CompletableFuture 创建异步任务

CompletableFuture实现了CompletionStage接口和Future接口，前者是对后者的一个扩展，增加了异步回调、流式处理、多个Future组合处理的能力，使Java在处理多任务的协同工作时更加顺畅便利。

submit

通常的线程池接口类ExecutorService，其中execute方法的返回值是void，即无法获取异步任务的执行状态，3个重载的submit方法的返回值是Future，可以据此获取任务执行的状态和结果，示例如下：

@Test
public void test3() throws Exception {
    // 创建异步执行任务:
    ExecutorService executorService= Executors.newSingleThreadExecutor();
    Future<Double> cf = executorService.submit(()->{
        System.out.println(Thread.currentThread()+" start,time->"+System.currentTimeMillis());
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }
        if(false){
            throw new RuntimeException("test");
        }else{
            System.out.println(Thread.currentThread()+" exit,time->"+System.currentTimeMillis());
            return 1.2;
        }
    });
    System.out.println("main thread start,time->"+System.currentTimeMillis());
    //等待子任务执行完成,如果已完成则直接返回结果
    //如果执行任务异常，则get方法会把之前捕获的异常重新抛出
    System.out.println("run result->"+cf.get());
    System.out.println("main thread exit,time->"+System.currentTimeMillis());
}

子线程是异步执行的，主线程休眠等待子线程执行完成，子线程执行完成后唤醒主线程，主线程获取任务执行结果后退出。

很多博客说使用不带等待时间限制的get方法时，如果子线程执行异常了会导致主线程长期阻塞，这其实是错误的，子线程执行异常时其异常会被捕获，然后修改任务的状态为异常结束并唤醒等待的主线程，get方法判断任务状态发生变更，就终止等待了，并抛出异常。get方法抛出异常导致主线程异常终止。

supplyAsync / runAsync

supplyAsync表示创建带返回值的异步任务的，相当于ExecutorService submit(Callable task) 方法，runAsync表示创建无返回值的异步任务，相当于ExecutorService submit(Runnable task)方法，这两方法的效果跟submit是一样的，测试用例如下：

@Test
public void test2() throws Exception {
    // 创建异步执行任务，有返回值
    CompletableFuture<Double> cf = CompletableFuture.supplyAsync(()->{
        System.out.println(Thread.currentThread()+" start,time->"+System.currentTimeMillis());
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }
        if(true){
            throw new RuntimeException("test");
        }else{
            System.out.println(Thread.currentThread()+" exit,time->"+System.currentTimeMillis());
            return 1.2;
        }
    });
    System.out.println("main thread start,time->"+System.currentTimeMillis());
    //等待子任务执行完成
    System.out.println("run result->"+cf.get());
    System.out.println("main thread exit,time->"+System.currentTimeMillis());
}

@Test
public void test4() throws Exception {
    // 创建异步执行任务，无返回值
    CompletableFuture cf = CompletableFuture.runAsync(()->{
        System.out.println(Thread.currentThread()+" start,time->"+System.currentTimeMillis());
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }
        if(false){
            throw new RuntimeException("test");
        }else{
            System.out.println(Thread.currentThread()+" exit,time->"+System.currentTimeMillis());
        }
    });
    System.out.println("main thread start,time->"+System.currentTimeMillis());
    //等待子任务执行完成
    System.out.println("run result->"+cf.get());
    System.out.println("main thread exit,time->"+System.currentTimeMillis());
}

这两方法各有一个重载版本，可以指定执行异步任务的Executor实现，如果不指定，默认使用ForkJoinPool.commonPool()，如果机器是单核的，则默认使用ThreadPerTaskExecutor，该类是一个内部类，每次执行execute都会创建一个新线程。测试用例如下：

@Test
public void test2() throws Exception {
    ForkJoinPool pool=new ForkJoinPool();
    // 创建异步执行任务:
    CompletableFuture<Double> cf = CompletableFuture.supplyAsync(()->{
        System.out.println(Thread.currentThread()+" start,time->"+System.currentTimeMillis());
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }
        if(true){
            throw new RuntimeException("test");
        }else{
            System.out.println(Thread.currentThread()+" exit,time->"+System.currentTimeMillis());
            return 1.2;
        }
    },pool);
    System.out.println("main thread start,time->"+System.currentTimeMillis());
    //等待子任务执行完成
    System.out.println("run result->"+cf.get());
    System.out.println("main thread exit,time->"+System.currentTimeMillis());
}

@Test
public void test4() throws Exception {
    ExecutorService executorService= Executors.newSingleThreadExecutor();
    // 创建异步执行任务:
    CompletableFuture cf = CompletableFuture.runAsync(()->{
        System.out.println(Thread.currentThread()+" start,time->"+System.currentTimeMillis());
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }
        if(false){
            throw new RuntimeException("test");
        }else{
            System.out.println(Thread.currentThread()+" exit,time->"+System.currentTimeMillis());
        }
    },executorService);
    System.out.println("main thread start,time->"+System.currentTimeMillis());
    //等待子任务执行完成
    System.out.println("run result->"+cf.get());
    System.out.println("main thread exit,time->"+System.currentTimeMillis());
}