ThreadPoolExecutor参数详解

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue) {

1、corePoolSize —new ThreadPoolExecutor的时候创建的的线程,
如果ThreadPoolExecutor.execute的任务数>corePoolSize 则会把任务放到BlockingQueue 、如果BlockingQueue满了会继续创建新线程、一直到线程个数大于第二个参数maximumPoolSize
2、maximumPoolSize 允许创建的最大线程数、如果BlockingQueue已经满了、线程数==maximumPoolSize 则会执行ThreadPoolExecutor的拒绝策略
3、keepAliveTime 如果BlockingQueue从满了到未满、则大于corePoolSize, 小于maximumPoolSize的线程, 会在keepAliveTime的时间内存活、否则会被回收

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参考
https://my.oschina.net/u/2250599/blog/498787

一、使用Executors创建线程池
之前创建线程的时候都是用的Executors的newFixedThreadPool(),newSingleThreadExecutor(),newCachedThreadPool()这三个方法。当然Executors也是用不同的参数去new ThreadPoolExecutor

1. newFixedThreadPool()

创建线程数固定大小的线程池。 由于使用了LinkedBlockingQueue所以maximumPoolSize 没用，当corePoolSize满了之后就加入到LinkedBlockingQueue队列中。每当某个线程执行完成之后就从LinkedBlockingQueue队列中取一个。所以这个是创建固定大小的线程池。

public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue());
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
2.newSingleThreadPool()

创建线程数为1的线程池，由于使用了LinkedBlockingQueue所以maximumPoolSize 没用，corePoolSize为1表示线程数大小为1,满了就放入队列中，执行完了就从队列取一个。

public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue()));
}

3.newCachedThreadPool()

创建可缓冲的线程池。没有大小限制。由于corePoolSize为0所以任务会放入SynchronousQueue队列中，SynchronousQueue只能存放大小为1，所以会立刻新起线程，由于maxumumPoolSize为Integer.MAX_VALUE所以可以认为大小为2147483647。受内存大小限制。

public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue());
}
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}

二、使用ThreadPoolExecutor创建线程池
ThreadPoolExecutor的构造函数
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
参数：
1、corePoolSize核心线程数大小，当线程数

    2、maximumPoolSize 最大线程数， 当线程数 >= corePoolSize的时候，会把runnable放入workQueue中

    3、keepAliveTime  保持存活时间，当线程数大于corePoolSize的空闲线程能保持的最大时间。

    4、unit 时间单位

    5、workQueue 保存任务的阻塞队列

    6、threadFactory 创建线程的工厂

    7、handler 拒绝策略

任务执行顺序：
1、当线程数小于corePoolSize时，创建线程执行任务。

    2、当线程数大于等于corePoolSize并且workQueue没有满时，放入workQueue中

    3、线程数大于等于corePoolSize并且当workQueue满时，新任务新建线程运行，线程总数要小于maximumPoolSize

    4、当线程总数等于maximumPoolSize并且workQueue满了的时候执行handler的rejectedExecution。也就是拒绝策略。

ThreadPoolExecutor默认有四个拒绝策略：
1、ThreadPoolExecutor.AbortPolicy() 直接抛出异常RejectedExecutionException

    2、ThreadPoolExecutor.CallerRunsPolicy()    直接调用run方法并且阻塞执行

    3、ThreadPoolExecutor.DiscardPolicy()   直接丢弃后来的任务

    4、ThreadPoolExecutor.DiscardOldestPolicy()  丢弃在队列中队首的任务

当然可以自己继承RejectedExecutionHandler来写拒绝策略.

int corePoolSize = 1;
int maximumPoolSize = 2;
int keepAliveTime = 10;
// BlockingQueue workQueue = new LinkedBlockingQueue();
BlockingQueue workQueue = new ArrayBlockingQueue(5);
ThreadFactory threadFactory = Executors.defaultThreadFactory();
//线程池和队列满了之后的处理方式
//1.跑出异常
RejectedExecutionHandler handler = new ThreadPoolExecutor.AbortPolicy();
RejectedExecutionHandler handler2 = new ThreadPoolExecutor.CallerRunsPolicy();
RejectedExecutionHandler handler3 = new ThreadPoolExecutor.DiscardPolicy();
RejectedExecutionHandler handler4 = new ThreadPoolExecutor.DiscardOldestPolicy();

	ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, TimeUnit.SECONDS, workQueue, threadFactory, handler2);
	
	
	for (int j = 1; j < 15; j++) {
		threadPoolExecutor.execute(new Runnable() {
			
			public void run() {
				
				try {
					System.out.println(Thread.currentThread().getName());
					TimeUnit.SECONDS.sleep(1);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				
				
			}
		});
	}
	
	System.out.println(threadPoolExecutor);
	
}