Runnable和Callable用法解析

Z时代
2024-01-10
分类：综合

Runnable

概述

众所周知，在Java中创建线程的2种方式是，继承thread重写run方法，实现Runnable接口。你可曾听闻有第三种方式叫做callable？先来探讨一下前两种方式是怎么实现的。Runnable接口是对异步任务的抽象，Thread对象实现了Runnable接口，并提供了多个构造方式用来创建线程，以下是Runnable接口的源码和Thread类的部分源码。

@FunctionalInterface
public interface Runnable {
    public abstract void run();
}

public class Thread implements Runnable {
    private Runnable target;
    public Thread(Runnable target) {
        init(null, target, "Thread-" + nextThreadNum(), 0);
    }
    @Override
    public void run() {
        if (target != null) {
            target.run();
        }
    }
    public synchronized void start() {
        boolean started = false;
        try {
            start0();//native method
            started = true;
        }
     }
    //忽略其余代码
}

用法示例

通过thread的源码，我们可以得知：

在构造方式传入Runnable对象，调用start()方法可以创建线程，这种方式使用了默认的run()方法，并调用target的run()方法。

    new Thread(new Runnable() {
        @Override
        public void run() {
        }
    }).start();

通过继承Thread重写run()方法，调用start()方法可以创建线程，这种方式省去了把异步代码委托在Runnable对象中。

    new Thread(){
        @Override
        public void run() {
        }
    }.start();

Callable

/**
 * A task that returns a result and may throw an exception.
 * Implementors define a single method with no arguments called
 * {@code call}.
 *
 * <p>The {@code Callable} interface is similar to {@link
 * java.lang.Runnable}, in that both are designed for classes whose
 * instances are potentially executed by another thread.  A
 * {@code Runnable}, however, does not return a result and cannot
 * throw a checked exception.
 *
 * <p>The {@link Executors} class contains utility methods to
 * convert from other common forms to {@code Callable} classes.
 *
 * @see Executor
 * @since 1.5
 * @author Doug Lea
 * @param <V> the result type of method {@code call}
 */
@FunctionalInterface
public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

概述

从源码和注释得知，Callable也是一种类似Runnable的异步任务抽象，但是可以返回结果，或抛出异常的，也属于jdk1.5版本加入的Executor任务执行模块的一部分。我们知道Runnable是配合Thread类使用的，那么Callable呢？从Thread类源码中，并没有看到接受callable接口的参数，所以callable的不会与Thread直接联系使用，答案是FutureTask，FutureTask包含传入Callable和Runnable接口的构造方法，及判断是否执行完成，取消任务等接口，get（）方法用于获取结果，并且是阻塞的，也就是说任务没有执行完成会一直等待。下面是FutureTask的继承关系和方法列表：

使用场景

FutureTask可用于异步任务获取执行结果或取消执行任务的场景。通过传入Runnable或者Callable的任务给FutureTask，直接结合Thread对象或者放入线程池执行，之后可以在外部通过FutureTask的get方法异步获取执行结果，因此，FutureTask非常适合用于耗时的计算，主线程可以在完成自己的任务后，再去获取结果。比如在需要聚合接口的场景，可以通过FutureTask并行远程调用多个接口，最后在主线程汇总结果返回给前端。

用法示例

最简单的用法

    public void test1() {
        FutureTask<String> stringFutureTask = new FutureTask<>(() -> {
            Thread.sleep(1000);
            return "200 ok";
        });
        new Thread(stringFutureTask).start();
        try {
            System.out.println("等待获取结果");
            System.out.println(stringFutureTask.get());
        } catch (ExecutionException | InterruptedException e) {
            e.printStackTrace();
        }
    }

等待获取结果 200 ok

配合线程池用法

public void test2() throws InterruptedException {
    ExecutorService executorService = Executors.newFixedThreadPool(10);
    List<Future> futureTasks = new ArrayList<>();
    for (int i = 0; i < 10; i++) {
        final int taskNum = i;
        //异步批量提交任务执行
        Future<String> task = executorService.submit(new Callable<String>() {
            @Override
            public String call() throws Exception {
                Thread.sleep(1000);//模拟任务执行
                return "200 ok" + taskNum;
            }
        });
        //保存结果钩子
        futureTasks.add(task);
        System.out.println("任务" + taskNum + "添加到队列。");
    }
    //模拟主线程执行
    Thread.sleep(2000);
    futureTasks.forEach(future -> {
        try {
            //获取异步任务结果
            System.out.println(future.get());
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }
    });
}

任务0添加到队列。任务1添加到队列。任务2添加到队列。任务3添加到队列。任务4添加到队列。任务5添加到队列。任务6添加到队列。任务7添加到队列。任务8添加到队列。任务9添加到队列。 200 ok0 200 ok1 200 ok2 200 ok3 200 ok4 200 ok5 200 ok6 200 ok7 200 ok8 200 ok9

使用CompletionService

public static void test3() throws InterruptedException, ExecutionException {
    ExecutorService executorService = Executors.newFixedThreadPool(10);
    ExecutorCompletionService<String> completionService = new ExecutorCompletionService<>(executorService);
    for (int i = 0; i < 10; i++) {
        final int taskNum = i;
        completionService.submit(() -> "200 ok" + taskNum);
    }
    for (int i = 0; i < 10; i++) {
        Future<String> take = completionService.take();
        System.out.println(take.get());
    }
}