CountDownLatch介绍

在项目中，我们通常会执行一些耗时操作，有的时候是同时执行若干个，然后执行完了还要等结果。通常我们会怎么做呢？Handler，或者runOnUIThread。但是有没有别的选择了呢？有的，就是CountDownLatch

先来看个例子

public class MainActivity extends Activity {

    private ArrayList result = new ArrayList<>();
    private TextView mTv1;
    private ThreadPoolExecutor executor = new ThreadPoolExecutor(10, 20, 5, TimeUnit.SECONDS, new SynchronousQueue());

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mTv1 = (TextView) findViewById(R.id.txt1);

        testCountDownLatch();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < result.size(); ++i) {
            sb.append(i + "\n");
        }
        mTv1.setText(sb.toString());
    }

    private void testCountDownLatch() {

        int threadCount = 10;

        final CountDownLatch latch = new CountDownLatch(threadCount);

        for (int i = 0; i < threadCount; i++) {

            final int index = i;

            executor.execute(new Runnable() {

                @Override
                public void run() {

                    try {
                        //模拟耗时操作
                        Thread.sleep(10000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                    // 理论上这里需要加锁
                    result.add(index);

                    latch.countDown();
                }
            });
        }

        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

主要看着一段代码：

        testCountDownLatch();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < result.size(); ++i) {
            sb.append(i + "\n");
        }
        mTv1.setText(sb.toString());

这里会把0~9都显示在textview上。
CountDownLatch的作用就是主线程在等待所有其它的子线程完成后再往下执行。它的好处就是异步写法换成了同步写法

CountDownLatch源码解析
https://www.cnblogs.com/will-shun/p/7392619.html