ThreadPoolExecutor

ThreadPoolExecutor

线程池对象，如果里面添加了任务，如果不手动调用 shutdown 方法，那么这个对象永远不会被 GC

首先看一下 ThreadPoolExecutor 的源码结构

ThreadPoolExecutor 里面有一个内部类 Worker 这个内部类实现了 Runnable 接口

这个 Worker 类里有一个 runWorker 方法，是保证线程不断运行的原理

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        while (task != null || (task = getTask()) != null) {
            w.lock();
            // If pool is stopping, ensure thread is interrupted;
            // if not, ensure thread is not interrupted.  This
            // requires a recheck in second case to deal with
            // shutdownNow race while clearing interrupt
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    task.run();
                } catch (RuntimeException x) {
                    thrown = x; throw x;
                } catch (Error x) {
                    thrown = x; throw x;
                } catch (Throwable x) {
                    thrown = x; throw new Error(x);
                } finally {
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

最外层的 while 循环不断调用 getTask() 方法从队列中取任务，如果取不到，就属于异常情况，把 completedAbruptly 设置为 false，进入 processWorkerExit 方法