java并发初探ThreadPoolExecutor拒绝策略

ThreadPoolExecuter构造器

corePoolSize是核心线程池，就是常驻线程池数量；
maximumPoolSize是最大线程池数量，如果队列满了，就会创建新的线程任务。如果与corePoolSize一样大小，
那么就是固定大小的鹅线程池；
keepAliveTime表示线程池中线程空闲时间，当空闲时间达到keepAliveTime，线程会被销毁，直到剩下corePoolSize，
默认情况只有当前的线程数大于corePoolSize时keepAliveTime才会起作用；
unit时间单位；
BlockingQueue阻塞队列。当请求线程数大于corePoolSize时，线程会进入阻塞队列，当达到阻塞队列的上限是，线程池会创建新的
线程池，最大的线程数就是maximumPoolSize；
其他参数：
threadFactory线程工厂，默认是Executors.defaultThreadFactory()；
RejectedExecutionHandler拒绝策略的执行对象，当线程池数量大于maximumPoolSize，拒绝策略就会执行；

    /*** Creates a new {@code ThreadPoolExecutor} with the given initial* parameters and default thread factory and rejected execution handler.* It may be more convenient to use one of the {@link Executors} factory* methods instead of this general purpose constructor.** @param corePoolSize the number of threads to keep in the pool, even*        if they are idle, unless {@code allowCoreThreadTimeOut} is set* @param maximumPoolSize the maximum number of threads to allow in the*        pool* @param keepAliveTime when the number of threads is greater than*        the core, this is the maximum time that excess idle threads*        will wait for new tasks before terminating.* @param unit the time unit for the {@code keepAliveTime} argument* @param workQueue the queue to use for holding tasks before they are*        executed.  This queue will hold only the {@code Runnable}*        tasks submitted by the {@code execute} method.* @throws IllegalArgumentException if one of the following holds:<br>*         {@code corePoolSize < 0}<br>*         {@code keepAliveTime < 0}<br>*         {@code maximumPoolSize <= 0}<br>*         {@code maximumPoolSize < corePoolSize}* @throws NullPointerException if {@code workQueue} is null*/public ThreadPoolExecutor(int corePoolSize,int maximumPoolSize,long keepAliveTime,TimeUnit unit,BlockingQueue<Runnable> workQueue) {this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,Executors.defaultThreadFactory(), defaultHandler);}

拒绝策略

AbortPolicy
默认拒绝策略，丢弃任务，抛出异常RejectedExecutionException

DiscardPolicy
丢弃任务
DiscardOldestPolicy
把最旧的等待任务抛弃，放入等待队列

CallerRunsPolicy
拒绝任务在当前正在执行的线程运行

    /*** A handler for rejected tasks that throws a* {@code RejectedExecutionException}.*/public static class AbortPolicy implements RejectedExecutionHandler/*** A handler for rejected tasks that silently discards the* rejected task.*/public static class DiscardPolicy implements RejectedExecutionHandler/*** 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/*** 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     

例子

package com.java.javabase.thread.base.threadpool;import lombok.extern.slf4j.Slf4j;import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;/*** @author*/
@Slf4j
public class ThreadPoolAbortPolicyTest {public static void main(String[] args) {ThreadPoolExecutor pool =new ThreadPoolExecutor(1,2,0, TimeUnit.SECONDS,new ArrayBlockingQueue<Runnable>(1));//pool.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());//pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy());pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardOldestPolicy());//pool.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());pool.execute(() -> log.info("thread {} 0", Thread.currentThread().getName()));pool.execute(() -> log.info("thread {} 1", Thread.currentThread().getName()));pool.execute(() -> log.info("thread {} 2", Thread.currentThread().getName()));pool.execute(() -> log.info("thread {} 3", Thread.currentThread().getName()));pool.execute(() -> log.info("thread {} 4", Thread.currentThread().getName()));pool.shutdown();}
}

DiscardOldestPolicy结果

2019-08-14 15:56:18,972   [pool-1-thread-1] INFO  ThreadPoolAbortPolicyTest  - thread pool-1-thread-1 0
2019-08-14 15:56:18,972   [pool-1-thread-1] INFO  ThreadPoolAbortPolicyTest  - thread pool-1-thread-1 4
2019-08-14 15:56:18,972   [pool-1-thread-2] INFO  ThreadPoolAbortPolicyTest  - thread pool-1-thread-2 2

CallerRunsPolicy结果

2019-08-14 15:58:03,439   [pool-1-thread-1] INFO  ThreadPoolAbortPolicyTest  - thread pool-1-thread-1 0
2019-08-14 15:58:03,439   [pool-1-thread-2] INFO  ThreadPoolAbortPolicyTest  - thread pool-1-thread-2 2
2019-08-14 15:58:03,454   [pool-1-thread-1] INFO  ThreadPoolAbortPolicyTest  - thread pool-1-thread-1 1
2019-08-14 15:58:03,439   [main] INFO  ThreadPoolAbortPolicyTest  - thread main 3
2019-08-14 15:58:03,454   [pool-1-thread-1] INFO  ThreadPoolAbortPolicyTest  - thread pool-1-thread-1 4