线程池

ThreadPoolExecutor
1、线程池5个运行状态
RUNNING:  Accept new tasks and process queued tasks

SHUTDOWN: Don't accept new tasks, but process queued tasks

STOP:     Don't accept new tasks, don't process queued and interrupt in-progress tasks
TIDYING:  All tasks have terminated, workerCount is zero, the thread transitioning to state TIDYING will run the terminated() hook method
TERMINATED: terminated() has completed

 RUNNING -> SHUTDOWN
    On invocation of shutdown(), perhaps implicitly in finalize()
 (RUNNING or SHUTDOWN) -> STOP
    On invocation of shutdownNow()
 SHUTDOWN -> TIDYING
    When both queue and pool are empty
 STOP -> TIDYING
    When pool is empty
 TIDYING -> TERMINATED
    When the terminated() hook method has completed

2、几个重要属性

corePoolSize、maximumPoolSize、threadFactory、keepAliveTime、workQueue.

When a new task is submitted in method {@link #execute(Runnable)},  
and fewer than corePoolSize threads are running, a new thread is  
created to handle the request, even if other worker threads are 
idle.  If there are more than corePoolSize but less than
 maximumPoolSize threads running, a new thread will be created only
 if the queue is full.  By setting corePoolSize and maximumPoolSize
 the same, you create a fixed-size thread pool. By setting
 maximumPoolSize to an essentially unbounded value such as {@code
 Integer.MAX_VALUE}, you allow the pool to accommodate an arbitrary
 number of concurrent tasks. Most typically, core and maximum pool
 sizes are set only upon construction, but they may also be changed
 dynamically using {@link #setCorePoolSize} and {@link
 #setMaximumPoolSize}. </dd>