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FutureTask 略谈

FutureTask 略谈

作者: sha4yu0119 | 来源:发表于2020-01-21 15:53 被阅读0次

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<T> 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<String> callable = new Callable<String>() {
            @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<T> 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<T> 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。

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