Question

我有一台服务器接受具有stop()方法的客户端关闭服务器，导致我想要解决的java.nio.AsynchronousCloseException。在不同的线程上调用stop()方法，这是导致我相信的竞争条件的原因。

这是我的代码：

public void run() {
    InetSocketAddress addr = new InetSocketAddress(provider.getConnection(), 12354);
    try {
        server = ServerSocketChannel.open();
        server.configureBlocking(true);
        server.socket().bind(addr);
        parent.setIP(addr.getAddress().getHostAddress().toString());
        password = generatePassword();
        parent.setPassword(password);
        parent.setStatus("Ready.");
    } catch (IOException e) {
        parent.die("Could not start server: " + e.getMessage());
        runner = null;
    }
    while (runner == Thread.currentThread()) {
        try {
            SocketChannel sc = server.accept();
            if (available) {
                session = new ReceiveSession(this, sc, password, addr.getAddress());
                session.start();
                available = false;
            } else {
                new ReceiveBusyHandler(sc).start();
            }
        } catch (IOException e) {
            synchronized (swallowException) {
                if (!swallowException) {
                    parent.showError(e.toString());
                }
                available = true;
            }
        }
    }
}

public void stop() throws IOException {
    synchronized (swallowException) {
        swallowException = true;
        runner = null;
        if (server != null) {
            server.socket().close();
            server.close();
        }

        swallowException = false;
        System.out.println("Server down");
    }
}

（仅供参考，swallowException是Boolean，你可以看到我尝试过同步它。）

看起来stop()方法正在将swallowException设置为true，然后在我的服务器循环中的异常处理程序有机会访问它之前返回false。

更新：我介绍了一个新的Object用作锁定，并使用wait()/notify()来解决我的问题：

public void run() {
        InetSocketAddress addr = new InetSocketAddress(provider.getConnection(), 12354);
        try {
            server = ServerSocketChannel.open();
            server.configureBlocking(true);
            server.socket().bind(addr);
            parent.setIP(addr.getAddress().getHostAddress().toString());
            password = generatePassword();
            parent.setPassword(password);
            parent.setStatus("Ready.");
        } catch (IOException e) {
            parent.die("Could not start server: " + e.getMessage());
            runner = null;
        }
        while (runner == Thread.currentThread()) {
            try {
                SocketChannel sc = server.accept();
                if (available) {
                    session = new ReceiveSession(this, sc, password, addr.getAddress());
                    session.start();
                    available = false;
                } else {
                    new ReceiveBusyHandler(sc).start();
                }
            } catch (IOException e) {
                synchronized (lock) {
                    if (!swallowException) {
                        parent.showError(e.toString());

                    }
                    lock.notify();
                    available = true;
                }
            }
        }
    }

    public void stop() throws IOException {
        synchronized (lock) {
            swallowException = true;
            runner = null;
            if (server != null) {
                server.socket().close();
                server.close();
            }
            while (swallowException) {
                try {
                    lock.wait();
                    swallowException = false;
                } catch (InterruptedException e) {
                }
            }
            //swallowException = false;
            System.out.println("Server down");
        }
    }

Answer 1

此部分未正确同步：

synchronized (swallowException) {
    swallowException = true;

您正在同步一个实例（false）并立即更改swallowException 引用以指向其他实例（true）。输入stop的下一个帖子不会阻止。

要么在不会被换出的实例上同步（这些方法的所有者），要么使用java.util.concurrent中的其他锁定机制。

Answer 2

在Java中，同步是在对象上完成的，而不是在变量上完成的。在swallowException上同步时，您会同步其值（Boolean.TRUE或Boolean.FALSE）。这不是你想要的。您应该同步包含swallowException的对象。

Answer 3

我强烈建议您重构代码（甚至是您作为更新发布的解决方案），因为它不清楚发生了什么。

从您的说明，您似乎只想要一种线程安全的方法来停止您的服务器。我建议您这样做，只需call close() on the ServerSocket，就可以抓住SocketException：

private boolean cont = true;

// this can safely be called from any thread
public synchronized void stop() {
    cont = false;
    if (server != null) {
       server.socket().close();
    }
}
private synchronized void setContinue(boolean value) {
    cont = value;
}
private synchronized boolean shouldContinue() {
    return cont;
}
private synchronized void openChannel() {
    server = ServerSocketChannel.open();
}

public void run() {
    InetSocketAddress addr = new InetSocketAddress(provider.getConnection(), 12354);
    try {
        openChannel();
        server.configureBlocking(true);
        server.socket().bind(addr);
        parent.setIP(addr.getAddress().getHostAddress().toString());
        password = generatePassword();
        parent.setPassword(password);
        parent.setStatus("Ready.");
    } catch (IOException e) {
        parent.die("Could not start server: " + e.getMessage());
        setContinue(false);
    }

    while (shouldContinue()) {
        try {
            SocketChannel sc = server.accept();
            if (shouldContinue()) {
                if (available) {
                    session = new ReceiveSession(this, sc, password, addr.getAddress());
                    session.start();
                    available = false;
                } else {
                    new ReceiveBusyHandler(sc).start();
                }
            }
        } catch (SocketException se) {
            // normal shutdown from stop()
        } catch (IOException e) {
            parent.showError(e.toString()); 
            available = true;               
        }
    }
    System.out.println("Server down");
}

See more about this technique for stopping a server here

我如何解决这种竞争状况？

3 个答案: