Executors.newCachedThreadPool 源碼解析

Executors 還有個常用靜態方法newCachedThreadPool()，來構造線程池
今天我們其源碼實現，探一探究竟

//底層還是調用ThreadPoolExecutor，不過參數有變化
//corePoolSize 竟然為0，maximumPoolSize為默認的最大值
//當任務隊列滿時，就會判斷maximumPoolSize大小
//keepAliveTime 空閑線程的最大等待時間，，60s后立馬銷毀線程了
public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

SynchronousQueue

注意這個隊列

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，實際上它不是一個真正的隊列，因為它不會為隊列中元素維護存儲空間。與其他隊列不同的是，它維護一組線程，這些線程在等待着把元素加入或移出隊列。
• 在使用SynchronousQueue作為工作隊列的前提下，客戶端代碼向線程池提交任務時，而線程池中又沒有空閑的線程能夠從SynchronousQueue隊列實例中取一個任務，那么相應的offer方法調用就會失敗（即任務沒有被存入工作隊列）。此時，ThreadPoolExecutor會新建一個新的工作者線程用於對這個入隊列失敗的任務進行處理（假設此時線程池的大小還未達到其最大線程池大小maximumPoolSize）。

newFixedThreadPool 和 newCachedThreadPool最大差別就是 隊列，線程回收的時間

newFixedThreadPool 應用場景

 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 線程池的數量是不確定的，可以無限大。
它比較適合處理執行時間比較小的任務