ThreadPoolExecutor自定义线程池

Posted 2020-11-23 1994july

1.ThreadPoolExecutor创建线程池的构造函数

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.acc = System.getSecurityManager() == null ? null : AccessController.getContext(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler; }

参数简介：

- corePoolSize：核心线程数量

- maximumPoolSize：最大线程数量

- keepAliveTime：没有执行任务的线程，最大空闲时间

- unit：时间单位,TimeUnit.DAYS,TimeUnit.HOURS,TimeUnit.MINUTES,TimeUnit.SECONDS,TimeUnit.MILLISECONDS
			TimeUnit.MICROSECONDS,TimeUnit.NANOSECONDS,七种

- BlockingQueue<Runnable> workQueue：一个阻塞队列，用来存储等待执行的任务，
			ArrayBlockingQueue，LinkedBlockingQueue，SynchronousQueue三种

- ThreadFactory threadFactory：创建线程的工厂类

- RejectedExecutionHandler handler：拒绝策略

2.拒绝策略 拒绝策略： 四种拒绝策略,实现：RejectedExecutionHandler接口 1.AbortPolicy(中止策略,抛出异常,默认的策略)

源码： /** * 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](https://my.oschina.net/u/2303379) r the runnable task requested to be executed
     * [@param](https://my.oschina.net/u/2303379) e the executor attempting to execute this task
     * [@throws](https://my.oschina.net/throws) RejectedExecutionException always
     */
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        throw new RejectedExecutionException("Task " + r.toString() +
                                             " rejected from " +
                                             e.toString());
    }
}

2.CallerRunsPolicy(由调用线程处理该任务)

源码： /** * 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();
        }
    }
}

3.DiscardOldestPolicy(丢弃队列最前面的任务，然后重新提交被拒绝的任务)

源码： /** * 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();
            e.execute(r);
        }
    }
}

4.DiscardPolicy(丢弃任务，但是不抛出异常。如果线程队列已满，则后续提交的任务都会被丢弃，且是静默丢弃)

源码： /** * 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) {
    }
}

根据实际需求，制定合适的拒绝策略

来源：江苏网站优化