Java 网络 : evented Socket/InputStream

Question

我正在 Java 的套接字上实现一个面向事件的层，我想知道是否有一种方法可以确定是否有待读取的数据。

我通常的方法是从套接字读取到缓冲区，并在缓冲区填充给定字节数时调用提供的回调(如果每次到达时都需要触发回调，则可以为 0 )，但我怀疑 Java 已经在为我做缓冲。

InputStream 的 available() 方法是否可靠？我应该只 read() 并在 Socket 之上做我自己的缓冲吗？还是有别的办法？

Answer 1

简而言之，不。 available() 不可靠(至少不适合我)。我推荐使用 java.nio.channels.SocketChannel 连接 Selector 和 SelectionKey。这个解决方案有点基于事件，但比普通套接字更复杂。

对于客户:

1. 构造套接字 channel (socket)，打开一个选择器(selector = Selector.open();)。
2. 使用非阻塞socket.configureBlocking(false);
3. 为连接注册选择器socket.register(selector, SelectionKey.OP_CONNECT);
4. 连接 socket.connect(new InetSocketAddress(host, port));
5. 看看有没有新的selector.select();
6. 如果“new”是指连接成功，则为OP_READ注册选择器；如果"new"指的是可用数据，则直接从套接字中读取。

但是，为了使其异步，您需要设置一个单独的线程(尽管套接字被创建为非阻塞，线程无论如何都会阻塞)来检查是否有东西到达。

对于服务器，有 ServerSocketChannel，您可以为它使用 OP_ACCEPT。

供引用，这是我的代码(客户端)，应该给你一个提示:

 private Thread readingThread = new ListeningThread();

 /**
  * Listening thread - reads messages in a separate thread so the application does not get blocked.
  */
 private class ListeningThread extends Thread {
  public void run() {
   running = true;
   try {
    while(!close) listen();
    messenger.close();
   }
   catch(ConnectException ce) {
    doNotifyConnectionFailed(ce);
   }
   catch(Exception e) {
//    e.printStackTrace();
    messenger.close();
   }
   running = false;
  }
 }

 /**
  * Connects to host and port.
  * @param host Host to connect to.
  * @param port Port of the host machine to connect to.
  */
 public void connect(String host, int port) {
  try {
   SocketChannel socket = SocketChannel.open();
   socket.configureBlocking(false);
   socket.register(this.selector, SelectionKey.OP_CONNECT);
   socket.connect(new InetSocketAddress(host, port));
  }
  catch(IOException e) {
   this.doNotifyConnectionFailed(e);
  }
 }

 /**
  * Waits for an event to happen, processes it and then returns.
  * @throws IOException when something goes wrong.
  */
 protected void listen() throws IOException {
  // see if there are any new things going on
  this.selector.select();
  // process events
  Iterator<SelectionKey> iter = selector.selectedKeys().iterator();
  while(iter.hasNext()) {
   SelectionKey key = iter.next();
   iter.remove();
   // check validity
   if(key.isValid()) {
    // if connectable...
    if(key.isConnectable()) {
     // ...establish connection, make messenger, and notify everyone
     SocketChannel client = (SocketChannel)key.channel();
     // now this is tricky, registering for OP_READ earlier causes the selector not to wait for incoming bytes, which results in 100% cpu usage very, very fast
     if(client!=null && client.finishConnect()) {
      client.register(this.selector, SelectionKey.OP_READ);
     }
    }
    // if readable, tell messenger to read bytes
    else if(key.isReadable() && (SocketChannel)key.channel()==this.messenger.getSocket()) {
     // read message here
    }
   }
  }
 }

 /**
  * Starts the client.
  */
 public void start() {
  // start a reading thread
  if(!this.running) {
   this.readingThread = new ListeningThread();
   this.readingThread.start();
  }
 }

 /**
  * Tells the client to close at nearest possible moment.
  */
 public void close() {
  this.close = true;
 }

对于服务器:

 /**
  * Constructs a server.
  * @param port Port to listen to.
  * @param protocol Protocol of messages.
  * @throws IOException when something goes wrong.
  */
 public ChannelMessageServer(int port) throws IOException {
  this.server = ServerSocketChannel.open();
  this.server.configureBlocking(false);
  this.server.socket().bind(new InetSocketAddress(port));
  this.server.register(this.selector, SelectionKey.OP_ACCEPT);
 }

 /**
  * Waits for event, then exits.
  * @throws IOException when something goes wrong.
  */
 protected void listen() throws IOException {
  // see if there are any new things going on
  this.selector.select();
  // process events
  Iterator<SelectionKey> iter = selector.selectedKeys().iterator();
  while(iter.hasNext()) {
   SelectionKey key = iter.next();
   // do something with the connected socket
   iter.remove();
   if(key.isValid()) this.process(key);
  }
 }

 /**
  * Processes a selection key.
  * @param key SelectionKey.
  * @throws IOException when something is wrong.
  */
 protected void process(SelectionKey key) throws IOException {
  // if incoming connection
  if(key.isAcceptable()) {
   // get client
   SocketChannel client = (((ServerSocketChannel)key.channel()).accept());
    try {
     client.configureBlocking(false);
     client.register(this.selector, SelectionKey.OP_READ);
    }
    catch(Exception e) {
     // catch
    }
  }
  // if readable, tell messenger to read
  else if(key.isReadable()) {
  // read
  }
 }

希望这对您有所帮助。