Creating a simple server using Apache MINA

Apache MINA is an open source library for writing client-server Java applications. The latest version to date - 2.0.4 - was released on June 14, 2011.

In this article I will describe the process of creating a simple server using this library.

What are we writing?

In this article we will write the simplest server to which you can connect using telnet. When connected, the server will greet the client, with the command time will tell how many seconds have passed since the connection, and with the command q it will say goodbye and close the connection.
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So, let's begin.

Connect to the project

In addition to the main mina-core library, you also need to connect the slf4j (Simple Logging Facade for Java) library, which MINA uses as an interface to various logging systems, for example, Apache log4j. Connect requires slf4j-api and one of the implementations of logging. In our example, logging will not be used, and therefore you can use slf4j-nop .

Entry point and ioacceptor

In the main method of the application, we need to create and configure an IoAcceptor — an object that will accept incoming connections. We will need the following methods ( IoAcceptor extends the IoService interface and some methods are declared in it):

• DefaultIoFilterChainBuilder getFilterChain() 

  - returns the so-called "filter chain";

•  void setHandler(IoHandler handler) 

  - sets the handler - the object in which the main server logic is located;

•  void bind(SocketAddress localAddress) 

  - begins to listen to the specified address.

For our example, the NioSocketAcceptor is suitable, which is a TCP / IP IoAcceptor implementation.

Filter Chain and Protocol

Before getting into the handler, all events (connection acceptance, data acquisition, etc.) go through the so-called “filter chain”. Filters can perform any actions that might be in the handler, but they are not very appropriate - logging, etc.

One of the filters should be considered separately - this is ProtocolCodecFilter . The task of this filter is to convert data from a certain object into a sequence of bytes during transmission and into a decorated object from a sequence of bytes during reception. One of the constructors of the filter looks like this:

 ProtocolCodecFilter(ProtocolCodecFactory factory) 

In this article I will not deal with creating my own “factory”, my own protocol, for our example one of the simplest and already implemented in MINA protocols - TextLine (“factory” - TextLineCodecFactory ), which converts a sequence of bytes into a string ( String ) and vice versa . In addition, when sending to a line, the end of line character will be added, and when received, the line will be transferred further along the “filter chain” or to the handler only when the same end of line character is received. The encoding and end-of-line character can be specified in the constructor:

 TextLineCodecFactory(Charset charset, String encodingDelimiter, String decodingDelimiter) 

Thus, the handler will have to deal with already decorated objects, in our case - strings.

The DefaultIoFilterChainBuilder object has several methods for adding filters, but we need only one:

 void addLast(String name, IoFilter filter) 

This method adds a filter to the end of the chain.

So now we can write the main method:

 public static void main(String[] args) throws IOException { IoAcceptor acceptor = new NioSocketAcceptor(); acceptor.getFilterChain().addLast("codec", new ProtocolCodecFilter(new TextLineCodecFactory(Charset.defaultCharset(), System.lineSeparator(), System.lineSeparator()))); acceptor.setHandler(new MyMinaServerHandler()); acceptor.bind(new InetSocketAddress(12345)); } 

Handler, IoHandler and IoSession

A handler is an implementation of the IoHandler interface, which contains the following methods:

•  void sessionCreated(IoSession session) 

  - called when accepting a connection;

•  void sessionOpened(IoSession session) 

  - called after sessionCreated ;

•  void sessionClosed(IoSession session) 

  - called when closing the connection;

•  void sessionIdle(IoSession session, IdleStatus status) 

  - called when idle (by timeout);

•  void messageReceived(IoSession session, Object message) 

  - called when receiving data;

•  void messageSent(IoSession session, Object message) 

  - called when sending data;

•  void exceptionCaught(IoSession session, Throwable cause) 

  - called when an exception occurs.

In our example, it is enough to implement only a couple of them - sessionOpened and messageReceived , so instead of implementing the interface directly, we can extend the IoHandlerAdapter class, which is an empty interface implementation, and override the methods we need.

The IoSession object is a session view (also known as a connection). We will need some of his methods:

•  WriteFuture write(Object message) 

  - sends data;

•  CloseFuture close(boolean immediately) 

  - closes the connection. If immediately is false, the connection will be closed only after all the data waiting to be sent will be sent.

IoSession also allows us to store connection-related data, and we will use this opportunity to save connection time. To do this, we will use two methods:

•  Object setAttribute(Object key, Object value) 

•  Object getAttribute(Object key) 

So now we can write and handler:

 public class MyMinaServerHandler extends IoHandlerAdapter { public void sessionOpened(IoSession session) { session.setAttribute("time", System.currentTimeMillis()); session.write("Hello!"); } public void messageReceived(IoSession session, Object message) { switch (((String) message).trim()) { case "time": session.write(String.format("You connected %d seconds ago", (System.currentTimeMillis() - (Long) session.getAttribute("time")) / 1000)); break; case "q": session.write("Bye!"); session.close(false); break; } } } 

That's all

Run the newly assembled server and write on the command line or terminal:

 telnet localhost 12345 

The server responds to the commands:

 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. Hello! time You connected 11 seconds ago q Bye! Connection closed by foreign host. 

The full source code can be downloaded here .
Thank you for reading!