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Practical Session 11. Multi Client-Server Java NIO. Supporting Multiple Clients. The basic flow of logic in such a server is this: while (true) { accept a connection; create a thread to deal with the client; }

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practical session 11

Practical Session 11

Multi Client-Server

Java NIO

supporting multiple clients
Supporting Multiple Clients
  • The basic flow of logic in such a server is this:

while (true) {

accept a connection;

create a thread to deal with the client;


  • The thread reads from and writes to the client connection as necessary.
the server interface
The Server Interface
  • interface ServerProtocol:
    • String processMessage(String msg);          
    • booleanisEnd(String msg);
  • In order to organize the work of the Server, and to allow several protocols, we create an interface:
    • processMessage: a function that decides what to do with the content received.
    • isEnd: returns true if the message equals our ‘exit’ command.
  • In order to allow more than one client to connect to our server, we need to run each connection in its own thread.
  • ConnectionHandler is a Runnable that handles one connection.
  • Each client that wishes to connect to our server initiates the connection.
  • The server accepts the connection from the client.
  • The socket returned is sent to the ConnectionHandler object.
  • ConnectionHandler is run as a Thread.
  • Server goes back to blocking mode, waiting for a new connection.
multiclient server implementation
MultiClient – Server Implementation
  • ServerProtocol  [Interface]
    • processMessage
    • isEnd
  • EchoProtocol implements ServerProtocol
    • processMessage: message received is returned
    • isEnd: returns true if message is ‘bye’
  • ConnectionHandler[Runnable]
    • Receives messages from client [msg = in.readLine()]
    • Processes message [using process()] using EchoProtocol
      • If message is ‘bye’, exits thread [protocol.isEnd(msg)]
      • Else, message is sent back to client [out.println(response)]
    • Sends returned result from processing to client.
  • MultipleClientProtocolServer [Runnable]
    • Our server – runs as a thread
    • Creates ServerSocket, listens to a port
    • Runs ConnectionHandler in a thread once accept() returns a socket.

java non blocking io
Java Non-blocking IO
  • In our examples, the server gets stuck on
    • msg = in.readLine()
    • clientSocket = serverSocket.accept()
    • out.println(msg) //writing faster than the other side can read
  • You cannot do something else while these methods are blocking. (process client messages, handle other clients etc.).
java non blocking io1
Java Non-blocking IO
  • Solution? java.nio.
  • The package is an efficient InputOutput package, which supports Non-blocking IO.
  • NIO Concepts:
    • Channels
    • Buffers
    • Selectors
  • Tutorial:
channels our sockets
Channels [our sockets]
  • An Object you can read from and write to.
  • Channels can be either blocking (by default) or non-blocking.
  • SocketChannel:
    • Same as regular Socket object.
    • Difference: read(), write() can be non-blocking.
  • ServerSocketChannel:
    • accept() returns SocketChannel
    • Same as regular ServerSocket object.
    • Difference: accept() can be non-blocking.
  • Does not block until a client connects.
  • Checks if a client is trying to connect, if so returns a new socket, otherwise returns null!
setting up serversocketchannel and socketchannel
Setting up ServerSocketChannel and SocketChannel
  • ServerSocketChannel :
    • int port = 9999;
    • ServerSocketChannel ssChannel =;
    • ssChannel.configureBlocking(false);
    • ssChannel.socket().bind(new InetSocketAddress(port));
  • SocketChannel:
    • SocketChannel;
    • sChannel.connect(new InetSocketAddress("host/ip", 9999));
    • sChannel.configureBlocking(false);

buffers our containers
Buffers [our containers]
  • The objects which hold the data to be sent and data received.
  • Channels know how to read and write into Buffers, and buffers can read and write into other buffers.
  • Java NIO comes with the following Buffer types:
    • ByteBuffer
    • CharBuffer
    • DoubleBuffer
    • FloatBuffer
    • IntBuffer
    • LongBuffer
    • ShortBuffer

creating buffers
Creating Buffers
  • We'll be using ByteBuffer.
  • These are buffers that hold bytes.
  • Creating a new ByteBuffer: [Method I]
    • final int NUM_OF_BYTES = 1024;
    • ByteBuffer buffer = ByteBuffer.allocate(NUM_OF_BYTES);
  • Creating a new ByteBuffer: [Method II]
    • String message = “Sentence to write into buffer”;
    • ByteBuffer buffer = ByteBuffer.wrap(message.getBytes(),”UTF-8”);
buffer markers
Buffer Markers
  • Each buffer has capacity, limit, and position markers.
  • Capacity:
    • Being a memory block, a Buffer has a certain fixed size, also called its "capacity".
    • You can only write capacity bytes, longs, chars etc. into the Buffer.
    • Once the Buffer is full, you need to empty it (read the data, or clear it) before you can write more data into it.
  • Position:
    • Writing data to buffer:
      • Initially the position is 0.
      • When a byte, long etc. has been written into the Buffer the position is advanced to point to the next cell in the buffer to insert data into.
      • Position can maximally become capacity - 1.
    • Reading data from buffer:
      • When you flip a Buffer from writing mode to reading mode, the position is reset to 0.
      • As you read data from the Buffer you do so from position, and position is advanced to next position to read.
  • Limit:
    • In write mode:
      • The limit of a Buffer is the limit of how much data you can write into the buffer.
      • The limit is equal to the capacity of the Buffer.
    • When flipping the Buffer into read mode:
      • The limit means the limit of how much data you can read from the buffer.
      • When flipping a Buffer into read mode, limit is set to write position of the write mode.
      • In other words, you can read as many bytes as were written (limit is set to the number of bytes written, which is marked by position).
buffer flipping
Buffer Flipping
  • The flip() method switches a Buffer from writing mode to reading mode.
  • Calling flip() sets the position back to 0, and sets the limit to where position just was.
  • The position marker now marks the reading position, and limit marks how many bytes were written into the buffer - the limit of how many bytes, chars etc. that can be read.
  • Example:
    • You create a ByteBuffer.
    • Write data into the buffer.
    • Flip()
    • Send the buffer to the channel.
buffer io operations
Buffer IO Operations
  • Readingfrom a channel to a buffer:

numBytesRead =;

    • Contents found in socketChannel are read from their internal container object to our buffer.
  • Writingfrom a buffer to a channel:

numBytesWritten = socketChannel.write(buf);

    • Contents from our buf object are written to the socketChannel’s internal container to be sent.
  • If read or write returns -1, it means that the channel is closed.
  • Read and write operations on Buffers update the position marker accordingly.
more bytebuffer methods
More ByteBuffer Methods
  • clear():
    • Makes a buffer ready for a new sequence of channel-read or relative put operations.
    • Sets the limit to the capacity .
    • Sets the position to zero.
  • rewind():
    • Makes a buffer ready for re-reading the data that it already contains.
    • Leaves the limit unchanged.
    • Sets the position to zero.
stringmessagetokenizer interface
StringMessageTokenizer Interface
  • void addBytes(ByteBuffer bytes);
    • Receives a Buffer of bytes containing data to be converted to chars.
  • booleanhasMessage();
      • Is there a complete message ready?
  • String nextMessage();
    • Get the next complete message if it exists, advancing the tokenizer to the next message.
  • FixedSeparatorMessageTokenizer(String separator, Charset charset)
    • The constructor.
    • Between two messages in the buffer we have a separator.
    • Messages are encoded [chars to bytes] and decoded [bytes to chars] using the given charset [ASCII, UTF-8]
  • public synchronized void addBytes(ByteBuffer bytes):
    • Array of bytes received is converted to string and concatenated to the ones before it.
  • public synchronized booleanhasMessage():
    • Checks if the buffer has a complete message, if so true, otherwise false. [done by checking if the separator exists in the string array]
  • public synchronized String nextMessage()
    • Returns the next complete message in the buffer.
nio echo client
NIO Echo Client
c echo client
C++ Echo Client
  • Connect a socket to a host and port:

boost::asio::ip::tcp::socket socket_;

boost::asio::ip::tcp::endpoint endpoint(boost::asio::ip::address::from_string(host_), port_);

boost::system::error_code error;

socket_.connect(endpoint, error);

  • Read from socket:

boost::asio::buffer(startingPointPointer, SizeToReadInBytes, exception) //exception is optional

//tmp holds the number of bytes read so far; bytes: an array of chars to read the received bytes from

socket_.read_some(boost::asio::buffer(bytes+tmp, bytesToRead-tmp), error)

  • Write to socket:

socket_.write_some(boost::asio::buffer(bytes + tmp, bytesToWrite - tmp), error)

  • Example:
    • 05_Boost_Echo_Client