摘要:所以,并不代表线程池就一定立即就能退出,它也可能必须要等待所有正在执行的任务都执行完成了才能退出。
本文主要分析java.util.concurrent.ThreadPoolExecutor的实现原理,首先看它的构造函数:
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueueworkQueue, 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; }
corePoolSize:线程池中稳定保存的线程数(一开始会小于这个数)
maximumPoolSize:线程池中最大线程数
keepAliveTime and unit:大于最小线程数的线程空闲后存活时间
workQueue:用于存放任务的阻塞队列
threadFactory:用于创建线程的工厂类
handler:当任务队列满了且线程数达到了最大时的饱和策略
对于IO密集型任务,线程数一般设为CPU数*2,对于计算密集型任务,线程数一般设为CPU数。
当调用execute方法时:
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); }
其流程如图:
创建线程是通过addWorker创建内部Worker类,其中调用getThreadFactory().newThread(this)来创建执行自己的线程,之后在addWorker中start该线程,执行Worker run方法中的runWorker会不断的从任务队列中获取任务或阻塞,并且每次执行任务前会执行beforeExecute,之后会afterExecute,可以通过重写beforeExecute方法来给执行线程重命名。
线程池状态变化如图:
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
shutdownNow终止线程的方法是通过调用Thread.interrupt()方法来实现的:
*If this thread is blocked in an invocation of the {@link * Object#wait() wait()}, {@link Object#wait(long) wait(long)}, or {@link * Object#wait(long, int) wait(long, int)} methods of the {@link Object} * class, or of the {@link #join()}, {@link #join(long)}, {@link * #join(long, int)}, {@link #sleep(long)}, or {@link #sleep(long, int)}, * methods of this class, then its interrupt status will be cleared and it * will receive an {@link InterruptedException}. * *
If this thread is blocked in an I/O operation upon an {@link * java.nio.channels.InterruptibleChannel InterruptibleChannel} * then the channel will be closed, the thread"s interrupt * status will be set, and the thread will receive a {@link * java.nio.channels.ClosedByInterruptException}. * *
If this thread is blocked in a {@link java.nio.channels.Selector} * then the thread"s interrupt status will be set and it will return * immediately from the selection operation, possibly with a non-zero * value, just as if the selector"s {@link * java.nio.channels.Selector#wakeup wakeup} method were invoked. * *
If none of the previous conditions hold then this thread"s interrupt * status will be set.
可以看到如果线程处于正常活动状态,那么会将该线程的中断标志设置为true,而无法中断当前的线程。所以,shutdownNow并不代表线程池就一定立即就能退出,它也可能必须要等待所有正在执行的任务都执行完成了才能退出。
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