一、ThreadPoolExecutor核心参数说明
1、corePoolSize：核心线程数
* 核心线程会一直存活，及时没有任务需要执行
* 当线程数小于核心线程数时，即使有线程空闲，线程池也会优先创建新线程处理
* 设置allowCoreThreadTimeout=true（默认false）时，核心线程会超时关闭
2、queueCapacity：任务队列容量（阻塞队列）
* 当核心线程数达到最大时，新任务会放在队列中排队等待执行
3、maxPoolSize：最大线程数
* 当线程数>=corePoolSize，且任务队列已满时。线程池会创建新线程来处理任务
* 当线程数=maxPoolSize，且任务队列已满时，线程池会拒绝处理任务而抛出异常
4、 keepAliveTime：线程空闲时间
* 当线程空闲时间达到keepAliveTime时，线程会退出，直到线程数量=corePoolSize
* 如果allowCoreThreadTimeout=true，则会直到线程数量=0
5、allowCoreThreadTimeout：允许核心线程超时
6、rejectedExecutionHandler：任务拒绝处理器
* 两种情况会拒绝处理任务：
- - 当线程数已经达到maxPoolSize，切队列已满，会拒绝新任务
- - 当线程池被调用shutdown()后，会等待线程池里的任务执行完毕，再shutdown。如果在调用shutdown()和线程池真正shutdown之间提交任务，会拒绝新任务
* 线程池会调用rejectedExecutionHandler来处理这个任务。如果没有设置默认是AbortPolicy，会抛出异常
* ThreadPoolExecutor类有几个内部实现类来处理这类情况：
- - AbortPolicy 丢弃任务，抛运行时异常
- - CallerRunsPolicy 执行任务
- - DiscardPolicy 忽视，什么都不会发生
- - DiscardOldestPolicy 从队列中踢出最先进入队列（最后一个执行）的任务
* 实现RejectedExecutionHandler接口，可自定义处理器
二、ThreadPoolExecutor执行顺序
线程池按以下行为执行任务
1. 当线程数小于核心线程数时，创建线程。
2. 当线程数大于等于核心线程数，且任务队列未满时，将任务放入任务队列。
3. 当线程数大于等于核心线程数，且任务队列已满
- -1 若线程数小于最大线程数，创建线程
- -2 若线程数等于最大线程数，抛出异常，拒绝任务
三、ThreadPoolExecutor如何设置参数
1、默认值
* corePoolSize=1 * queueCapacity=Integer.MAX_VALUE * maxPoolSize=Integer.MAX_VALUE * keepAliveTime=60s * allowCoreThreadTimeout=false * rejectedExecutionHandler=AbortPolicy()
2、如何来设置
* 需要根据几个值来决定
- - tasks ：每秒的任务数，假设为1000
- - taskcost：每个任务花费时间，假设为0.1s
- - responsetime：系统允许容忍的最大响应时间，假设为1s
* 做几个计算
- corePoolSize = 每秒需要多少个线程处理？
* 一颗CPU核心同一时刻只能执行一个线程，然后操作系统切换上下文，核心开始执行另一个线程的代码，以此类推，超过cpu核心数，就会放入队列，如果队列也满了，就另起一个新的线程执行，所有推荐：corePoolSize = ((cpu核心数 * 2) + 有效磁盘数)，java可以使用Runtime.getRuntime().availableProcessors()获取cpu核心数
- queueCapacity = (coreSizePool/taskcost)*responsetime
* 计算可得 queueCapacity = corePoolSize/0.1*1。意思是队列里的线程可以等待1s，超过了的需要新开线程来执行
* 切记不能设置为Integer.MAX_VALUE，这样队列会很大，线程数只会保持在corePoolSize大小，当任务陡增时，不能新开线程来执行，响应时间会随之陡增。
- maxPoolSize = (max(tasks)- queueCapacity)/(1/taskcost)
* 计算可得 maxPoolSize = (1000-corePoolSize)/10，即(每秒并发数-corePoolSize大小) / 10
* （最大任务数-队列容量）/每个线程每秒处理能力 = 最大线程数
- - rejectedExecutionHandler：根据具体情况来决定，任务不重要可丢弃，任务重要则要利用一些缓冲机制来处理
- - keepAliveTime和allowCoreThreadTimeout采用默认通常能满足
ThreadPoolExecutor参数allowCoreThreadTimeOut
ThreadPoolExecutor的执行流程有一点可能被吐槽过，就是只有缓存队列已经满了的时候才会使用到maxPoolSize创建新的线程.也就是说如果corePoolSize设为0的时候，要等到队列满了，才会创建线程去执行任务
之前有被问到，希望没有任务的时候线程池里的线程可以停掉。可能对性能和资源有过考虑的人都会想到这个问题吧
今天看JDK源码的时候发现了ThreadPoolExecutor在1.6的时候已经支持了
allowCoreThreadTimeOut参数就是为此设计的
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/**
* Sets the policy governing whether core threads may time out and
* terminate if no tasks arrive within the keep-alive time, being
* replaced if needed when new tasks arrive. When false, core
* threads are never terminated due to lack of incoming
* tasks. When true, the same keep-alive policy applying to
* non-core threads applies also to core threads. To avoid
* continual thread replacement, the keep-alive time must be
* greater than zero when setting {@code true}. This method
* should in general be called before the pool is actively used.
*
* @param value {@code true} if should time out, else {@code false}
* @throws IllegalArgumentException if value is {@code true}
* and the current keep-alive time is not greater than zero
*
* @since 1.6
*/
public void allowCoreThreadTimeOut(boolean value) {
if (value && keepAliveTime <= 0)
throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
if (value != allowCoreThreadTimeOut) {
allowCoreThreadTimeOut = value;
if (value)
interruptIdleWorkers();
}
}
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在ThreadPoolExecutor构造函数的注释上也有明确说明：corePoolSize 的数量会一直保持，即使这些线程是空闲的，除非设置了allowCoreThreadTimeOut
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/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters and default thread factory and rejected execution handler.
* It may be more convenient to use one of the {@link Executors} factory
* methods instead of this general purpose constructor.
*
* @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.
* @throws IllegalArgumentException if one of the following holds:<br>
* {@code corePoolSize < 0}<br>
* {@code keepAliveTime < 0}<br>
* {@code maximumPoolSize <= 0}<br>
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}
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当然，在Executors的静态工厂里面的newCachedThreadPool提供了另外一种思路
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coreSize为0
SynchronousQueue最多只能有一个任务在队列里面
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也就是说这个线程池的任务会被立即分配一个线程去处理，如果没有空闲的线程会立即创建线程。
在空闲的时候，线程数量会减少直至为0，这一点倒是满足了要求。可是队列中最多只会缓存一个任务，当任务的处理速度慢于任务进入线程池的速度时，线程数量就会不断膨胀。如果maxPoolSize设置成一个比较小的数字时，可能就会有大量任务被拒绝策略处理。
所以正如注释中所说，newCachedThreadPool只适合于任务处理速度很快的场景下。比如做一些计算，不需要依赖其它服务
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/**
* Creates a thread pool that creates new threads as needed, but
* will reuse previously constructed threads when they are
* available. These pools will typically improve the performance
* of programs that execute many short-lived asynchronous tasks.
* Calls to {@code execute} will reuse previously constructed
* threads if available. If no existing thread is available, a new
* thread will be created and added to the pool. Threads that have
* not been used for sixty seconds are terminated and removed from
* the cache. Thus, a pool that remains idle for long enough will
* not consume any resources. Note that pools with similar
* properties but different details (for example, timeout parameters)
* may be created using {@link ThreadPoolExecutor} constructors.
*
* @return the newly created thread pool
*/
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
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