ThreadPoolExecutor运行原理

线程池用来管理工作线程，它包含一个队列用来存放等待执行的任务。java的java.util.concurrent.Executors提供了实现java.util.concurrent.Executor这个接口的方法，用来创建线程池。

下面从一个简单的例子解释它的运行机制。
创建Runnable类

public class WorkerThread implements Runnable {    private String command;    public WorkerThread(String s){        this.command=s;    }    @Override    public void run() {        System.out.println(Thread.currentThread().getName()+' Start. Command = '+command);        processCommand();        System.out.println(Thread.currentThread().getName()+' End.');    }    private void processCommand() {        try {            Thread.sleep(5000);        } catch (InterruptedException e) {            e.printStackTrace();        }    }    @Override    public String toString(){        return this.command;    }}

给出测试程序，使用Executors框架创建固定大小的线程池。

import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class SimpleThreadPool {    public static void main(String[] args) {        ExecutorService executor = Executors.newFixedThreadPool(5);        for (int i = 0; i < 10; i++) {            Runnable worker = new WorkerThread('' + i);            executor.execute(worker);          }        executor.shutdown();        while (!executor.isTerminated()) {        }        System.out.println('Finished all threads');    }}

上面的程序中，创建了固定大小的线程池，能容纳5个工作线程，然后向这个线程池中提交10个任务，因为线程池的大小为5，先执行其中的5个任务，剩余的5个任务处于等待状态，只要其中的一个任务完成结束，处于等待队列的任务的工作线程取出并执行。上面程序的运行结果如下：

pool-1-thread-2 Start. Command = 1
pool-1-thread-4 Start. Command = 3
pool-1-thread-1 Start. Command = 0
pool-1-thread-3 Start. Command = 2
pool-1-thread-5 Start. Command = 4
pool-1-thread-4 End.
pool-1-thread-5 End.
pool-1-thread-1 End.
pool-1-thread-3 End.
pool-1-thread-3 Start. Command = 8
pool-1-thread-2 End.
pool-1-thread-2 Start. Command = 9
pool-1-thread-1 Start. Command = 7
pool-1-thread-5 Start. Command = 6
pool-1-thread-4 Start. Command = 5
pool-1-thread-2 End.
pool-1-thread-4 End.
pool-1-thread-3 End.
pool-1-thread-5 End.
pool-1-thread-1 End.
Finished all threads

Executors类使用ThreadPoolExecutor
提供了ExecutorService的简单实现1，但是ThreadPoolExecutor提供了更多的特征。在代码中，当创建ThreadPoolExecutor实例时，可以控制存活线程的数量、限制线程池的大小和实现RejectedExecutionHandler处理不能在等待队列中容纳的任务2。

下面先实现RejectedExecutionHandler接口的类。

import java.util.concurrent.RejectedExecutionHandler;import java.util.concurrent.ThreadPoolExecutor;public class RejectedExecutionHandlerImpl implements RejectedExecutionHandler {    @Override    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor)     {        System.out.println(r.toString() + ' is rejected');    }}

下面创建监控线程用来打印不同时间段executor的执行情况。

import java.util.concurrent.ThreadPoolExecutor;public class MyMonitorThread implements Runnable{    private ThreadPoolExecutor executor;    private int seconds;    private boolean run=true;    public MyMonitorThread(ThreadPoolExecutor executor, int delay)    {        this.executor = executor;        this.seconds=delay;    }    public void shutdown(){        this.run=false;    }    @Override    public void run()    {        while(run){                System.out.println(                    String.format('[monitor] [%d/%d] Active: %d, Completed: %d, Task: %d, isShutdown: %s, isTerminated: %s',                        this.executor.getPoolSize(),                        this.executor.getCorePoolSize(),                        this.executor.getActiveCount(),                        this.executor.getCompletedTaskCount(),                        this.executor.getTaskCount(),                        this.executor.isShutdown(),                        this.executor.isTerminated()));                try {                    Thread.sleep(seconds*1000);                } catch (InterruptedException e) {                    e.printStackTrace();                }        }    }}

下面使用ThreadPoolExecutor创建线程池。

import java.util.concurrent.ArrayBlockingQueue;import java.util.concurrent.Executors;import java.util.concurrent.ThreadFactory;import java.util.concurrent.ThreadPoolExecutor;import java.util.concurrent.TimeUnit;public class WorkerPool {    public static void main(String args[]) throws InterruptedException{        //RejectedExecutionHandler implementation        RejectedExecutionHandlerImpl rejectionHandler = new RejectedExecutionHandlerImpl();        //Get the ThreadFactory implementation to use        ThreadFactory threadFactory = Executors.defaultThreadFactory();        //creating the ThreadPoolExecutor        ThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(2), threadFactory, rejectionHandler);        //start the monitoring thread        MyMonitorThread monitor = new MyMonitorThread(executorPool, 3);        Thread monitorThread = new Thread(monitor);        monitorThread.start();        //submit work to the thread pool        for(int i=0; i<10; i++){            executorPool.execute(new WorkerThread('cmd'+i));        }        Thread.sleep(30000);        //shut down the pool        executorPool.shutdown();        //shut down the monitor thread        Thread.sleep(5000);        monitor.shutdown();    }}

在初始化ThreadPoolExecutor时，设置线程池的大小corePoolSize为2，这时没有工作线程（work thread）。随着ThreadPoolExecutor 对任务的提交（submit），新的工作线程被创建，这些线程不会被空闲（idle），直到线程的数量达到corePoolSize = 2。接下来新来的任务会被放在工作队列（work queue）中，直到工作队列满了（等待队列大小为2），这时新的工作线程又被创建，线程的数量直到达到maximumPoolSize =4，超出的任务有RejectedExecutionHandlerImpl处理。

在创建ThreadPoolExecutor时，应该注意上述参数的设置3。

运行结果：

pool-1-thread-1 Start. Command = cmd0
pool-1-thread-4 Start. Command = cmd5
cmd6 is rejected
pool-1-thread-3 Start. Command = cmd4
pool-1-thread-2 Start. Command = cmd1
cmd7 is rejected
cmd8 is rejected
cmd9 is rejected
[monitor] [0/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, >isTerminated: false
pool-1-thread-4 End.
pool-1-thread-1 End.
pool-1-thread-2 End.
pool-1-thread-3 End.
pool-1-thread-1 Start. Command = cmd3
pool-1-thread-4 Start. Command = cmd2
[monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown: false, isTerminated: false
pool-1-thread-1 End.
pool-1-thread-4 End.
[monitor] [4/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, >isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown: true, isTerminated: true
[monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown: true, isTerminated: true

ThreadPoolExecutor 构造方法

 public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler)

参数 含义 corePoolSize 线程池的基本大小，如果队列中任务已满，并且当前线程个数小于maximumPoolSize，那么会创建新的线程来执行任务 maximumPoolSize 线程池中允许的最大线程数，当线程数量大于maximumPoolSize，超出的线程由丢弃机制来处理 workQueue 等待队列，当线程的数量达到corePoolSize时，就向该等待队列放入线程信息（默认为一个LinkedBlockingQueue） keepAliveTime 当线程池中的线程数量大于corePoolSize的时候，多余的线程会等待keepAliveTime长的时间，如果无请求可处理就自行销毁 threadFactory4 用来构造Thread对象，主要包括线程名称，守护线程的状态，线程的优先级以及设置UncaughtExceptionHandler handler 超过maximumPoolSize之后丢弃处理的方法，java提供了5种丢弃处理的方法，也可以自定义，实现RejectedExecutionHandler接口

参考资料：
Java Thread Pool Example using Executors and ThreadPoolExecutor

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1. A. Executors.newCachedThreadPool() 无限大小的线程池，线程会自动重用。相当于new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>())，SynchronousQueue 可以存储元素，但是总是报“I am full”，导致 ThreadPoolExecutor 创建新的线程去SynchronousQueue 中拉取任务并运行该任务。线程数量是无界的，队列是有界的，用它处理大批量任务时比较危险的。
   B. Executors.newFixedThreadPool(int) 固定线程数的线程池。相当于new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()) ，线程数量是有界的，而工作队列是无界的，如果负载超过了当前的处理能力，那么工作队列会堆积越来越多的任务，有可能会导致内存不足的问题。
   C. Executors.newSingleThreadExecutor() 单线程执行器。 ↩
2. 如果线程池中线程数目已达到maximumPoolSize，则提交的任务交由RejectedExecutionHandler处理丢弃。对于任务丢弃，ThreadPoolExecutor以内部类的形式实现了4个策略：
   - CallerRunsPolicy：提交任务的线程自己负责执行这个任务；
   - AbortPolicy：使Executor抛出异常，通过异常做处理；
   - DiscardPolicy：丢弃提交的任务；
   - DiscardOldestPolicy：丢弃掉队列中最早加入的任务。
   在调用构造方法时，参数中未指定RejectedExecutionHandler情况下，默认采用AbortPolicy。 ↩
3. 创建ThreadPoolExecutor 应该避免设置corePoolSize过小，maximumPoolSize过大以及过大的无界队列LinkedBlockingQueue，容易造成工作队列未满，而抛出OutOfMemoryError 异常。 ↩
4. 给线程设置一个好的有状态的名字，如Thread.currentThread().setName(Context + TID + Params + current Time,..) ，具体请参考这里。当线程中运行一个任务时发生未捕获异常，默认情况下线程终止，异常信息打印在终端，这种默认的行为并不恰当。在创建线程时可以定制未捕获异常的处理。如：new UncaughtExceptionHandler() {
@Override public void uncaughtException(Thread t, Throwable e) {
System.err.println(String.format( "Thread %s threw exception - %s", t.getName(), e.getMessage())) } } ↩