Java线程池 ThreadPoolExecutor类

什么是线程池?

java线程池是将大量的线程集中管理的类, 包括对线程的创建, 资源的管理, 线程生命周期的管理。
当系统中存在大量的异步任务的时候就考虑使用java线程池管理所有的线程, 从而减少系统资源的开销。

阿里的开发手册规范

线程池不允许使用 Executors 去创建，而是通过 ThreadPoolExecutor 的方式，这样的处理方式让写的人更加明确线程池的运行规则，规避资源耗尽的风险。
Executors 返回的线程池对象的弊端如下：
- FixedThreadPool 和 SingleThreadPool: 允许的请求队列长度为 Integer.MAX_VALUE，可能会堆积大量的请求，从而导致 OOM。
- CachedThreadPool 和 ScheduledThreadPool: 允许的创建线程数量为 Integer.MAX_VALUE，可能会创建大量的线程，从而导致 OOM。

线程池的创建

new ThreadPoolExecutor(int corePoolSize, int maximumPoolSize,long keepAliveTime, TimeUnit unit,BlockingQueue workQueue,RejectedExecutionHandler handler)
- corePoolSize: 线程池维护线程的最少数量
- maximumPoolSize: 线程池维护线程所允许的空闲时间
- unit: 线程池维护线程锁允许的空闲时间的单位
- workQueue: 线程池锁使用的缓冲队列
- handler: 线程池对拒绝任务的处理策略

添加任务到线程池

通过execute(Runnable) 方法添加到线程池, 任务就是一个Runnable类型的对象, 任务的执行方法就是Runnable类型对象的run()方法。
当一个任务通过execute(Runnable) 方法欲添加到线程池时:
- 如果此时线程池中线程的数量小于corePoolSize, 即使线程池中的线程都处于空闲状态，也要创建新的线程来处理被添加的任务。
- 如果此时线程池中的数量等于 corePoolSize，但是缓冲队列 workQueue未满，那么任务被放入缓冲队列
- 如果此时线程池中的数量大于corePoolSize，缓冲队列workQueue满，并且线程池中的数量小于maximumPoolSize，建新的线程来处理被添加的任务。
- 如果此时线程池中的数量大于corePoolSize，缓冲队列workQueue满，并且线程池中的数量等于maximumPoolSize，那么通过 handler所指定的策略来处理此任务。
也就是说:
- 处理任务的优先级为：
  - 核心线程corePoolSize、任务队列workQueue、最大线程maximumPoolSize，如果三者都满了，使用handler处理被拒绝的任务。
当线程池中的线程数量大于 corePoolSize时，如果某线程空闲时间超过keepAliveTime，线程将被终止。这样，线程池可以动态的调整池中的线程数。
unit可选的参数为java.util.concurrent.TimeUnit中的几个静态属性：NANOSECONDS、MICROSECONDS、MILLISECONDS、SECONDS。
workQueue常用的是：java.util.concurrent.ArrayBlockingQueue

handler的四个选择:

ThreadPoolExecutor.AbortPolicy() [直译过来流产计划?]

抛出 java.util.concurrent.RejectedExecutionException异常

 /**
     * 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());
        }
    }

ThreadPoolExecutor.CallerRunsPolicy()

重试添加当前的任务，它会自动重复调用execute()方法

/**
     * 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.
         * 在调用者的线程中执行任务r, 除非执行器被关闭, 任务才会被抛弃。
         *
         * @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();
            }
        }
    }

ThreadPoolExecutor.DiscardOldestPolicy()

抛弃旧的任务

 /**
     * 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.
         * 获取并忽视下一个执行器会执行的任务, 如果其中一个是当前可获取的, 并且
         *多次重试执行过任务r。除非执行器被关闭, 任务才会被抛弃。
         *
         * @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);
            }
        }
    }
}

ThreadPoolExecutor.DiscardPolicy()

抛弃当前的任务

/**
     * 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.
         * 什么都不做, 就有抛弃任务r的效果
         *
         * @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) {
        }
    }

测试线程池Demo

package com.ronnie;import java.io.Serializable;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;public class ThreadPoolDemo {
    private static int produceTaskSleepTime = 5;
    private static int consumeTaskSleepTime = 5000;
    private static int produceTaskMaxNumber = 20; // 定义最大添加20个线程到线程池中    /**
     *  线程池执行的任务
     */
   public static class ThreadPoolTask implements Runnable, Serializable{
       private static final long serialVersionUID = 0;
        // 保存任务所需要的数据
       private Object threadPoolTaskData;
       ThreadPoolTask(Object works){
           this.threadPoolTaskData = works;
       }
       @Override
       public void run() {
           // 处理一个任务
           System.out.println(threadPoolTaskData + "started......");
           try {
               // 便于观察, 等待一段时间
               Thread.sleep(consumeTaskSleepTime);
           } catch (Exception e){
               e.printStackTrace();
           }
           threadPoolTaskData = null;
       }       public Object getTask(){
           return  this.threadPoolTaskData;
       }
   }    public static void main(String[] args) {
        // 构造一个线程池
        ThreadPoolExecutor threadPool = new ThreadPoolExecutor(6, 12, 9,
                TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(6), new ThreadPoolExecutor.DiscardOldestPolicy());        for (int i = 1; i <= produceTaskMaxNumber; i++){
            try {
                // 创建一个任务并将其加入线程池
                String work = "work@: " + i;
                System.out.println("put: " + work);
                threadPool.execute(new ThreadPoolTask(work));
                // 等待一会儿, 便于观察
                Thread.sleep(produceTaskSleepTime);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }    }
}

执行结果

put: work@: 1
work@: 1started......
put: work@: 2
work@: 2started......
put: work@: 3
work@: 3started......
put: work@: 4
work@: 4started......
put: work@: 5
work@: 5started......
put: work@: 6
work@: 6started......
put: work@: 7
put: work@: 8
put: work@: 9
put: work@: 10
put: work@: 11
put: work@: 12
put: work@: 13
work@: 13started......
put: work@: 14
work@: 14started......
put: work@: 15
work@: 15started......
put: work@: 16
work@: 16started......
put: work@: 17
work@: 17started......
put: work@: 18
work@: 18started......
put: work@: 19
put: work@: 20
work@: 9started......
work@: 10started......
work@: 11started......
work@: 12started......
work@: 19started......
work@: 20started......

什么是线程池?

阿里的开发手册规范

线程池的创建

添加任务到线程池

测试线程池Demo

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