java - Java ThreadPool 构造 - “何时创建线程”？

Question

成员们，我正在使用 ThreadPool 构造（ThreadPoolExecutors），而不是每次都实例化新线程。我的问题是：

• JVM 什么时候真正创建和启动新线程？
• JVM 如何将新的 Runnable 任务分配给已经启动的线程？

标签： javaconcurrency

Answer 1

JVM 什么时候真正创建并启动新线程？

这取决于您使用的框架。如果您使用ThreadPoolExecutor的是自 1.5 以来位于 JDK 的那个，线程池是在第一个任务提交时创建的，而不是在创建时ThreadPoolExecutor创建的。下面的代码片段来自java.util.concurrent.ThreadPoolExecutor：

    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);
    }

JVM 如何将新的 Runnable 任务分配给已经启动的线程？

例如ThreadPoolExecutor，它首先将任务放入workQueue，它是 aBlockingQueue并且在创建时ThreadPoolExecutor创建，然后，当线程池中的一个工作线程完成最后一个任务时，它会尝试从workQueue.

但请注意，所有的工人都是首先创建一个初始任务，他们在一次创建时完成工作。

    private boolean addWorker(Runnable firstTask, boolean core) {