问题描述

在《Java并发编程实战》第7章，作者通过newTaskFor方法封装实现了线程的取消。给出的demo如下：

public abstract class SocketUsingTask <T> implements CancellableTask<T> {
    @GuardedBy("this") private Socket socket;

    protected synchronized void setSocket(Socket s) {
        socket = s;
    }

    public synchronized void cancel() {
        try {
            if (socket != null)
                socket.close();
        } catch (IOException ignored) {
        }
    }

    public RunnableFuture<T> newTask() {
        return new FutureTask<T>(this) {
            public boolean cancel(boolean mayInterruptIfRunning) {
                try {
                    SocketUsingTask.this.cancel();
                } finally {
                    return super.cancel(mayInterruptIfRunning);
                }
            }
        };
    }
}


interface CancellableTask <T> extends Callable<T> {
    void cancel();

    RunnableFuture<T> newTask();
}


@ThreadSafe
class CancellingExecutor extends ThreadPoolExecutor {
    public CancellingExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
    }

    public CancellingExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory);
    }

    public CancellingExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, handler);
    }

    public CancellingExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, handler);
    }

    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        if (callable instanceof CancellableTask)
            return ((CancellableTask<T>) callable).newTask();
        else
            return super.newTaskFor(callable);
    }
}

我觉得这段代码有点绕，感觉很不直白，我个人感觉可以换一种方法实现：

public abstract class SocketUsingTask<T> extends FutureTask<T> implements Callable<T> {
    @GuardedBy("this") private Socket socket;
    
    protected synchronized void setSocket(Socket s) { socket = s; }
    
    public synchronized void cancel() {
        try {
            if (socket != null)
                socket.close();
        } catch (IOException ignored) { }
    }

    @Override
    public boolean cancel(boolean mayInterruptIfRunning) {
        try {
            cancel();
        } finally {
            return super.cancel(mayInterruptIfRunning);
        }
    }
}

我想知道《Java并发编程实战》中给出的这个demo的最大的优点在哪？为什么要这样实现？因为感觉存在一些多余的操作。谢谢