ThreadPoolExecutor线程池submit() 和 excute()区别

旧城等待， 2022-03-14 11:26 381阅读 0赞

在研究ThreadPoolExecutor线程池的时候，发现可以有两种启动线程的方法：submit(Runnable runnable)，excute(Runnable runnable)   
先说个结论吧：

submit()方法，可以提供Future < T > 类型的返回值。   
excute()方法，无返回值。

首先，我们看一下ThreadPoolExecutor的类图：

ThreadPoolExecutor的抽象父类AbstractExecutorService实现了接口ExecutorService和Executor，而submit方法是ExecutorService中的接口，executor方法是Executor中的接口。

查看AbstractExecutorService源码，发现submit方法是在此抽象父类中实现：

/**
         * @throws RejectedExecutionException {@inheritDoc}
         * @throws NullPointerException       {@inheritDoc}
         */
        public Future<?> submit(Runnable task) {
            if (task == null) throw new NullPointerException();
            RunnableFuture<Void> ftask = newTaskFor(task, null);
            execute(ftask);
            return ftask;
        }
    
        /**
         * @throws RejectedExecutionException {@inheritDoc}
         * @throws NullPointerException       {@inheritDoc}
         */
        public <T> Future<T> submit(Runnable task, T result) {
            if (task == null) throw new NullPointerException();
            RunnableFuture<T> ftask = newTaskFor(task, result);
            execute(ftask);
            return ftask;
        }
    
        /**
         * @throws RejectedExecutionException {@inheritDoc}
         * @throws NullPointerException       {@inheritDoc}
         */
        public <T> Future<T> submit(Callable<T> task) {
            if (task == null) throw new NullPointerException();
            RunnableFuture<T> ftask = newTaskFor(task);
            execute(ftask);
            return ftask;
        }

可以看到，submit方法是先构造出一个RunnableFuture(FutureTask) 然后调用execute方法。不管你submit的时候传入的是Runnable还是Callable最后RunnableFuture(FutureTask)里面都会生成Callable对象。任务调用的时候调用RunnableFuture(FutureTask)的run方法，run方法调用Callable对象的call方法。

submit()方法，可以提供Future < T > 类型的返回值。

ThreadPoolExecutor中的execute方法：

public void execute(Runnable command) {
            if (command == null)
                throw new NullPointerException();
            /*
             * Proceed in 3 steps:
             *
             * 1. If fewer than corePoolSize threads are running, try to
             * start a new thread with the given command as its first
             * task.  The call to addWorker atomically checks runState and
             * workerCount, and so prevents false alarms that would add
             * threads when it shouldn't, by returning false.
             *
             * 2. If a task can be successfully queued, then we still need
             * to double-check whether we should have added a thread
             * (because existing ones died since last checking) or that
             * the pool shut down since entry into this method. So we
             * recheck state and if necessary roll back the enqueuing if
             * stopped, or start a new thread if there are none.
             *
             * 3. If we cannot queue task, then we try to add a new
             * thread.  If it fails, we know we are shut down or saturated
             * and so reject the task.
             */
            int c = ctl.get();
            if (workerCountOf(c) < corePoolSize) {
                if (addWorker(command, true))
                    return;
                c = ctl.get();
            }
            if (isRunning(c) && workQueue.offer(command)) {
                int recheck = ctl.get();
                if (! isRunning(recheck) && remove(command))
                    reject(command);
                else if (workerCountOf(recheck) == 0)
                    addWorker(null, false);
            }
            else if (!addWorker(command, false))
                reject(command);
        }

excute()方法，无返回值。