FutureTask 略谈

在Java中，线程(或者任务？)分为两类。

• Runnable 无返回值，执行特定的逻辑。包入Thread或者线程池使用
• Callable 用户自定义返回值。与线程池结合使用。异步化的 java.util.function.Supplier

FutureTask 其实与Callable是息息相关的，在我们submit一个任务后，线程池会返回一个FutureTask实例，用于同步获取异步执行任务的返回值以及其状态。

这里我们先考虑使用Runnable自行实现一个带返回值的任务。

v1.0

public class FutureTask implements Runnable {
    private volatile T outcome;
    private final Object lock = new Object();
    private Callable callable;
    @Override
    public void run() {
        // biz operation
        synchronized (lock) {
            outcome = callable.call();
            lock.notifyAll();
        }
    }

    public T get() throws Exception {
        synchronized (lock) {
            if (outcome == null) {
                lock.wait();
            }
            return outcome;
        }
    }

    // 这里的实现非常简易 0.0 便于理解  
    public static void main(String[] args) throws Exception {
        Callable callable = new Callable() {
            @Override
            protected String call() {
                return "Hello world.";
            }
        };
        new Thread(callable).start();
        callable.get();
    }
}

这里我们发现好像有点问题，线程任务是能跑起来了，但是好像无法取消？

v2.0

那我们加上取消功能

public class Callable implements Runnable {
    private volatile T outcome;
    private final Object lock = new Object();
    private Thread runner;
    private Callable callable;
    @Override
    public void run() {
        synchronized (lock) {
            if (runner != null)
                return;
            else
                runner = Thread.currentThread();
        }
        // biz operation
        synchronized (lock) {
            outcome = callable.call();
            lock.notifyAll();
        }
    }

    public T get() throws Exception {
        synchronized (lock) {
            if (outcome == null) {
                lock.wait();
            }
            return outcome;
        }
    }

    public void cancel() {
        if (runner != null && !runner.isInterrupted() && outcome == null)
            runner.interrupt();
    }

}

现在已经支持了取消，但好像还缺少状态管理？

v3.0

public abstract class Callable implements Runnable {
    private static final int NEW = 0;
    private static final int COMPLETING = 1;
    private static final int CANCELLED = 2;

    private final Object lock = new Object();
    private int state = NEW;
    private volatile T outcome;
    private Thread runner;
    private Callable callable;

    @Override
    public void run() {
        synchronized (lock) {
            if (runner != null)
                return;
            else
                runner = Thread.currentThread();
        }
        // biz operation
        synchronized (lock) {
            outcome = callable.call();
            lock.notifyAll();
            state = COMPLETING;
        }
    }

    public int getState() {
        return state;
    }

    public T get() throws Exception {
        synchronized (lock) {
            if (outcome == null) {
                lock.wait();
            }
            return outcome;
        }
    }

    public void cancel() {
        synchronized (lock) {
            if (outcome == null && state == COMPLETING) {
                state = CANCELLED;
            }
            if (runner != null && !runner.isInterrupted() && outcome == null) {
                runner.interrupt();
            }
        }
    }

}

这里我们添加了简单的状态跟踪，状态简单分为新建，完成，取消。

不过好像还缺点东西，状态还可以细化分为正常，业务异常，中断等等，不过再细化下去便是一个FutureTask了。

现在我们对FutureTask有了个整体直观的印象。FutureTask便是JDK提供的“Callable”的官方实现。支持状态查询，运行时操作，同步获取返回值等。和我们简易版的Callable相比，FutureTask 使用Unsafe实现粒度更细的锁，细化了状态与程序报错信息等。