java - FutureTask的两种用法有什么区别？

Question

使用线程池:

ExecutorService exec = Executors.newFixedThreadPool(5);
FutureTask<Integer> ft = new FutureTask<Integer>(() -> rpcMethod());
exec.submit(ft);

调用run方法:

FutureTask<Integer> ft = new FutureTask<Integer>(() -> rpcMethod());
ft.run()

我看了一下run方法源码，好像是同步调用，好像没有新建线程。之所以出现这个问题，是因为我查看了Dubbo的RpcContext#asyncCall的源码，发现它是直接使用FutureTask#run的。下面是部分代码。

public <T> Future<T> asyncCall(Callable<T> callable) {
        try {
            try {
                setAttachment(Constants.ASYNC_KEY, Boolean.TRUE.toString());
                final T o = callable.call();
                //local调用会直接返回结果.
                if (o != null) {
                    FutureTask<T> f = new FutureTask<T>(new Callable<T>() {
                        public T call() throws Exception {
                            return o;
                        }
                    });
                    f.run();
                    return f;
                } else {

                }

Answer 1

FutureTask 是 Runnable 的实现。

区别是

如果您使用exec.submit(ft)；它的异步，在两个不同的线程中执行。

如果你使用ft.run()；它是在同一个线程中执行的。

如果有任何疑问，请尝试此操作。我将 getResult() 放在另一个方法中，这样您就有了更明确的调用堆栈:

public class FutureExample {

private Callable<Long> call = new Callable<Long>() {

    private Long getResult(){
        return Thread.currentThread().getId();
    }

    @Override
    public Long call() {
        return getResult();
    }
};

private Long callByRun() throws ExecutionException, InterruptedException {
    FutureTask<Long> ft = new FutureTask<>(call);
    ft.run();
    return ft.get();
}

private Long callByExec() throws ExecutionException, InterruptedException {
    ExecutorService pool = Executors.newFixedThreadPool(5);
    FutureTask<Long> ft = new FutureTask<>(call);
    pool.submit(ft);
    Long result = ft.get();
    pool.shutdown();
    return result;
}

public static void main(String[] args) throws ExecutionException, InterruptedException {
    Long mainThreadId = Thread.currentThread().getId();
    System.out.println(String.format("main: %d callByRun %d", mainThreadId, new FutureExample().callByRun()));
    System.out.println(String.format("main: %d callByExec %d", mainThreadId, new FutureExample().callByExec()));
}

}