ThreadPoolExecutor 线程池的实现

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ThreadPoolExecutor继承自 AbstractExecutorService。AbstractExecutorService实现了 ExecutorService 接口。

首先是ThreadPoolExecutor的构造方法：

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

回到 Executors 构建线程池的方法

• 构建固定大小的线程池

  public static ExecutorService More ...newFixedThreadPool(int nThreads) {
      return new ThreadPoolExecutor(nThreads, nThreads,
                                          0L, TimeUnit.MILLISECONDS,
                                          new LinkedBlockingQueue<Runnable>());
  } 

• 构建缓冲池，需要的时候才新建线程，新建出来的线程不会被回收而是用于后面的重用

  public static ExecutorService More ...newCachedThreadPool() {
      return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                         60L, TimeUnit.SECONDS,
                                         new SynchronousQueue<Runnable>());
  }

再看ThreadPoolExecutor的execute()方法，该方法是Executor接口定义的方法（ExecutorService接口继承自Executor）：

    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn‘t, by returning false.
         * 当池子大小小于corePoolSize就新建线程，并处理请求
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         * 如果任务可以被插入到workQueue，需要复查当前运行状态
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         * 如果任务无法被插入到workQueue，则调用reject方法
         */
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }