詳解Java中CountDownLatch的用法

更新時間：2023年05月17日 10:39:15 作者：tizzybepeacejoy

這篇文章主要為大家詳細介紹了Java中CountDownLatch類的用法，本文通過一些簡單的示例進行了簡單介紹，感興趣的小伙伴可以跟隨小編一起了解一下

CountDownLatch使用場景

線程計數(shù)器用于線程執(zhí)行任務(wù),計數(shù) 等待線程結(jié)束

用法一: 等待所有的事情都做完

        //程序計數(shù)器
        CountDownLatch countDownLatch = new CountDownLatch(10000);
        //2個線程
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        AtomicInteger count = new AtomicInteger(0);
        for (int i = 0; i < 10000; i++) {
            executorService.submit(() -> {
                count.getAndIncrement();//自增
                System.out.println(Thread.currentThread().getName() + " : " + count.get());
                countDownLatch.countDown();
            });
        }
        //線程池 等待10s
        executorService.awaitTermination(10, TimeUnit.SECONDS);
        //關(guān)閉線程 其實是將線程狀態(tài)設(shè)置為中斷標志  必須等待所有線程處理完任務(wù),才能完全關(guān)閉
        executorService.shutdown();
        //必須等待兩個線程執(zhí)行完   會一直等待下去，當然也可以設(shè)置指定時間等待超時 await(timeout);
        countDownLatch.await();
    }

始終是2個線程在做事情，等2個線程做完事情才會停止下來。

用法二：假設(shè)2個線程做事情,剛開始并行做事情,等一個執(zhí)行完成之后,另一個才能執(zhí)行(實際還是計數(shù))

        //程序計數(shù)器
        CountDownLatch countDownLatch = new CountDownLatch(1);
        Thread thread1 =new Thread(()->{
                System.out.println(" ---------------- 1  準備 ---------------- ");
                try {
                    countDownLatch.await();
                    System.out.println(" ---------------- 1  finsh  ---------------- ");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
        });
        thread1.start();
        Thread thread2 = new Thread(() -> {
                System.out.println(" ---------------- 2  準備 ---------------- ");
                try {
                    Thread.sleep(1_000);
                    System.out.println(" ---------------- 異步做事情  ---------------- ");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }finally {
                    countDownLatch.countDown();
                }
        });
        thread2.start();
        //main 在等main 結(jié)束  死循環(huán)
        Thread.currentThread().join();

剛開始一起在準備狀態(tài)，然后分開做事情

用法三：退出條件

中斷一個線程 count 到0

        //程序計數(shù)器
        CountDownLatch countDownLatch = new CountDownLatch(1);
        Thread thread = Thread.currentThread();
        Thread thread1 = new Thread(() -> {
            try {
             Thread.sleep(10_000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // 1 中斷條件1
		    countDownLatch.countDown();
        });
        thread1.start();
        countDownLatch.await();
        System.out.println(" ----------------- ");
    }

等待時間中斷

        //程序計數(shù)器
        CountDownLatch countDownLatch = new CountDownLatch(1);
        Thread thread = Thread.currentThread();
        Thread thread1 = new Thread(() -> {
            try {
             Thread.sleep(10_000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        thread1.start();
        //2 中斷條件3
        countDownLatch.await(5, TimeUnit.SECONDS);
        System.out.println(" ----------------- ");
    }

就中斷條件而言: 當前還可以父線程中斷

        //程序計數(shù)器
        Thread thread1 = new Thread(() -> {
            try {
             Thread.sleep(10_000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // 中斷條件3
            thread.interrupt();
        });
        thread1.start();
        System.out.println(" ----------------- ");

用法四：封裝結(jié)束完后通知事件

封裝結(jié)束完后通知事件參考 CyclicBarrier 通知

public class CountDownLatchTest4 extends CountDownLatch {
    private Runnable runnable;
    public CountDownLatchTest4(int count, Runnable runnable) {
        super(count);
        this.runnable = runnable;
    }
    @Override
    public void countDown() {
        super.countDown();
        if (super.getCount()==0){
            runnable.run();
        }
    }
    public static void main(String[] args) throws InterruptedException {
        //程序計數(shù)器
        CountDownLatchTest4 countDownLatch = new CountDownLatchTest4(1,()->{
            System.out.println(" 計數(shù)結(jié)束 .... ");
        });
        Thread thread1 = new Thread(() -> {
            try {
                Thread.sleep(2_000);
                countDownLatch.countDown();
                System.out.println(" thread 1 do something ");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Thread thread2 = new Thread(() -> {
            try {
                Thread.sleep(1_000);
                System.out.println(" thread 2 do something ");
                countDownLatch.countDown();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        thread1.start();
        thread2.start();
        countDownLatch.await();
        System.out.println("  -----------------  main 結(jié)束 ----------------- ");
    }
}

可以看到運行結(jié)束，通知事件

自定義計數(shù)器

當然我們也可以實現(xiàn)自己的計數(shù)器

/**
 * 自定義 CountDown 計數(shù)器
 */
public class CountDown {
    //計數(shù)器
    private int count = 0;
    private final int total;
    public CountDown(int total) {
        this.total = total;
    }
    public void countDown() {
        synchronized (this) {
            this.count++;
            //鎖住 ++ 通知其他線程
            this.notifyAll();
        }
    }
    public void aWait() throws InterruptedException {
        synchronized (this) {
            while (total != count) {
                //不等于 則 繼續(xù)等待
                this.wait();
            }
        }
    }
}

測試

        CountDown countDown = new CountDown( 5);
        System.out.println(" 準備多線程處理任務(wù) ");
        IntStream.rangeClosed(1, 5).forEach(x -> {
            new Thread(() -> {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(" 線程開始 -----  " + Thread.currentThread().getName());
                countDown.countDown();
            }, x + "").start();
        });
        try {
            countDown.aWait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(" 準備多線程處理任務(wù) 結(jié)束 ");
        System.out.println(" ---------------------- ");
        System.out.println(" 結(jié)束 mian ---------- ");
    }

測試結(jié)果