Executors.newCachedThreadPool 源码解析

Executors 还有个常用静态方法newCachedThreadPool()，来构造线程池
今天我们其源码实现，探一探究竟

//底层还是调用ThreadPoolExecutor，不过参数有变化
//corePoolSize 竟然为0，maximumPoolSize为默认的最大值
//当任务队列满时，就会判断maximumPoolSize大小
//keepAliveTime 空闲线程的最大等待时间，，60s后立马销毁线程了
public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

SynchronousQueue

注意这个队列

A {@linkplain BlockingQueue blocking queue} in which each insert
 * operation must wait for a corresponding remove operation by another
 * thread, and vice versa.  A synchronous queue does not have any
 * internal capacity, not even a capacity of one. 

• SynchronousQueue，实际上它不是一个真正的队列，因为它不会为队列中元素维护存储空间。与其他队列不同的是，它维护一组线程，这些线程在等待着把元素加入或移出队列。
• 在使用SynchronousQueue作为工作队列的前提下，客户端代码向线程池提交任务时，而线程池中又没有空闲的线程能够从SynchronousQueue队列实例中取一个任务，那么相应的offer方法调用就会失败（即任务没有被存入工作队列）。此时，ThreadPoolExecutor会新建一个新的工作者线程用于对这个入队列失败的任务进行处理（假设此时线程池的大小还未达到其最大线程池大小maximumPoolSize）。

newFixedThreadPool 和 newCachedThreadPool最大差别就是 队列，线程回收的时间

newFixedThreadPool 应用场景

 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 <tt>execute</tt> 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.

newFixedThreadPool 线程池的数量是不确定的，可以无限大。
它比较适合处理执行时间比较小的任务