Java NIO server socket channel is again a selectable type channel used for stream oriented data flow connecting sockets.Server Socket channel can be created by invoking its static open() method,providing any pre-existing socket is not already present.Server Socket channel is created by invoking open method but not yet bound.In order to bound socket channel bind() method is to be called.

One point to be mentioned here is if channel is not bound and any I/O operation is tried to be attempted then NotYetBoundException is thrown by this channel.So one must be ensure that channel is bounded before performing any IO operation.

Incoming connections for the server socket channel are listen by calling the ServerSocketChannel.accept() method. When the accept() method returns, it returns a SocketChannel with an incoming connection. Thus, the accept() method blocks until an incoming connection arrives.If the channel is in non-blocking mode then accept method will immediately return null if there are no pending connections. Otherwise it will block indefinitely until a new connection is available or an I/O error occurs.

The new channel”s socket is initially unbound; it must be bound to a specific address via one of its socket”s bind methods before connections can be accepted.Also the new channel is created by invoking the openServerSocketChannel method of the system-wide default SelectorProvider object.

Like socket channel server socket channel could read data using read() method.Firstly the buffer is allocated. The data read from a ServerSocketChannel is stored into the buffer.Secondly we call the ServerSocketChannel.read() method and it reads the data from a ServerSocketChannel into a buffer. The integer value of the read() method returns how many bytes were written into the buffer

Similarly data could be written to server socket channel using write() method using buffer as a parameter.Commonly uses write method in a while loop as need to repeat the write() method until the Buffer has no further bytes available to write.

Important methods of Socket channel

• bind(SocketAddress local) − This method is used to bind the socket channel to the local address which is provided as the parameter to this method.

• accept() − This method is used to accepts a connection made to this channel”s socket.

• connect(SocketAddress remote) − This method is used to connect the socket to the remote address.

• finishConnect() − This method is used to finishes the process of connecting a socket channel.

• getRemoteAddress() − This method return the address of remote location to which the channel”s socket is connected.

• isConnected() − As already mentioned this method returns the status of connection of socket channel i.e whether it is connected or not.

• open() − Open method is used open a socket channel for no specified address.This convenience method works as if by invoking the open() method, invoking the connect method upon the resulting server socket channel, passing it remote, and then returning that channel.

• read(ByteBuffer dst) − This method is used to read data from the given buffer through socket channel.

• setOption(SocketOption<T> name, T value) − This method sets the value of a socket option.

• socket() − This method retrieves a server socket associated with this channel.

• validOps() − This method returns an operation set identifying this channel”s supported operations.Server-socket channels only support the accepting of new connections, so this method returns SelectionKey.OP_ACCEPT.

Example

The following example shows the how to send data from Java NIO ServerSocketChannel.

C:/Test/temp.txt

Hello World!

Client: SocketChannelClient.java

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.EnumSet;

public class SocketChannelClient {
   public static void main(String[] args) throws IOException {
      ServerSocketChannel serverSocket = null;
      SocketChannel client = null;
      serverSocket = ServerSocketChannel.open();
      serverSocket.socket().bind(new InetSocketAddress(9000));
      client = serverSocket.accept();
      System.out.println("Connection Set:  " + client.getRemoteAddress());
      Path path = Paths.get("C:/Test/temp1.txt");
      FileChannel fileChannel = FileChannel.open(path, 
         EnumSet.of(StandardOpenOption.CREATE, 
            StandardOpenOption.TRUNCATE_EXISTING,
            StandardOpenOption.WRITE)
         );      
      ByteBuffer buffer = ByteBuffer.allocate(1024);
      while(client.read(buffer) > 0) {
         buffer.flip();
         fileChannel.write(buffer);
         buffer.clear();
      }
      fileChannel.close();
      System.out.println("File Received");
      client.close();
   }
}

Output

Running the client will not print anything until server starts.

Server: SocketChannelServer.java

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.SocketChannel;
import java.nio.file.Path;
import java.nio.file.Paths;

public class SocketChannelServer {
   public static void main(String[] args) throws IOException {
      SocketChannel server = SocketChannel.open();
      SocketAddress socketAddr = new InetSocketAddress("localhost", 9000);
      server.connect(socketAddr);
      Path path = Paths.get("C:/Test/temp.txt");
      FileChannel fileChannel = FileChannel.open(path);
      ByteBuffer buffer = ByteBuffer.allocate(1024);
      while(fileChannel.read(buffer) > 0) {
         buffer.flip();
         server.write(buffer);
         buffer.clear();
      }
      fileChannel.close();
      System.out.println("File Sent");
      server.close();
   }
}

Output

Running the server will print the following.

Connection Set:  /127.0.0.1:49558
File Received