Java线程池详解

线程池简介

线程池就是首先创建一些线程，它们的集合称为线程池。使用线程池可以很好地提高性能，线程池在系统启动时即创建大量空闲的线程，程序将一个任务传给线程池，线程池就会启动一条线程来执行这个任务，执行结束以后，该线程并不会死亡，而是再次返回线程池中成为空闲状态，等待执行下一个任务。

工作流程

三大常用方式

public class Demo01 {
    public static void main(String[] args) {
        // 线程池常用的三大方法
        ExecutorService es = Executors.newFixedThreadPool(5);// 1池固定线程
        ExecutorService es2 = Executors.newSingleThreadExecutor();     // 1池1线程
        ExecutorService es3 = Executors.newCachedThreadPool();         // 1池N线程
    }
}

七大参数

JDK内置四种拒绝策略

AbortPolicy（默认）：直接抛出 RejectedExecutionException 异常阻止系统正常运行

CallerRunsPolicy：“调用者运行”一种调节机制，该策略既不会抛弃任务，也不会抛出异常，而是将某些任务回退到调用者，从而降低新任务的流量

DiscardOldestPolicy：抛弃队列中等待最久的任务，然后把当前任务加入队列中尝试再次提交当前任务

DiscardPolicy：直接丢弃任务，不予任何处理也不抛出异常。如果允许任务丢失，这是最好的一种方案　

 以上内置拒绝策略均实现了RejectedExecutionHandler接口

线程池五大状态

线程池属性标识

RUNNING

正常运行状态，可以接受任务，会处理阻塞队列中的任务，可以处理当前线程中的任务

SHUTDOWN

关闭状态，不可以接受任务，但是会处理阻塞队列中的任务，且可以处理当前线程中的任务

STOP

停止状态，不可以接受任务，也不会处理阻塞队列中的任务，还会中断正在处理任务的线程

TIDYING

过渡状态，代表当前线程池即将Game Over

TERMINATED

结束状态，要执行terminated()方法，真的凉凉了

源码

/**
     * The main pool control state, ctl, is an atomic integer packing
     * two conceptual fields
     *   workerCount, indicating the effective number of threads
     *   runState,    indicating whether running, shutting down etc
     *
     * In order to pack them into one int, we limit workerCount to
     * (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
     * billion) otherwise representable. If this is ever an issue in
     * the future, the variable can be changed to be an AtomicLong,
     * and the shift/mask constants below adjusted. But until the need
     * arises, this code is a bit faster and simpler using an int.
     *
     * The workerCount is the number of workers that have been
     * permitted to start and not permitted to stop.  The value may be
     * transiently different from the actual number of live threads,
     * for example when a ThreadFactory fails to create a thread when
     * asked, and when exiting threads are still performing
     * bookkeeping before terminating. The user-visible pool size is
     * reported as the current size of the workers set.
     *
     * The runState provides the main lifecycle control, taking on values:
     *
     *   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 tasks,
     *             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
     *
     * The numerical order among these values matters, to allow
     * ordered comparisons. The runState monotonically increases over
     * time, but need not hit each state. The transitions are:
     *
     * 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
     *
     * Threads waiting in awaitTermination() will return when the
     * state reaches TERMINATED.
     *
     * Detecting the transition from SHUTDOWN to TIDYING is less
     * straightforward than you'd like because the queue may become
     * empty after non-empty and vice versa during SHUTDOWN state, but
     * we can only terminate if, after seeing that it is empty, we see
     * that workerCount is 0 (which sometimes entails a recheck -- see
     * below).
     */
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

    // runState is stored in the high-order bits
    private static final int RUNNING    = -1 << COUNT_BITS;
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    private static final int STOP       =  1 << COUNT_BITS;
    private static final int TIDYING    =  2 << COUNT_BITS;
    private static final int TERMINATED =  3 << COUNT_BITS;

图示