线程池源码解析

2024-07-13 05:10| 来源: 网络整理| 查看: 265

上篇博文《线程池源码解析》中，遗留了一个问题，拒绝策略。

这篇文章，大概说一下。

ThreadPoolExecutor 类中，线程池的拒绝策略有四种

AbortPolicy 默认策略，丢弃任务，直接抛出异常。CallerRunsPolicy 由提交任务的线程，来执行任务。DiscardPolicy 丢弃任务，不抛异常，实际是啥也不做。DiscardOldestPolicy 将队列中老的任务丢弃，然后提交当前任务。

现在看下它们各自的源码

public void execute(Runnable command) { if (command == null) throw new NullPointerException(); int c = ctl.get(); if (workerCountOf(c) // 2、将任务放入阻塞队列 int recheck = ctl.get(); if (! isRunning(recheck) && remove(command)) reject(command); else if (workerCountOf(recheck) == 0) addWorker(null, false); } else if (!addWorker(command, false)) // 3、队满时，继续创建线程，直至工作线程数量达到最大值。 reject(command); // 4、执行拒绝策略 }

先回顾下，线程池的运行，

工作线程数量小于核心线程数时，优先创建线程。工作线程数量等于核心线程数时，将任务往阻塞队列中存在。阻塞队列放不下时，继续创建工作线程，直到达到最大线程数。以上三步还是处理不过来，执行拒绝策略。 final void reject(Runnable command) { handler.rejectedExecution(command, this); } private static final RejectedExecutionHandler defaultHandler = new AbortPolicy();

这里的handle 是在创建线程池时，指定的一个参数。默认的拒绝策略是AbortPolicy

/** * A handler for rejected tasks that throws a * {@code RejectedExecutionException}. */ public static class AbortPolicy implements RejectedExecutionHandler { /** * Creates an {@code AbortPolicy}. */ public AbortPolicy() { } /** * Always throws RejectedExecutionException. * * @param r the runnable task requested to be executed * @param e the executor attempting to execute this task * @throws RejectedExecutionException always */ public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { throw new RejectedExecutionException("Task " + r.toString() + " rejected from " + e.toString()); } }

代码很明了，传入的任务，没有做任何处理，只是抛出一个异常。

CallerRunsPolicy /** * A handler for rejected tasks that runs the rejected task * directly in the calling thread of the {@code execute} method, * unless the executor has been shut down, in which case the task * is discarded. */ public static class CallerRunsPolicy implements RejectedExecutionHandler { /** * Creates a {@code CallerRunsPolicy}. */ public CallerRunsPolicy() { } /** * Executes task r in the caller's thread, unless the executor * has been shut down, in which case the task is discarded. * * @param r the runnable task requested to be executed * @param e the executor attempting to execute this task */ public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { if (!e.isShutdown()) { r.run(); } } }

这个处理还是比较合理的，线程池处理不过来的时候，由哪个线程提交的任务，就由哪个线程来执行。

DiscardPolicy /** * A handler for rejected tasks that silently discards the * rejected task. */ public static class DiscardPolicy implements RejectedExecutionHandler { /** * Creates a {@code DiscardPolicy}. */ public DiscardPolicy() { } /** * Does nothing, which has the effect of discarding task r. * * @param r the runnable task requested to be executed * @param e the executor attempting to execute this task */ public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { } }

这个方法处理，很是懒，什么也没做，只是个空方法。

DiscardOldestPolicy /** * A handler for rejected tasks that discards the oldest unhandled * request and then retries {@code execute}, unless the executor * is shut down, in which case the task is discarded. */ public static class DiscardOldestPolicy implements RejectedExecutionHandler { /** * Creates a {@code DiscardOldestPolicy} for the given executor. */ public DiscardOldestPolicy() { } /** * Obtains and ignores the next task that the executor * would otherwise execute, if one is immediately available, * and then retries execution of task r, unless the executor * is shut down, in which case task r is instead discarded. * * @param r the runnable task requested to be executed * @param e the executor attempting to execute this task */ public void rejectedExecution(Runnable r, ThreadPoolExecutor e) { if (!e.isShutdown()) { e.getQueue().poll(); // 队头worker出队，给新任务腾个地方 e.execute(r); // 执行任务 } } }

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