现象描述

线上的定时任务使用了corntab来定时启动console应用，最近出现应用运行完成后，不自动关闭的情况，因为进程不停止，占用了大量内存。

使用jstack查看线程信息，发现几个非守护进程，处于WAITING状态。

$ jstack pid

"background_executor-3-thread-7" #87 prio=5 os_prio=0 tid=0x00007fdfcc17d000 nid=0x6423 waiting on condition [0x00007fdf566ed000]
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x00000000f1ee9850> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
        at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)
        at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)

问题分析

在代码中搜索关键字 “background_executor”，找到是线程池里自定义了ThreadFactory，创建线程时同时给了线程名称，并且设定了线程为非守护进线程 t.setDaemon(true) 。
同时线程池初始化设置了
corePoolSize = 8
maximumPoolSize = 20
keepAliveTime = 30 second
这致使线程池在完成任务空闲时，仍会保持8个线程处于WAITING状态，等待新任务。除非设置
allowCoreThreadTimeOut = true。
参照线程池构造函数的说明:

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue,
                          RejectedExecutionHandler handler)

Creates a new ThreadPoolExecutor with the given initial parameters and default thread factory.
Parameters:
corePoolSize - the number of threads to keep in the pool, even if they are idle, unless allowCoreThreadTimeOut is set
maximumPoolSize - the maximum number of threads to allow in the pool
keepAliveTime - when the number of threads is greater than the core, this is the maximum time that excess idle threads will wait for new tasks before terminating.
unit - the time unit for the keepAliveTime argument
workQueue - the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute method.
handler - the handler to use when execution is blocked because the thread bounds and queue capacities are reached

解决方法

将线程池的线程设置为守护进程肯定不行，那样会导致任务没有执行完就退出。llowCoreThreadTimeOut = true ，线程池在执行完任务后，经过超时时间，将所有空闲的线程都释放掉，进程池这样进程就可以退出。

扩展方法

那是否还有其他方法呢？当然线程池还提供了 Shutdown 方法，在提交完所有任务后，调用Shutdown关闭线程池，线程池完成所有任务后，进程池会被回收。

Shutdown调用后，线程池从Running状态变为Shutdown状态，不会接收新任务，但会将队列的任务执行完，然后进入Tidying状态，在执行完terminated()钩子后，变成Terminated状态，线程池可以被回收。

线程池中使用一个int值的前3位标识线程池状态，后面29位表示线程数量。这样在设置线程数量的时候，可以同时设置线程池的状态，避免加锁。
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));

    /*
     * 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).
     */