A bare minimum web server for android platform

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Since today’s mobile phone are having more computing power, running a web server on mobile phone can do lot more. There are lots of  Android Web Server applications out there that you can try. But I decided to create one my own.  We can create HTTP Server using org.apache.http packages, the only thing is that we could not run the server with default port, i.e. port 80. I think this is a Linux restriction and noting to do with Android platform.

To create a typical Web Server, first you want to create a server socket and listen to the desired port. Then accept the connection and finally process the HTTP request and send the response to the client.

To create a server socket we use the class java.net.ServerSocket. The constructor of this class accepts a port number to listen for the incoming connections. Once the ServerSocket object is created, we accept the incoming connection using ServerSocket.accept() method. This method is blocking so we create a separate thread to accept and process the incoming connections. The accept() method returns a java.net.Socket object which represents the accepted connection. Once the connection established, next we want to process the HTTP request. This can be done using org.apache.http.protocol.HttpService class. This class provides a minimal HTTP processor server side implementation. Following is the HttpService constructor:

HttpService(
	HttpProcessor proc,
	ConnectionReuseStrategy connStrategy,
	HttpResponseFactory responseFactory)

This class is from the Apache implementation for Android. Explanation of Apache implementation is out of scope of this article. For more information about this please follow this link.

The first parameter org.apache.http.protocol.HttpProcessor is an interface that is used to process the requests and response. The class org.apache.http.protocol.BasicHttpProcessor is a default implementation of this interface.

The second parameter org.apache.http.ConnectionReuseStrategy determines the connection reuse strategy. There two different implementation of this interface; org.apache.http.impl.DefaultConnectionReuseStrategy and org.apache.http.impl.NoConnectionReuseStrategy. The first one re-uses the connection and the second one never re-use the connection. In normal cases we use the first one.

The HttpService class relies on the last parameter to process and send the HTTP response. org.apache.http.HttpResponseFactory is a factory interface that process and send the response to the client. The class org.apache.http.impl.DefaultHttpResponseFactory is a default implementation of this class.

To handle different HTTP requests, we use a handler map based on the URI pattern. For example for all the URIs which starts with /message will be handled by a class and URIs which starts with /dir will be handled by another class. For this purpose we use org.apache.http.protocol.HttpRequestHandlerRegistry class to create a map based on the URI pattern. This class implements org.apache.http.protocol.HttpRequestHandlerResolver which handles different request handlers. Using org.apache.http.protocol.HttpRequestHandlerRegistry we can register different URIs patterns and corresponding org.apache.http.protocol.HttpRequestHandler to handle the requests. HttpRequestHandler is a class that handles the HTTP request. We can create different HttpRequestHandler classes to handle HTTP requests that matches a particular pattern. To register a URI pattern we use the HttpRequestHandlerRegistry.register() method. The syntax of this method is:

public void register (String pattern, HttpRequestHandler handler)

The first parameter is the URI pattern. This can be one of the following:

• * – handles all requests
• *<uri> – handles all requests that has <uri> at the end
• <uri>* – handles all requests that starts with <uri>

The HttpService class will determine which handler should use based on the URI that is received. To handle the HTTP requests we use HttpService.handleRequest() method. The syntax of the method is:

public void handleRequest (HttpServerConnection conn, HttpContext context)

The first parameter is org.apache.http.HttpServerConnection interface for use on the server side. We can us the org.apache.http.impl.DefaultHttpServerConnection class for this purpose which can be created by passing the Socket we received. Second parameter is an org.apache.http.protocol.HttpContext interface, again the org.apache.http.protocol.DefaultedHttpContext implementation can be used.

Sample Application

The sample application is a bare minimum web server. The server provided two functionality; send a message to the device and folder/file listing. To access the server we should know the IP address of the device, then we can use the URL :/”>:/”>http://<deviceip>:<port>/. This will present the home page. This one provides you a very minimum functionality. I will be adding more functionality in future and will post the changes here.

You know, it’s very hard to explain a complicated subject in a short article like this. But I hope this gives an introduction to the web server implementation in Android.