并发编程（四）TaskFuture

并发编程（四）TaskFuture

ExecutorService executorService = Executors.newSingleThreadExecutor();
Future<Object> future = executorService.submit(() -> {
    TimeUnit.SECONDS.sleep(5);
    return 5;
});
Object result = future.get();

ExecutorService 异步执行任务返回一个 Future，本节重点分析 Future 的 get 方法是如何拿到返回结果的呢？

public <T> Future<T> submit(Callable<T> task) {
    if (task == null) throw new NullPointerException();
    RunnableFuture<T> ftask = newTaskFor(task);
    execute(ftask);
    return ftask;
}

protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
    return new FutureTask<T>(callable);
}

下面我们重点分析 FutureTask 类

(1) FutureTask 的执行状态

/*
 * FutureTask 可能的状态变化如下：
 * NEW -> COMPLETING -> NORMAL
 * NEW -> COMPLETING -> EXCEPTIONAL
 * NEW -> CANCELLED
 * NEW -> INTERRUPTING -> INTERRUPTED
 */
private volatile int state;
private static final int NEW          = 0;
private static final int COMPLETING   = 1;
private static final int NORMAL       = 2;
private static final int EXCEPTIONAL  = 3;
private static final int CANCELLED    = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED  = 6;

(2) UNSAFE

Unsafe 是 JDK 底层的类库，位于 sun.misc.Unsafe 中，在 java.util.concurrent 广泛使用。

private static final UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long stateOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("state"));

// 更新 state 状态
UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)

参考：

1. 《并发编程 Promise, Future 和 Callback》：https://ifeve.com/promise-future-callback/

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每天用心记录一点点。内容也许不重要，但习惯很重要！