Editing web.xml 
Use
You edit the deployment descriptor in the relevant multipage editor. The multipage editor ensures that the elements in the generated XML file are complete and in the correct order.
The tabs of the multipage editor are organized to reflect the structure of the descriptor being edited. The final tab, Source, displays the generated XML file. You can also directly edit the XML file here (for example, using cut and paste).
Here, the general procedure for editing the file is described. For description of specific tasks, follow the links listed below.
Prerequisites
An Web Application project exists.
Procedure
1. In the J2EE Explorer or the J2EE DC Explorer view, select the web.xml node.
2. From the context menu, choose Open.
The multipage editor is displayed.
3. Select the relevant tab and enter your changes.
4. Choose Save from the toolbar.
Related Procedures
The procedures are grouped under the relevant tab names.
General
Specifying Web Application Display Information
Context
Defining Servlet Context Parameters
Web Objects
Declaring and Configuring Web Components
Mapping
Mapping Servlets and JSP Pages
Resource
Configuring Resource Environment Entries
Configuring Resource References
Security
Specifying Security Constraints
Security Roles
Defining Web Application Security Roles
Environment
Configuring Environment Entries
EJBs
Configuring Enterprise Bean References
Others
Configuring Custom Error Response Pages