简单解析execute和submit有什么区别

1、execute 方法位于 java.util.concurrent.Executor 中

void execute(Runnable command);

2、execute 的具体实现

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);
  }

3、submit 方法位于 java.util.concurrent.AbstractExecutorService 中

/**
   * @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;
  }

4、submit 方式使用 Runnable 入参时的具体实现

static final class RunnableAdapter<T> implements Callable<T> {
    final Runnable task;
    final T result;
    RunnableAdapter(Runnable task, T result) {
      this.task = task;
      this.result = result;
    }
    public T call() {
      task.run();
      return result;
    }
  }

5、submit 方式使用 Callable 入参时的具体实现

public FutureTask(Callable<V> callable) {
    if (callable == null)
      throw new NullPointerException();
    this.callable = callable;
    this.state = NEW;    // ensure visibility of callable
  }

  //重写run方法
  public void run() {
    if (state != NEW ||
      !UNSAFE.compareAndSwapObject(this, runnerOffset,
                     null, Thread.currentThread()))
      return;
    try {
      Callable<V> c = callable;
      if (c != null && state == NEW) {
        V result;
        boolean ran;
        try {
          result = c.call();
          ran = true;
        } catch (Throwable ex) {
          result = null;
          ran = false;
          setException(ex);
        }
        if (ran)
          set(result);
      }
    } finally {
      // runner must be non-null until state is settled to
      // prevent concurrent calls to run()
      runner = null;
      // state must be re-read after nulling runner to prevent
      // leaked interrupts
      int s = state;
      if (s >= INTERRUPTING)
        handlePossibleCancellationInterrupt(s);
    }
  }

总结：

1、根据源码可以看到 execute 仅可以接受Runnable类型，而 submit 重载了三个方法，参数可以是 Runnable 类型、Runnable 类型+泛型T 、Callable 类型接口。

2、从上面源码可以看出 submit 方法实际上如果用Runnable类型的接口可以有返回值，也可以没有返回值。

3、传递Runnable类型接口加泛型T会被进一步封装，在 Executors 这个类里面有个内部类 RunnableAdapter 实现了 Callable 接口。

4、看submit方法可以看出，submit最终也是在调用 execute 方法，无论是 Runnable 还是 Callable 类型接口，都会被封装成 FutureTask 继续执行。

5、如果使用submit方法提交，会进一步封装成FutureTask，执行execute方法，在FutureTask里面重写的run方法里面调用 Callable 接口的call方法。

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