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支持生产阻塞的线程池

在生产 - 消费者问题中

newFixedThreadPool的构造参数里的nThreads是最大同时工作的线程数，如果工作线程已满，新提交的任务会被放到一个无界的LinkedBlockingQueue里（等待队列）

如果生产速度大于消费速度，那么会发生任务堆积，等待队列会扩展到内存耗尽

naive的想法是，自定义线程池，将等待队列设置为有界的BlockingQueue，那么新提交的任务会被block住，直到工作线程空出来为止

但是如果去看J.U.C的源码，ThreadPoolExecutor.execute(Runnable)方法

 1     public void execute(Runnable command) {
 2         if (command == null)
 3             throw new NullPointerException();
 4         /*
 5          * Proceed in 3 steps:
 6          *
 7          * 1. If fewer than corePoolSize threads are running, try to
 8          * start a new thread with the given command as its first
 9          * task.  The call to addWorker atomically checks runState and
10          * workerCount, and so prevents false alarms that would add
11          * threads when it shouldn't, by returning false.
12          *
13          * 2. If a task can be successfully queued, then we still need
14          * to double-check whether we should have added a thread
15          * (because existing ones died since last checking) or that
16          * the pool shut down since entry into this method. So we
17          * recheck state and if necessary roll back the enqueuing if
18          * stopped, or start a new thread if there are none.
19          *
20          * 3. If we cannot queue task, then we try to add a new
21          * thread.  If it fails, we know we are shut down or saturated
22          * and so reject the task.
23          */
24         int c = ctl.get();
25         if (workerCountOf(c) < corePoolSize) {
26             if (addWorker(command, true))
27                 return;
28             c = ctl.get();
29         }
30         if (isRunning(c) && workQueue.offer(command)) {
31             int recheck = ctl.get();
32             if (! isRunning(recheck) && remove(command))
33                 reject(command);
34             else if (workerCountOf(recheck) == 0)
35                 addWorker(null, false);
36         }
37         else if (!addWorker(command, false))
38             reject(command);
39     }

可以看到，第30行调用的是workQueue的非阻塞的offer方法。

所以如果队列已满，新提交的任务不会被block住，反而会调用后续的reject流程。

如果我们想要达到阻塞生产者的目的的话，要么用信号量之类的东西限制同时进入线程池等待队列的任务数
要么加入线程池的等待策略

可以使用CallerRunsPolicy，让提交任务的线程来运行任务
也可以自定义等待策略，把新加入的任务put到等待队列里，这样就可以阻塞住生产者

范例代码如下

 1         int corePoolSize = 30;
 2         int waitQueueSize = 100000;
 3         ExecutorService pool = new ThreadPoolExecutor(corePoolSize, corePoolSize, 1, TimeUnit.DAYS,
 4                 new LinkedBlockingQueue<Runnable>(waitQueueSize), new RejectedExecutionHandler() {
 5             @Override
 6             public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
 7                 if (!executor.isShutdown()) {
 8                     try {
 9                         executor.getQueue().put(r);
10                     } catch (InterruptedException e) {
11                         // should not be interrupted
12                     }
13                 }
14             }
15         });