Java NIO - Socket Channel


Java NIO socket channel is a selectable type channel which means it can be multiplexed using selector, used for stream oriented data flow connecting sockets.Socket channel can be created by invoking its static open() method,providing any pre-existing socket is not already present.Socket channel is created by invoking open method but not yet connected.In order to connect socket channel connect() method is to be called.One point to be mentioned here is if channel is not connected and any I/O operation is tried to be attempted then NotYetConnectedException is thrown by this channel.So one must be ensure that channel is connected before performing any IO operation.Once channel is get connected,it remains connected until it is closed.The state of socket channel may be determined by invoking its isConnected method.

The connection of socket channel could be finished by invoking its finishConnect() method.Whether or not a connection operation is in progress may be determined by invoking the isConnectionPending method.By default socket channel supports non-blocking connection.Also it support asynchronous shutdown, which is similar to the asynchronous close operation specified in the Channel class.

Socket channels are safe for use by multiple concurrent threads. They support concurrent reading and writing, though at most one thread may be reading and at most one thread may be writing at any given time. The connect and finishConnect methods are mutually synchronized against each other, and an attempt to initiate a read or write operation while an invocation of one of these methods is in progress will block until that invocation is complete.

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.

  • 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() and open((SocketAddress remote) − Open method is used open a socket channel for no specified address while parameterized open method open channel for specified remote address and also connects to it.This convenience method works as if by invoking the open() method, invoking the connect method upon the resulting 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.

  • isConnectionPending() − This method tells whether or not a connection operation is in progress on this channel.

Example

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

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
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