Android套接字已连接但无法写入

Question

I was trying to get some networking going in my app, but i encountered some issues. It seems that I cant write to the OutputStream object. Though my server recieves the connection, it does not recieve any data. I've tried using Writer, DataOutputStream among others. none seemed to work. My app uses asynctasks that call this object with a Socket object and a message. (The socket object has already been used to set Streams after initialisation using the setStreams method.) can someone please try and find the problem? I will be very thankful.

public class NetworkingUtils {
private OutputStream out = null;
private InputStream in = null;

//set streams
public void setStreams(Socket sock){
    if (sock.isConnected()) {
        try {
            this.out = (OutputStream) sock.getOutputStream();
            this.in = (InputStream) sock.getInputStream();
        } catch (Throwable e) {
            Log.d("SOCKET", "FAILED TO SET STREAMS");
            e.printStackTrace();
        }
    }
}

//send \n terminated messages to pre defined socket
public void sendMessage(Socket sock, String message) throws Throwable {
    if (sock.isConnected()) {
        try {
            this.out.write(message.getBytes());
            Log.d("SOCKET","WRITING COMPLETE. " + message);
        } catch (Throwable e) {
            throw e;
        }
    }
}

public String recvMessage(Socket sock) throws Throwable {
    //receives \n terminated message from pre defined socket
    String answer = null;
    if (sock.isConnected()){
        try{
            answer = this.convertStreamToString(this.in);
            Log.d("SOCKET","READING COMPLETE");
        }
        catch (Throwable e){
            Log.d("socket",e.getLocalizedMessage());
            throw e;
        }
    }
    else{
        Log.d("socket","is not connected!!!");
    }
    if (answer.length() == 0){
        //empty string answer from server
        throw new IOException();
    }
    else {
        return answer;
    }

}

private String convertStreamToString(java.io.InputStream is) {
    java.util.Scanner s = null;
    try{
     s = new java.util.Scanner(is).useDelimiter("\r\n");}
    catch (Throwable e){
        e.printStackTrace();
    }
    return s.hasNext() ? s.next() : "";
}

}

Answer 1

I can only see one client-side that might cause this ... and I'm doubtful about it. (That is to say: try this, just in case it makes a differences, but I don't think it will.)

  this.out.write(message.getBytes());
  Log.d("SOCKET","WRITING COMPLETE. " + message);

The potential problem is that if out is a "buffered" stream, then a write may only result in the bytes being written to the buffer. It may be necessary to call this.out.flush() to "push" to the server.

But I am doubtful it will help, because (to my knowledge) a socket output stream isn't buffered in Java. I think it is more likely that the real problem is on the server side.

If you are stumped with figuring out which side the problem is occuring, I suggest you try using a network monitoring / packet sniffing tool (on the server side) to check if the data is reaching the server host.

---

While I have your attention, your exception code is really, really bad.

1. Don't declare methods as throws Throwable (or throws Exception ). That basically says "this method may throw ANY exception, and I'm not telling you which one". When you do that, the caller code has to cope with any exception, which is basically impossible to do intelligently.

   What you should do is to declare the method as throwing the checked exceptions that the code can throw. For example, in your case, IOException is probably sufficient.

2. It is not a good idea to catch an exception, log it, and then rethrow it. Why? Because further up the stack there are probably other methods that will see the exception. They can't know if the exception has already been logged or not. So should they log it (possibly resulting in duplicate logs events for the same problem) or not (possibly resulting in the exception going unlogged.)

3. Don't throw exceptions without a message:

     throw new IOException(); 

   It is lazy. You should always include a simple message that can (at least) be grep'd or googled for.

In addition, your testing of Socket.isConnected() all over the place is unnecessary. According to the javadoc:

Returns: true if the socket was successfuly connected to a server

Note: Closing a socket doesn't clear its connection state, which means this method will return true for a closed socket (see isClosed()) if it was successfuly connected prior to being closed.

So repeatedly testing isConnected is nugatory. If it returns true once, it will will always return true from then on.

Even the initial isConnected test in setStreams is doubtful. I'd just call getInputStream without testing, and rely on the Socket API throwing an IOException if the socket is in the wrong state.

Answer 2

You're effectively reading lines with that obscure Scanner usage, but you're not writing lines. So the scanner will block until a line terminator or EOS arrives.

You need to append a line terminator when sending.