线程池源码分析（2）submit

线程池的话，其实就那么几种，前面已经分析过了。大体的流程已经清楚了。至于其他几种线程池的区别（其实区别就在于队列的放入，放出的处理不同）的话，就可以放在后面来慢慢看看区别。接下来，今天要看的是能够得到返回值的线程任务—Callable。

//先看一下平常的使用方式。
Callable job = new  YourJob();
Future<Object> future = executor.submit(job);
Object result = future.get();
//result就是结果

那么接下来就看看是如何执行的。

public <T> Future<T> submit(Callable<T> task) {
    if (task == null) throw new NullPointerException();
    //RunnableFuture 一听名字就知道实现了Runnable,future这两个接口
    RunnableFuture<T> ftask = newTaskFor(task);
    //这里就是和之前那篇文章一样的了。都是执行了
    execute(ftask);
    return ftask;
}
//突然发现,这里没什么难度了,那么接下来看一看future.get()方法
public V get() throws InterruptedException, ExecutionException {
    //代表线程的状态：正常是0,进行中是1,完成是2,取消，打断为4 5 6
    int s = state;
    if (s <= COMPLETING)
        s = awaitDone(false, 0L);
    return report(s);
}
//目的就是为了让run执行完
private int awaitDone(boolean timed, long nanos) throws InterruptedException {
    final long deadline = timed ? System.nanoTime() + nanos : 0L;
    WaitNode q = null;
    boolean queued = false;
    for (;;) {
        //检验该任务线程是否被打断了。
        if (Thread.interrupted()) {
            removeWaiter(q);
            throw new InterruptedException();
        }
        int s = state;
        if (s > COMPLETING) {
            if (q != null)
                q.thread = null;
            return s;
        }
        else if (s == COMPLETING) // cannot time out yet
            Thread.yield();
        else if (q == null)
            q = new WaitNode();
        else if (!queued)
            queued = UNSAFE.compareAndSwapObject(this, waitersOffset,q.next = waiters, q);
        else if (timed) {
            nanos = deadline - System.nanoTime();
            if (nanos <= 0L) {
                removeWaiter(q);
                return state;
            }
            LockSupport.parkNanos(this, nanos);
        }
        else
            LockSupport.park(this);
    }
}

//接下来继续看一看run()的方法
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;
            //这里可以看出run就是调用了call。
            //返回值要么是结果，要么就是异常
            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);
    }
}