Monday, February 18, 2013

Dissection of URL of HTTP Requests in Java Web Applications

Java web applications are request-driven. A client (i.e. a web browser) sends a HTTP request to a server (e.g. Tomcat) and gets a HTTP response back, and displays content of the response. Every HTTP request has a URL. This article is a brief explanation of the structure of such URLs.

Overview

A typical HTTP request URL is like: http://www.mycompany.com/my-web-appliction/my-servlet?param1=value1&param2=value2

It begins with "http" or "https" - the request protocol.

In above example, www.mycompany.com is the full host name of the server. The place can also been taken by the IP address of the server. If the protocol is http but the server port is not 80 (or the protocol is https but the server port is not 443), the port number has to be appended to the host name, like in www.mycompany.com:8080. Here 8080 is the server port.

In above example,  my-web-appliction is the context root of the web application. The way to specify the context root of a web application depends on the application server (e.g. WebLogic server). More details on this will come later.

In above example, my-servlet is the servlet URL. A servlet's URL is specified in the web.xml file for the web application (every servlet is declared in a web application). Details will come later.

In above example, param1=value1&param2=value2 are the request parameters. They are name/value pairs. Withing a pair, the name and value are separated by =, and pairs are separated by & (or ;). In this example, there are two request parameters, named param1 and param2, and their value are respectively value1 and value2. In a generic HTTP URL, this part is called a query string, and is separated from the main URL by a character ?.

Specifying Context Root of a Web Application  in Apache Tomcat

In Tomcat, the name of the base direction of a web application is the context root. For example, if the base directory of a web application is tomcat/webapps/mywebapp, where tomcat is the installation directory of Tomcat, the context root is mywebapp.


Specifying Context Root of a Web Application in JBoss Server

It is the value of the <module>/<web>/<context-root> element in application.xml.

Specifying Context Root of a Web Application in WebLogic Server

It is the value of the <context-root> element in weblogic.xml for the web applicaiton (i.e. in the same directory as web.xml for the web application)

Specifying Context Root of a Web Application in WebSphere Server

It is the value of the uri attribute of the <context-root> element in the ibm-web-ext.xml for the web application (i.e. in the same directory as web.xml for the web application)


Specifying  Servlet URL

An example (partial) web.xml for a web application:

<servlet>
    <servlet-name>the-servlet-name</servlet-name>
    <servlet-class>com.mycompany.XyzServlet</servlet>
</servlet>

<servlet-mapping>
    <servlet-name>the-servlet-name</servlet-name>
    <url-pattern>/my-servlet</url-pattern> 
</servlet-mapping>


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