关于Java线程池的解析与用法

继承关系图

ThreadPoolExecutor

进程池的几种常用创建类型

通过ThreadPoolExecutor构造函数创建

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public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory)

public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler)

public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler)

主要参数

  • corePoolSize:最大核心线程数(必须)
  • maximumPoolSize:线程池最大总线程数(必须)
  • keepAliveTime:空闲线程的最长存活时间(必须)
  • unit:时间单位(必须)
  • workQueue:任务储存队列(必须)
  • ThreadFactory:线程工厂(默认Executors.DefaultThreadFactory)
  • handler:拒绝策略(默认AbortPolicy)

拒绝策略

拒绝策略是线程池无法接受新任务时的处理机制

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public interface RejectedExecutionHandler {
    void rejectedExecution(Runnable var1, ThreadPoolExecutor var2);
}

常见拒绝策略:

  • ThreadPoolExecutor.AbortPolicy:抛出一个RejectedExecutionException错误
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public static class AbortPolicy implements RejectedExecutionHandler {
    public AbortPolicy() {
    }

    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        throw new RejectedExecutionException("Task " + r.toString() + " rejected from " + e.toString());
    }
}
  • ThreadPoolExecutor.DiscardOldestPolicy:丢弃队列中最早加入的任务
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public static class DiscardOldestPolicy implements RejectedExecutionHandler {
    public DiscardOldestPolicy() {
    }

    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            e.getQueue().poll();
            e.execute(r);
        }

    }
}
  • ThreadExecutor.DiscardPolicy:丢弃当前任务,即不做任何操作
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public static class DiscardPolicy implements RejectedExecutionHandler {
    public DiscardPolicy() {
    }

    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
    }
}
  • ThreadExecutor.CallerRunsPolicy:由提交任务的线程执行任务
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public static class CallerRunsPolicy implements RejectedExecutionHandler {
    public CallerRunsPolicy() {
    }

    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            r.run();
        }

    }
}

通过Executors框架工厂函数创建

  • Executors.newFixedThreadPool:创建一个固定线程的线程
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public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue());
}
  • Executors.SingleThreadExecutor:创建一个单线程线程池
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public static ExecutorService newSingleThreadExecutor() {
    return newSingleThreadExecutor(defaultThreadFactory());
}

public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
    return new AutoShutdownDelegatedExecutorService(new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue(), threadFactory));
}
  • Executors.newCachedThreadPool:创建一个无核心线程且线程上限为Integer.MAX_VALUE的线程池
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public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue());
}

注：以上工厂函数都可在最后加上ThreadFactory对象参数自定义线程工厂且任务队列最大容量为Integer.MAX_VALUE