Java并发22 线程池原理

1.线程池处理流程

当线程池提交一个新的任务时，线程池的处理流程如下：

1. 线程池判断核心线程池corePoolSize是否已满(是否都在执行任务)，未满则创建线程执行任务；已满则下一步
2. 线程池判断工作队列BlockingQueue是否已满，未满则将任务放进队列；已满则下一步
3. 线程池判断池中线程数是否达到maximumPoolSize(所有线程是否都已处在工作状态)，如果没有则创建新的线程执行任务；如果已满则交给饱和策略RejectedExecutionHandler处理。
   

2.execute()方法

有了上面的线程池处理流程，我们来看看线程池ThreadPoolExecutor执行execute方法流程，如下图：

1. 如果当前运行线程少于corePoolSize，则创建新县城来执行任务（这一步需要获取全局锁）
2. 如果运行线程等于多于corePoolSize，则将任务加入BlockingQueue
3. 如果BlockingQueue工作队列已满，则创建新的线程来处理任务（这一步需要获取全局锁）
4. 如果线程池饱和（工作队列已满，线程池数达到maximumPoolSize），任务将被拒绝，并调用饱和策略RejectedExecutionHandler.rejectExecution()处理
   下面结合上面的流程来瞅瞅大胡子 Doug Lea的源码
   

3.线程池源码

注释中3个steps也对应着上面的流程

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.
         *
         * 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.
         *
         * 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.
         */
        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);
    }

下一篇博客来详细讲一下线程池ThreadPoolExecutor