In earlier article, we went through basic concepts of Netty & a very simple example of Chat application.

Netty Project | Understanding Netty using simple real-world example of Chat server client | Good for beginners

Example in this article

In this article, we will enhance that same chat application & add a custom codec to encode/decode custom object with file read as explained below.

Use same chat application from earlier article.
Introduce special chat message (or command) which client can enter in format “file:<file-name>”.
Create custom POJO object with that command & name of client who ran command. Write that POJO object to channel.
Create custom codec which will take the custom POJO object & encode/decode. It will take file name from command, read the content of the file. Then it will create a String with client name, file name, file content & start-end marking of content. Then write this String to server. Decoding will be done as plain text.
This way other client will receive the content of the file.

Lets code

Here is the custom POJO object which we will use in client.

package com.itsallbinary.netty.chat.enhanced;

public class FileMessage {

	// Command format = file:<file-name>
	private String fileCommand;

	private String clientName;

	public FileMessage(String fileCommand, String clientName) {
		this.fileCommand = fileCommand;
		this.clientName = clientName;
	}

	String getClientName() {
		return clientName;
	}

	String getFileCommand() {
		return fileCommand;
	}

}

package com.itsallbinary.netty.chat.enhanced;

public class FileMessage {

// Command format = file:<file-name>

private String fileCommand;

private String clientName;

public FileMessage(String fileCommand, String clientName) {

this.fileCommand = fileCommand;

this.clientName = clientName;

}

String getClientName() {

return clientName;

}

String getFileCommand() {

return fileCommand;

}

Here is the custom codec which will take above custom POJO object & encode it as per above requirements of this example.

package com.itsallbinary.netty.chat.enhanced;

import java.io.File;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.util.List;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageCodec;

public class FileMessageCodec extends ByteToMessageCodec<FileMessage> {

	/*
	 * In case of message of type FileMessage, read file & create a String message
	 * with the text inside file. Write final string to buffer.
	 * 
	 * File command format file:<file-name>
	 */
	@Override
	protected void encode(ChannelHandlerContext ctx, FileMessage msg, ByteBuf out) throws Exception {

		File file = new File(msg.getFileCommand().replace("file:", ""));
		String fileData = Files.readString(file.toPath());
		String message = "[" + msg.getClientName() + "]: \n-------------------" + file.getName()
				+ "-------------------\n" + fileData + "\n-------------------End Of File-------------------\n";
		out.writeCharSequence(message, Charset.defaultCharset());

	}

	/*
	 * For decoding convert message to String.
	 */
	@Override
	protected void decode(ChannelHandlerContext ctx, ByteBuf msg, List<Object> out) throws Exception {
		out.add(msg.toString(Charset.defaultCharset()));
	}

}

package com.itsallbinary.netty.chat.enhanced;

import java.io.File;

import java.nio.charset.Charset;

import java.nio.file.Files;

import java.util.List;

import io.netty.buffer.ByteBuf;

import io.netty.channel.ChannelHandlerContext;

import io.netty.handler.codec.ByteToMessageCodec;

public class FileMessageCodec extends ByteToMessageCodec<FileMessage> {

* In case of message of type FileMessage, read file & create a String message

* with the text inside file. Write final string to buffer.

* File command format file:<file-name>

@Override

protected void encode(ChannelHandlerContext ctx, FileMessage msg, ByteBuf out) throws Exception {

File file = new File(msg.getFileCommand().replace("file:", ""));

String fileData = Files.readString(file.toPath());

String message = "[" + msg.getClientName() + "]: \n-------------------" + file.getName()

+ "-------------------\n" + fileData + "\n-------------------End Of File-------------------\n";

out.writeCharSequence(message, Charset.defaultCharset());

}

* For decoding convert message to String.

@Override

protected void decode(ChannelHandlerContext ctx, ByteBuf msg, List<Object> out) throws Exception {

out.add(msg.toString(Charset.defaultCharset()));

}

Now we will modify Chat client from earlier article to accept special command & based on that command create custom object & write that object to channel.

package com.itsallbinary.netty.chat.enhanced;

import java.io.File;
import java.util.Scanner;

import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;

public class EnhancedChatClient {

	static final String HOST = "127.0.0.1";
	static final int PORT = 8007;
	static String clientName;

	public static void main(String[] args) throws Exception {

		/*
		 * Get name of the user for this chat session.
		 */
		Scanner scanner = new Scanner(System.in);
		System.out.println("Please enter your name: ");
		if (scanner.hasNext()) {
			clientName = scanner.nextLine();
			System.out.println("Welcome " + clientName);
		}

		/*
		 * Configure the client.
		 */

		// Since this is client, it doesn't need boss group. Create single group.
		EventLoopGroup group = new NioEventLoopGroup();
		try {
			Bootstrap b = new Bootstrap();
			b.group(group) // Set EventLoopGroup to handle all eventsf for client.
					.channel(NioSocketChannel.class)// Use NIO to accept new connections.
					.handler(new ChannelInitializer<SocketChannel>() {
						@Override
						public void initChannel(SocketChannel ch) throws Exception {
							ChannelPipeline p = ch.pipeline();
							/*
							 * Socket/channel communication happens in byte streams. String decoder &
							 * encoder helps conversion between bytes & String.
							 */
							p.addLast(new StringDecoder());
							p.addLast(new StringEncoder());

							/*
							 * Add FileMessage codec. If message has format file:<file-name>, then this
							 * codec will read file & send content of file to other clients.
							 */
							p.addLast(new FileMessageCodec());

							// This is our custom client handler which will have logic for chat.
							p.addLast(new EnhancedChatClientHandler());

						}
					});

			// Start the client.
			ChannelFuture f = b.connect(HOST, PORT).sync();

			/*
			 * Iterate & take chat message inputs from user & then send to server.
			 */
			while (scanner.hasNext()) {
				String input = scanner.nextLine();
				Channel channel = f.sync().channel();

				if (input.startsWith("file:")) {
					channel.writeAndFlush(new FileMessage(input, clientName));
				} else {
					channel.writeAndFlush("[" + clientName + "]: " + input);
				}
				channel.flush();
			}

			// Wait until the connection is closed.
			f.channel().closeFuture().sync();
		} finally {
			// Shut down the event loop to terminate all threads.
			group.shutdownGracefully();
		}
	}
}

package com.itsallbinary.netty.chat.enhanced;

import java.io.File;

import java.util.Scanner;

import io.netty.bootstrap.Bootstrap;

import io.netty.channel.Channel;

import io.netty.channel.ChannelFuture;

import io.netty.channel.ChannelInitializer;

import io.netty.channel.ChannelPipeline;

import io.netty.channel.EventLoopGroup;

import io.netty.channel.nio.NioEventLoopGroup;

import io.netty.channel.socket.SocketChannel;

import io.netty.channel.socket.nio.NioSocketChannel;

import io.netty.handler.codec.string.StringDecoder;

import io.netty.handler.codec.string.StringEncoder;

public class EnhancedChatClient {

static final String HOST = "127.0.0.1";

static final int PORT = 8007;

static String clientName;

public static void main(String[] args) throws Exception {

* Get name of the user for this chat session.

Scanner scanner = new Scanner(System.in);

System.out.println("Please enter your name: ");

if (scanner.hasNext()) {

clientName = scanner.nextLine();

System.out.println("Welcome " + clientName);

}

* Configure the client.

// Since this is client, it doesn't need boss group. Create single group.

EventLoopGroup group = new NioEventLoopGroup();

try {

Bootstrap b = new Bootstrap();

b.group(group) // Set EventLoopGroup to handle all eventsf for client.

.channel(NioSocketChannel.class)// Use NIO to accept new connections.

.handler(new ChannelInitializer<SocketChannel>() {

@Override

public void initChannel(SocketChannel ch) throws Exception {

ChannelPipeline p = ch.pipeline();

* Socket/channel communication happens in byte streams. String decoder &

* encoder helps conversion between bytes & String.

p.addLast(new StringDecoder());

p.addLast(new StringEncoder());

* Add FileMessage codec. If message has format file:<file-name>, then this

* codec will read file & send content of file to other clients.

p.addLast(new FileMessageCodec());

// This is our custom client handler which will have logic for chat.

p.addLast(new EnhancedChatClientHandler());

}

});

// Start the client.

ChannelFuture f = b.connect(HOST, PORT).sync();

* Iterate & take chat message inputs from user & then send to server.

while (scanner.hasNext()) {

String input = scanner.nextLine();

Channel channel = f.sync().channel();

if (input.startsWith("file:")) {

channel.writeAndFlush(new FileMessage(input, clientName));

} else {

channel.writeAndFlush("[" + clientName + "]: " + input);

}

channel.flush();

}

// Wait until the connection is closed.

f.channel().closeFuture().sync();

} finally {

// Shut down the event loop to terminate all threads.

group.shutdownGracefully();

}

In order to test, also add below 2 text files at the root of the project.

This is content of RaviFile.txt. This text file belongs to Ravi.

1	This is content of RaviFile.txt. This text file belongs to Ravi.

This is content of JohnFile.txt. This text file belongs to John.

1	This is content of JohnFile.txt. This text file belongs to John.

Rest of the code can remain as it is. Here is the remaining code files.

package com.itsallbinary.netty.chat.enhanced;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;

public class EnhancedChatClientHandler extends SimpleChannelInboundHandler<String> {

	/*
	 * Print chat message received from server.
	 */
	@Override
	protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
		System.out.println("Message: " + msg);

	}

}

package com.itsallbinary.netty.chat.enhanced;

import io.netty.channel.ChannelHandlerContext;

import io.netty.channel.SimpleChannelInboundHandler;

public class EnhancedChatClientHandler extends SimpleChannelInboundHandler<String> {

* Print chat message received from server.

@Override

protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {

System.out.println("Message: " + msg);

}

package com.itsallbinary.netty.chat.enhanced;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;

public final class EnhancedChatServer {

	// Port where chat server will listen for connections.
	static final int PORT = 8007;

	public static void main(String[] args) throws Exception {

		/*
		 * Configure the server.
		 */

		// Create boss & worker groups. Boss accepts connections from client. Worker
		// handles further communication through connections.
		EventLoopGroup bossGroup = new NioEventLoopGroup(1);
		EventLoopGroup workerGroup = new NioEventLoopGroup();

		try {
			ServerBootstrap b = new ServerBootstrap();
			b.group(bossGroup, workerGroup) // Set boss & worker groups
					.channel(NioServerSocketChannel.class)// Use NIO to accept new connections.
					.childHandler(new ChannelInitializer<SocketChannel>() {
						@Override
						public void initChannel(SocketChannel ch) throws Exception {
							ChannelPipeline p = ch.pipeline();
							/*
							 * Socket/channel communication happens in byte streams. String decoder &
							 * encoder helps conversion between bytes & String.
							 */
							p.addLast(new StringDecoder());
							p.addLast(new StringEncoder());

							// This is our custom server handler which will have logic for chat.
							p.addLast(new EnhancedChatServerHandler());
						}
					});

			// Start the server.
			ChannelFuture f = b.bind(PORT).sync();
			System.out.println("Chat Server started. Ready to accept chat clients.");

			// Wait until the server socket is closed.
			f.channel().closeFuture().sync();
		} finally {
			// Shut down all event loops to terminate all threads.
			bossGroup.shutdownGracefully();
			workerGroup.shutdownGracefully();
		}
	}
}

package com.itsallbinary.netty.chat.enhanced;

import io.netty.bootstrap.ServerBootstrap;

import io.netty.channel.ChannelFuture;

import io.netty.channel.ChannelInitializer;

import io.netty.channel.ChannelPipeline;

import io.netty.channel.EventLoopGroup;

import io.netty.channel.nio.NioEventLoopGroup;

import io.netty.channel.socket.SocketChannel;

import io.netty.channel.socket.nio.NioServerSocketChannel;

import io.netty.handler.codec.string.StringDecoder;

import io.netty.handler.codec.string.StringEncoder;

public final class EnhancedChatServer {

// Port where chat server will listen for connections.

static final int PORT = 8007;

public static void main(String[] args) throws Exception {

* Configure the server.

// Create boss & worker groups. Boss accepts connections from client. Worker

// handles further communication through connections.

EventLoopGroup bossGroup = new NioEventLoopGroup(1);

EventLoopGroup workerGroup = new NioEventLoopGroup();

try {

ServerBootstrap b = new ServerBootstrap();

b.group(bossGroup, workerGroup) // Set boss & worker groups

.channel(NioServerSocketChannel.class)// Use NIO to accept new connections.

.childHandler(new ChannelInitializer<SocketChannel>() {

@Override

public void initChannel(SocketChannel ch) throws Exception {

ChannelPipeline p = ch.pipeline();

* Socket/channel communication happens in byte streams. String decoder &

* encoder helps conversion between bytes & String.

p.addLast(new StringDecoder());

p.addLast(new StringEncoder());

// This is our custom server handler which will have logic for chat.

p.addLast(new EnhancedChatServerHandler());

}

});

// Start the server.

ChannelFuture f = b.bind(PORT).sync();

System.out.println("Chat Server started. Ready to accept chat clients.");

// Wait until the server socket is closed.

f.channel().closeFuture().sync();

} finally {

// Shut down all event loops to terminate all threads.

bossGroup.shutdownGracefully();

workerGroup.shutdownGracefully();

}

package com.itsallbinary.netty.chat.enhanced;

import java.util.ArrayList;
import java.util.List;

import io.netty.channel.Channel;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;

/**
 * Handles a server-side channel.
 */
public class EnhancedChatServerHandler extends SimpleChannelInboundHandler<String> {

	// List of connected client channels.
	static final List<Channel> channels = new ArrayList<Channel>();

	/*
	 * Whenever client connects to server through channel, add his channel to the
	 * list of channels.
	 */
	@Override
	public void channelActive(final ChannelHandlerContext ctx) {
		System.out.println("Client joined - " + ctx);
		channels.add(ctx.channel());
	}

	/*
	 * When a message is received from client, send that message to all channels.
	 * FOr the sake of simplicity, currently we will send received chat message to
	 * all clients instead of one specific client. This code has scope to improve to
	 * send message to specific client as per senders choice.
	 */
	@Override
	public void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
		System.out.println("Server received - " + msg);
		for (Channel c : channels) {
			c.writeAndFlush("-> " + msg + '\n');
		}
	}

	/*
	 * In case of exception, close channel. One may chose to custom handle exception
	 * & have alternative logical flows.
	 */
	@Override
	public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
		System.out.println("Closing connection for client - " + ctx);
		ctx.close();
	}
}

package com.itsallbinary.netty.chat.enhanced;

import java.util.ArrayList;

import java.util.List;

import io.netty.channel.Channel;

import io.netty.channel.ChannelHandlerContext;

import io.netty.channel.SimpleChannelInboundHandler;

/**

* Handles a server-side channel.

public class EnhancedChatServerHandler extends SimpleChannelInboundHandler<String> {

// List of connected client channels.

static final List<Channel> channels = new ArrayList<Channel>();

* Whenever client connects to server through channel, add his channel to the

* list of channels.

@Override

public void channelActive(final ChannelHandlerContext ctx) {

System.out.println("Client joined - " + ctx);

channels.add(ctx.channel());

}

* When a message is received from client, send that message to all channels.

* FOr the sake of simplicity, currently we will send received chat message to

* all clients instead of one specific client. This code has scope to improve to

* send message to specific client as per senders choice.

@Override

public void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {

System.out.println("Server received - " + msg);

for (Channel c : channels) {

c.writeAndFlush("-> " + msg + '\n');

}

* In case of exception, close channel. One may chose to custom handle exception

* & have alternative logical flows.

@Override

public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {

System.out.println("Closing connection for client - " + ctx);

ctx.close();

}

Executing the enhanced chat application

Steps for execution are same as earlier article. Here are the console logs.

ChatServer console logs

Chat Server started. Ready to accept chat clients.
Client joined - ChannelHandlerContext(EnhancedChatServerHandler#0, [id: 0x04383fc2, L:/127.0.0.1:8007 - R:/127.0.0.1:51704])
Client joined - ChannelHandlerContext(EnhancedChatServerHandler#0, [id: 0xf7c75f20, L:/127.0.0.1:8007 - R:/127.0.0.1:51731])
Server received - [John]: Hi Ravi
Server received - [Ravi]: Hi John
Server received - [John]: 
-------------------JohnFile.txt-------------------
This is content of JohnFile.txt. This text file belongs to John.
-------------------End Of File-------------------

Server received - [Ravi]: 
-------------------RaviFile.txt-------------------
This is content of RaviFile.txt. This text file belongs to Ravi.
-------------------End Of File-------------------

Closing connection for client - ChannelHandlerContext(EnhancedChatServerHandler#0, [id: 0x04383fc2, L:/127.0.0.1:8007 - R:/127.0.0.1:51704])
Closing connection for client - ChannelHandlerContext(EnhancedChatServerHandler#0, [id: 0xf7c75f20, L:/127.0.0.1:8007 - R:/127.0.0.1:51731])

Chat Server started. Ready to accept chat clients.

Client joined - ChannelHandlerContext(EnhancedChatServerHandler#0, [id: 0x04383fc2, L:/127.0.0.1:8007 - R:/127.0.0.1:51704])

Client joined - ChannelHandlerContext(EnhancedChatServerHandler#0, [id: 0xf7c75f20, L:/127.0.0.1:8007 - R:/127.0.0.1:51731])

Server received - [John]: Hi Ravi

Server received - [Ravi]: Hi John

Server received - [John]:

-------------------JohnFile.txt-------------------

This is content of JohnFile.txt. This text file belongs to John.

-------------------End Of File-------------------

Server received - [Ravi]:

-------------------RaviFile.txt-------------------

This is content of RaviFile.txt. This text file belongs to Ravi.

-------------------End Of File-------------------

Closing connection for client - ChannelHandlerContext(EnhancedChatServerHandler#0, [id: 0x04383fc2, L:/127.0.0.1:8007 - R:/127.0.0.1:51704])

Closing connection for client - ChannelHandlerContext(EnhancedChatServerHandler#0, [id: 0xf7c75f20, L:/127.0.0.1:8007 - R:/127.0.0.1:51731])

First ChatClient console logs

Please enter your name: 
Ravi
Welcome Ravi
Message: -> [John]: Hi Ravi

Hi John
Message: -> [Ravi]: Hi John

Message: -> [John]: 
-------------------JohnFile.txt-------------------
This is content of JohnFile.txt. This text file belongs to John.
-------------------End Of File-------------------


file:RaviFile.txt
Message: -> [Ravi]: 
-------------------RaviFile.txt-------------------
This is content of RaviFile.txt. This text file belongs to Ravi.
-------------------End Of File-------------------

Please enter your name:

Ravi

Welcome Ravi

Message: -> [John]: Hi Ravi

Hi John

Message: -> [Ravi]: Hi John

Message: -> [John]:

-------------------JohnFile.txt-------------------

This is content of JohnFile.txt. This text file belongs to John.

-------------------End Of File-------------------

file:RaviFile.txt

Message: -> [Ravi]:

-------------------RaviFile.txt-------------------

This is content of RaviFile.txt. This text file belongs to Ravi.

-------------------End Of File-------------------

Second ChatClient console logs

Please enter your name: 
John
Welcome John
Hi Ravi
Message: -> [John]: Hi Ravi

Message: -> [Ravi]: Hi John

file:JohnFile.txt
Message: -> [John]: 
-------------------JohnFile.txt-------------------
This is content of JohnFile.txt. This text file belongs to John.
-------------------End Of File-------------------


Message: -> [Ravi]: 
-------------------RaviFile.txt-------------------
This is content of RaviFile.txt. This text file belongs to Ravi.
-------------------End Of File-------------------

Please enter your name:

John

Welcome John

Hi Ravi

Message: -> [John]: Hi Ravi

Message: -> [Ravi]: Hi John

file:JohnFile.txt

Message: -> [John]:

-------------------JohnFile.txt-------------------

This is content of JohnFile.txt. This text file belongs to John.

-------------------End Of File-------------------

Message: -> [Ravi]:

-------------------RaviFile.txt-------------------

This is content of RaviFile.txt. This text file belongs to Ravi.

-------------------End Of File-------------------

You can find complete code in GIT Repository.

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Netty Project | Create custom codec to encode decode custom object, file data | Enhanced Chat Application

Example in this article

Lets code

Executing the enhanced chat application

ChatServer console logs

First ChatClient console logs

Second ChatClient console logs

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