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