java并发：线程池之饱和策略

一、饱和策略（线程池任务拒绝策略）

ThreadPoolExecutor构造函数的RejectedExecutionHandler handler参数表示当提交的任务数超过maxmumPoolSize与workQueue之和时，任务会交给RejectedExecutionHandler来处理，此处我们来具体了解一下

二、源码分析

（1）ThreadPoolExecutor中默认的饱和策略定义如下：

    /**
     * The default rejected execution handler.
     */
    private static final RejectedExecutionHandler defaultHandler =
        new AbortPolicy();

（2）ThreadPoolExecutor中获取、设置饱和策略的方法如下：

    /**
     * Sets a new handler for unexecutable tasks.
     *
     * @param handler the new handler
     * @throws NullPointerException if handler is null
     * @see #getRejectedExecutionHandler
     */
    public void setRejectedExecutionHandler(RejectedExecutionHandler handler) {
        if (handler == null)
            throw new NullPointerException();
        this.handler = handler;
    }

    /**
     * Returns the current handler for unexecutable tasks.
     *
     * @return the current handler
     * @see #setRejectedExecutionHandler(RejectedExecutionHandler)
     */
    public RejectedExecutionHandler getRejectedExecutionHandler() {
        return handler;
    }

（3）RejectedExecutionHandler接口

RejectedExecutionHandler的定义如下：

public interface RejectedExecutionHandler{
    //被线程池丢弃的线程处理机制
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) ;
}

（4）AbortPolicy

此策略继承RejectedExecutionHandler接口，其源码如下：

public static class AbortPolicy implements RejectedExecutionHandler{

    public AbortPolicy(){}
    
    //直接抛出异常
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        throw new RejectedExecutionException("Task"+r.toString()+"rejected from"+executor.toString());
    }
    
}

（5）自定义饱和策略

实现RejectedExecutionHandler接口，代码如下：

package com.test;

import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;

public class RejectedExecutionHandlerDemo implements RejectedExecutionHandler{

    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        // TODO Auto-generated method stub
        System.out.println("线程信息"+r.toString()+"被遗弃的线程池："+executor.toString());
    }
    
}

Note：

针对线程池使用 FutureTask 时，如果饱和策略设置为 DiscardPolicy 和 DiscardOldestPolicy，并且在被拒绝的任务的 Future对象上调用了无参 get方法，那么调用线程会一直被阻塞。