CountDownLatch介紹

在項目中，我們通常會執行一些耗時操作，有的時候是同時執行若干個，然後執行完了還要等結果。通常我們會怎麼做呢？Handler，或者runOnUIThread。但是有沒有別的選擇了呢？有的，就是CountDownLatch

先來看個例子

public class MainActivity extends Activity {

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

    @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 {
                        //模擬耗時操作
                        Thread.sleep(10000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                    // 理論上這裏需要加鎖
                    result.add(index);

                    latch.countDown();
                }
            });
        }

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

主要看着一段代碼：

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

這裏會把0~9都顯示在textview上。
CountDownLatch的作用就是主線程在等待所有其它的子線程完成後再往下執行。它的好處就是異步寫法換成了同步寫法

CountDownLatch源碼解析
https://www.cnblogs.com/will-shun/p/7392619.html