITY);
       return t;
    }
}

5.3启动带有属性设置的线程
ExecFromFactory.java

package com.zj.concurrency.executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import com.zj.concurrency.executors.factory.DaemonThreadFactory;
import com.zj.concurrency.executors.factory.MaxPriorityThreadFactory;
import com.zj.concurrency.executors.factory.MinPriorityThreadFactory;
 
public class ExecFromFactory {
    public static void main(String[] args) throws Exception {
       ExecutorService defaultExec = Executors.newCachedThreadPool();
       ExecutorService daemonExec = Executors
              .newCachedThreadPool(new DaemonThreadFactory());
       ExecutorService maxPriorityExec = Executors
              .newCachedThreadPool(new MaxPriorityThreadFactory());
       ExecutorService minPriorityExec = Executors
              .newCachedThreadPool(new MinPriorityThreadFactory());
       for (int i = 0; i < 10; i++)
           daemonExec.execute(new MyThread(i));
       for (int i = 10; i < 20; i++)
           if (i == 10)
              maxPriorityExec.execute(new MyThread(i));
           else if (i == 11)
              minPriorityExec.execute(new MyThread(i));
           else
              defaultExec.execute(new MyThread(i));
    }
}

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