题目案例

已知一个业务查询操作涉及 3 个 RPC 服务调用 : query1, query2, query3, 其中query1 耗时约 1 秒， query2 耗时约 0.5 秒，query3 耗时约 0.6 秒，且query3查询条件依赖 query2 的查询结果.请编写代码，使该业务查询总体耗时最小:

考察点 1：异步
考察点 2：多线程并发控制

解题思路：
主要考察知识点异步和多线程控制。

如果采用串行执行，query1+query2+query3 总耗时为 2.1 秒。

采用多线程异步并行执行，使用线程 A 请求 query1，同时使用线程 B 请求query2后再请求 query3（query3 依赖 query2 结果只能串行执行），这样总耗时是 min(1, 0.5+0.6) = 1.1 秒。

需要熟悉 join, CountDownLatch 等线程协调控制方法，如果考生使用线程池则更佳

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public void queryTest1(){
    ThreadPoolExecutor poolExecutor = new ThreadPoolExecutor(2, 10, 60, TimeUnit.SECONDS,
            new ArrayBlockingQueue<>(5, true), new ThreadPoolExecutor.CallerRunsPolicy());
    long start = System.currentTimeMillis();

    Thread thread1 = new Thread(() -> {
        query1();
    });
    Thread thread2 = new Thread(() -> {
        query2();
    });
    Thread thread3 = new Thread(() -> {
        query3();
    });

    thread1.start();
    thread2.start();
    try {
        thread2.join();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    thread3.start();
    try {
        thread3.join();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    long time = System.currentTimeMillis() - start;
    System.out.println("耗时 " + time);
    poolExecutor.shutdown();
}


public void queryTest(){
    ThreadPoolExecutor poolExecutor = new ThreadPoolExecutor(2, 10, 60, TimeUnit.SECONDS,
            new ArrayBlockingQueue<>(5, true), new ThreadPoolExecutor.CallerRunsPolicy());
    CountDownLatch downLatch = new CountDownLatch(1);
    CountDownLatch mainLatch = new CountDownLatch(3);
    long start = System.currentTimeMillis();

    poolExecutor.execute(() -> {
        query1();
        mainLatch.countDown();
    });

    poolExecutor.execute(() -> {
        query2();
        downLatch.countDown();
        mainLatch.countDown();
    });

    poolExecutor.execute(() -> {
        try {
            downLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        query3();
        mainLatch.countDown();
    });

    try {
        mainLatch.await();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    long time = System.currentTimeMillis() - start;
    System.out.println("耗时 " + time);
    poolExecutor.shutdown();
}

public void query1(){
    System.out.println(DateUtil.date().toString("yyyy-MM-dd HH:mm:sss ") + Thread.currentThread().getName() + "query1");
    try {
        Thread.sleep(10000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}
public void query2(){
    System.out.println(DateUtil.date().toString("yyyy-MM-dd HH:mm:sss ") + Thread.currentThread().getName() + "query2");
    try {
        Thread.sleep(5000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}
public void query3(){
    System.out.println(DateUtil.date().toString("yyyy-MM-dd HH:mm:sss ") + Thread.currentThread().getName() + "query3");
    try {
        Thread.sleep(6000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}