為什么要使用拒絕策略？

等待隊(duì)列已經(jīng)排滿了，再也塞不下新任務(wù)了
同時(shí)，線程池中的max線程也達(dá)到了，無法繼續(xù)為新任務(wù)服務(wù)。

這個(gè)是時(shí)候我們就需要拒絕策略機(jī)制合理的處理這個(gè)問題。

四種拒絕策略介紹

jdk中自帶的拒絕策略均實(shí)現(xiàn)了java.util.concurrent.RejectedExecutionHandler接口

AbortPolicy

默認(rèn)的拒絕策略。直接拋出 java.util.concurrent.RejectedExecutionException異常

 new ThreadPoolExecutor.AbortPolicy()

import java.util.concurrent.*;

public class Test {

    public static void main(String[] args) {
        ExecutorService executorService = new ThreadPoolExecutor(2, 5, 2, TimeUnit.SECONDS, new LinkedBlockingQueue<>(3), Executors.defaultThreadFactory(), new ThreadPoolExecutor.AbortPolicy());
        try {
            for (int i = 1; i <= 9; i++) {
                executorService.execute(new Runnable() {
                    @Override
                    public void run() {
                        System.out.println(Thread.currentThread().getName() + " 辦理業(yè)務(wù)");
                    }
                });
            }
        } finally {
            executorService.shutdown();
        }

    }
}

CallerRunsPolicy

將任務(wù)返還給調(diào)用者線程執(zhí)行

new ThreadPoolExecutor.CallerRunsPolicy()

import java.util.concurrent.*;

public class Test {

    public static void main(String[] args) {
        ExecutorService executorService = new ThreadPoolExecutor(2, 5, 2, TimeUnit.SECONDS, new LinkedBlockingQueue<>(3), Executors.defaultThreadFactory(), new ThreadPoolExecutor.CallerRunsPolicy());
        try {
            for (int i = 1; i <= 9; i++) {
                executorService.execute(new Runnable() {
                    @Override
                    public void run() {
                        System.out.println(Thread.currentThread().getName() + " 辦理業(yè)務(wù)");
                    }
                });
            }
        } finally {
            executorService.shutdown();
        }

    }
}

執(zhí)行結(jié)果： 任務(wù)果然返還給main線程了

image.png

DiscardPolicy

直接拋棄無法處理的任務(wù)，不予處理不拋異常。如果業(yè)務(wù)匯總允許任務(wù)丟失，這是最好的策略

new ThreadPoolExecutor.DiscardPolicy()

查看執(zhí)行結(jié)果，只有8個(gè)。第9個(gè)被拋棄

image.png

DiscardOldestPolicy

拋棄隊(duì)列中等待最久的任務(wù)，然后把當(dāng)前任務(wù)加入隊(duì)列中嘗試再次提交當(dāng)前任務(wù)

new ThreadPoolExecutor.DiscardOldestPolicy()

image.png