Java线程池类ThreadPoolExecutor的使用 JDK自带的4种线程池

首先，继承关系: Executor -> ExecutorService -> AbstractExecutorService -> ThreadPoolExecutor

Interface: Executor.execute(Runnable)
Interface: ExecutorService.submit(Callable) -- ExecutorService implements Executor 

AbstractExecutorService实现了submit(Callable),这个需要调用execute(Runnable)来实现, 正是AbstractExecutorService的子类ThreadPoolExecutor中实现了execute(Runnable)

参数最全的构造方法

	 * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the
     *        pool
     * @param keepAliveTime when the number of threads is greater than
     *        the core, this is the maximum time that excess idle threads
     *        will wait for new tasks before terminating.
     * @param unit the time unit for the {@code keepAliveTime} argument
     * @param workQueue the queue to use for holding tasks before they are
     *        executed.  This queue will hold only the {@code Runnable}
     *        tasks submitted by the {@code execute} method.
     * @param threadFactory the factory to use when the executor
     *        creates a new thread
     * @param handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached
public ThreadPoolExecutor(
int corePoolSize, //
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler)

JDK自带的配置好的线程池：
    // 固定工作线程数量的线程池  
    ExecutorService executorService1 = Executors.newFixedThreadPool(3);  
      
    // 一个可缓存的线程池  
    ExecutorService executorService2 = Executors.newCachedThreadPool();  
      
    // 单线程化的Executor  
    ExecutorService executorService3 = Executors.newSingleThreadExecutor();  
      
    // 支持定时的以及周期性的任务执行  
    ExecutorService executorService4 = Executors.newScheduledThreadPool(3); 

1.CachedThreadPool

    CachedThreadPool首先会按照需要创建足够多的线程来执行任务(Task)。随着程序执行的过程，有的线程执行完了任务，可以被重新循环使用时，才不再创建新的线程来执行任务。我们采用《Thinking In Java》中的例子来分析。

    首先，任务定义如下(实现了Runnable接口，并且复写了run方法)：

    package net.jerryblog.concurrent; 
    public class LiftOff implements Runnable{ 
        protected int countDown = 10; //Default 
        private static int taskCount = 0; 
        private final int id = taskCount++;  
        public LiftOff() {} 
        public LiftOff(int countDown) { 
            this.countDown = countDown; 
        } 
        public String status() { 
            return "#" + id + "(" + 
                (countDown > 0 ? countDown : "LiftOff!") + ") "; 
        } 
        @Override 
        public void run() { 
            while(countDown-- > 0) { 
                System.out.print(status()); 
                Thread.yield(); 
            } 
             
        }    
    } 

    采用CachedThreadPool方式执行编写的客户端程序如下：

    package net.jerryblog.concurrent; 
    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 < 10; i++) { 
                exec.execute(new LiftOff()); 
            } 
            exec.shutdown();     
        } 
    } 

    上面的程序中，有10个任务，采用CachedThreadPool模式，exec没遇到一个LiftOff的对象(Task)，就会创建一个线程来处理任务。现在假设遇到到第4个任务时，之前用于处理第一个任务的线程已经执行完成任务了，那么不会创建新的线程来处理任务，而是使用之前处理第一个任务的线程来处理这第4个任务。接着如果遇到第5个任务时，前面那些任务都还没有执行完，那么就会又新创建线程来执行第5个任务。否则，使用之前执行完任务的线程来处理新的任务。

2.FixedThreadPool

     FixedThreadPool模式会使用一个优先固定数目的线程来处理若干数目的任务。规定数目的线程处理所有任务，一旦有线程处理完了任务就会被用来处理新的任务(如果有的话)。这种模式与上面的CachedThreadPool是不同的，CachedThreadPool模式下处理一定数量的任务的线程数目是不确定的。而FixedThreadPool模式下最多的线程数目是一定的。

    采用FixedThreadPool模式编写客户端程序如下：

    package net.jerryblog.concurrent; 
    import java.util.concurrent.ExecutorService; 
    import java.util.concurrent.Executors; 
    public class FixedThreadPool { 
        public static void main(String[] args) {
    //三个线程来执行五个任务 
            ExecutorService exec = Executors.newFixedThreadPool(3);    
            for(int i = 0; i < 5; i++) { 
                exec.execute(new LiftOff()); 
            } 
            exec.shutdown(); 
        } 
    } 

3.SingleThreadExecutor模式

    SingleThreadExecutor模式只会创建一个线程。它和FixedThreadPool比较类似，不过线程数是一个。如果多个任务被提交给SingleThreadExecutor的话，那么这些任务会被保存在一个队列中，并且会按照任务提交的顺序，一个先执行完成再执行另外一个线程。

    SingleThreadExecutor模式可以保证只有一个任务会被执行。这种特点可以被用来处理共享资源的问题而不需要考虑同步的问题。

    SingleThreadExecutor模式编写的客户端程序如下：

    package net.jerryblog.concurrent; 
    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 < 2; i++) { 
                exec.execute(new LiftOff()); 
            } 
        } 
    }