线程池 execute 和 submit 的区别

代码示例：

public class ThreadPool_Test {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        ExecutorService pool = Executors.newCachedThreadPool();
        pool.execute(new MyRunner());
        
        Future<String> future = pool.submit(new MyCaller());
        String ret = future.get();
        System.out.println(ret);
    }
}

class MyCaller implements Callable<String> {

    @Override
    public String call() throws Exception {
        System.out.println("calling");
        return "return_from_call";
    }
    
}

class MyRunner implements Runnable {
    @Override
    public void run() {
        System.out.println("running");
    }
    
}

execute 方法执行 runnable 任务，submit 方法执行 callable 任务，callable 任务有返回值，而 runnable 任务是 void 的，无返回值。

// void java.util.concurrent.ThreadPoolExecutor
final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        while (task != null || (task = getTask()) != null) {
            w.lock();
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    task.run();
                } catch (RuntimeException x) {
                    thrown = x; throw x;
                } catch (Error x) {
                    thrown = x; throw x;
                } catch (Throwable x) {
                    thrown = x; throw new Error(x);
                } finally {
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

对于 Runnable，task 是 MyRunner，对于 Callable，task 是 FutureTask。

submit 方法的调用栈：

创建 FutureTask 对象，把 Callable 对象包裹起来，在 run 方法中调用 Callable 对象的方法，并设置返回值。

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

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);
    }
}