Java ServerSocketChannel和SocketChannel（回调）

Question

我正在学习Java，希望能够实现一个简单的联网四子棋游戏和聊天功能。

我想要我的网络逻辑是非阻塞的，经过研究后，我发现SocketChannel符合我的需求。

然而，仍然让我感到困惑的是SocketChannels中缺少回调函数，这与C#中的实现方式不同。

我的问题是：如何将接收到的数据传递到聊天或游戏窗口（JFrame）中？

希望能够得到一些指导。

- iTEgg

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我建议您使用多线程而不是非阻塞I/O。 - Steve Emmerson

Answer 1

您需要使用选择器。首先创建一个选择器来接收事件：

Selector selector = Selector.open()

然后，您需要将ServerSocketChannel注册到选择器中：

SelectionKey acceptKey = server.register(selector, SelectionKey.OP_ACCEPT);

接下来，您需要使用选择器来处理事件（可以将其视为过程的“回调”部分）：

while(true){
  //how many channel keys are available
  int available = selector.select(); 
  //select is blocking, but should only return if available is >0, this is more of a sanity check
  if(available == 0) continue;

  Iterator<SelectionKey> keys = selector.selectedKeys().iterator();
  while(keys.hasNext()){
    SelectionKey key = keys.next();
    keys.remove();
    //someone is trying to connect to the server socket
    if(key.isAcceptable())  doAccept(key); 
    //someone is sending us data
    else if(key.isReadable()) doRead(key); 
    //we are trying to (and can) send data
    else if(key.isWritable()) doWrite(key);
}

肉在doAccept()、doRead()和doWrite()中。对于接受键，选择键将包含创建新套接字所需的信息。

doAccept(SelectionKey key){

//create the new socket
SocketChannel socket = ((ServerSocketChannel)key.channel()).accept(); 
//make it non-blocking as well
socket.configureBlocking(false);

...
//here you would likely have some code to init your game objects / communication protocol, etc. and generate an identifier object (used below).
//and be able to find the socket created above
...

//Since it is non blocking it needs a selector as well, and we register for both read and write events
SelectionKey socketKey = socket.register(selector, SelectionKey.OP_READ|SelectionKey.OP_WRITE);
// so we can identify the events as they come in
socketKey.attach(someSocketIndentifier);
}

最后一行代码向键中添加了一些对象，以便将从选择器接收到的事件归属于某个连接（例如，它可能是游戏中的玩家）。现在你可以接受新的连接，只需要读取和写入即可。

doRead(SelectionKey key){
  //here we retrieve the key we attached earlier, so we now what to do / wheer the data is coming from
  MyIdentifierType myIdentifier = (MyIdentifierType)key.attachment();
  //This is then used to get back to the SocketChannel and Read the Data
  myIdentifier.readTheData();
}

同样的，对于写入操作。

doWrite(SelectionKey key){
  //here we retrieve the key we attached earlier, so we now what to do / wheer the data is coming from
  MyIdentifierType myIdentifier = (MyIdentifierType)key.attachment();
  //This is then used to get back to the SocketChannel and Read the Data
  myIdentifier.getSocketHandler().writePendingData();
}

阅读操作相对简单，只需创建一个 ByteBuffer 对象，然后调用 SocketChannel 的 read(ByteBuffer) 方法（或其变体之一），将通道上准备好的数据读取直至为空。

写入操作有些棘手，因为通常需要缓冲要写入的数据直到接收到写入事件：

class MyNetworkClass{
  ByteBuffer writeBuffer = ByteBuffer.allocate(1024);
  SocketChannel commchannel; //from the server accept processing

  ...

  public void write(byte[] data){
    //here the class writeBuffer object is filled with the data
    //but it isn't actually sent over the socket
    ...
  }

  public void writePendingData(){
    //here actually write the data to the socket
    commchannel.write(writeBuffer);
  }
}

请注意，在类中管理缓冲区时需要适当的代码，以防它变满，或在写入挂起方法中适当地修改它，如果没有将缓冲区中的所有数据写出到套接字，则需要进行修改，还有可能在过程中抛出各种异常。希望这能帮助你入门。

- M. Jessup

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这非常有用，我现在更好地理解了这个过程。 - iTEgg