Question

I’m developing Server-Client application and I have a problem with waiting for input data on input stream.

I have thread dedicated to reading input data. Currently it uses while loop to hold until data is available. (N.B. protocol is as follow: send size of packet, say N, as int then send N bytes).

public void run(){
    //some initialization
    InputStream inStream = sock.getInputStream();
    byte[] packetData;
    //some more stuff
    while(!interrupted){
        while(inStream.available()==0);
        packetData = new byte[inStream.read()];
        while(inStream.available()<packetData.length);
        inStream.read(packetData,0,packetData.length);
        //send packet for procession in other thread
    }
}

It works but blocking the thread by while loop is IMO a bad idea. I could use Thread.sleep(X) to prevent resources being continously consumed by the loop, but there surely must be a better way.

Also I can not rely on InputStream.read to block the thread as part of the data may be sent by the server with delays. I have tried but it always resulted in unexpected behaviour.

I’d appreciate any ideas 🙂

Answer 1

You can use DataInputStream.readFully()

DataInputStream in = new DataInputStream(sock.getInputStream());
//some more stuff
while(!interrupted) {
    // readInt allows lengths of up to 2 GB instead of limited to 127 bytes.
    byte[] packetData = new byte[in.readInt()];
    in.readFully(packetData);
    //send packet for procession in other thread
}

I prefer to use blocking NIO which supports re-usable buffers.

SocketChannel sc = 
ByteBuffer bb = ByteBuffer.allocateDirect(1024 *1024); // off heap memory.

while(!Thread.currentThread.isInterrupted()) {
     readLength(bb, 4);
     int length = bb.getInt(0);
     if (length > bb.capacity()) 
         bb = ByteBuffer.allocateDirect(length);
     readLength(bb, length);
     bb.flip();
     // process buffer.
}



static void readLength(ByteBuffer bb, int length) throws EOFException {
     bb.clear();
     bb.limit(length);
     while(bb.remaining() > 0 && sc.read(bb) > 0);
     if (bb.remaining() > 0) throw new EOFException();
}