Question

我正在Java套接字上实现一个面向事件的层，我想知道是否有办法确定是否有待读的数据。

我的常规方法是从套接字读入缓冲区，并在缓冲区填充给定量的字节时调用提供的回调（如果每次到达时都需要触发回调，则可以为0）），但我怀疑Java已经为我做了缓冲。

InputStream的available()方法是否可靠？我应该read()并在Socket上做我自己的缓冲吗？或者还有另一种方式吗？

Answer 1

很快，没有。 available()不可靠（至少不适合我）。我建议使用与java.nio.channels.SocketChannel和Selector相关联的SelectionKey。这个解决方案在某种程度上基于事件，但比普通的套接字更复杂。

对于客户：

构造套接字通道（socket），打开选择器（selector = Selector.open();）。
使用非阻止socket.configureBlocking(false);
注册连接选择器socket.register(selector, SelectionKey.OP_CONNECT);
连接socket.connect(new InetSocketAddress(host, port));
查看是否有新的selector.select();
如果“new”表示连接成功，请注册OP_READ的选择器;如果“new”指的是可用数据，只需从套接字中读取即可。

但是，为了使它具有异步性，您需要设置一个单独的线程（尽管套接字被创建为非阻塞，但线程仍会阻塞），它会检查是否有某些东西到来。

对于服务器，有ServerSocketChannel，您可以使用OP_ACCEPT。

供参考，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
  }
 }

希望这有帮助。

Answer 2

available（）只会告诉您是否可以在不进入操作系统的情况下读取数据。它在这里不是很有用。

您可以根据需要执行阻止或非阻止读取。当没有数据要读取时，只会返回非阻塞读取，这可能就是您想要的。

Java网络：即兴的Socket / InputStream

2 个答案: