线程池拒绝策略

• 简介

AbortPolicy：该策略直接抛出异常，阻止系统正常工作
CallerRunsPolicy：只要线程池没有关闭，该策略直接在调用者线程中，执行当前被丢弃的任务
DiscardPolicy：直接把任务丢弃
DiscardOldestPolicy：丢弃最老的一个请求（任务队列里面的第一个），再尝试提交任务

• 查看类结构

# 查看ThreadPoolExecutor类
    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());
        }
    }

• 自定义线程拒绝策略

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

public class CustomPolicy implements RejectedExecutionHandler {
    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        // 执行其他操作，例如发送邮件告警
        System.out.println("线程池满了");
    }
}

# 测试使用
public class ThreadPoolDemo {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        // 创建队列，队列大小指定10
        LinkedBlockingQueue<Runnable> objects = new LinkedBlockingQueue<>(10);
        // 创建线程池，每次处理10个任务，3秒
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10, 20, 3L,
                TimeUnit.SECONDS, objects, new CustomPolicy());      // 使用自定义策略
        // 初始化核心线程
        threadPoolExecutor.prestartAllCoreThreads();
        // 执行50个线程
        for (int i = 0; i < 50; i++) {
            threadPoolExecutor.submit(()->{         // 提交任务
                try {
                    Thread.sleep(2000L);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(threadPoolExecutor.getActiveCount());    // 获取线程池中活跃的数量
            });
        }
    }

}