Introduction to Servlet technology

Servlet Filters in Java classes are used for following purposes:

• To check requests from client before they access resource at back end.
• To check responses from server before sent back to client.

The various types of filters are:

• Image Conversion Filters
  Logging and Auditing Filters
  MIME-TYPE Chain Filters
  Tokenizing Filters
  Authentication Filters
  Data compression Filters
  Encryption Filters

Filters are deployed in deployment descriptor file web.xml and then map to URL patterns or servlet names in application's deployment descriptor.

Servlet Filter Methods

The javax.servlet.Filter interface implements filter methods:

Exception handling 

When servlet throws an exception, web container searches configurations in web.xml that use exception-type element for a match with thrown exception type. 

web.xml Configuration

The ErrorHandler servlet will be called whenever there is any defined exception or error. Below is the entry created in web.xml.

<!-- definition servlet -->
<servlet>
        <servlet-name>ErrorHandler</servlet-name>
        <servlet-class>ErrorHandler</servlet-class>
</servlet>
<!—mappings servlet -->
<servlet-mapping>
        <servlet-name>ErrorHandler</servlet-name>
        <url-pattern>/ErrorHandler</url-pattern>
</servlet-mapping>
<error-page>
    <error-code>404</error-code>
    <location>/ErrorHandler</location>
</error-page>
<error-page>
    <error-code>403</error-code>
    <location>/ErrorHandler</location>
</error-page>
<error-page>
    <exception-type>
          javax.servlet.ServletException
    </exception-type >
    <location>/ErrorHandler</location>
</error-page>

<error-page>
    <exception-type>java.io.IOException</exception-type >
    <location>/ErrorHandler</location>
</error-page>

There can be generic Error Handler for all exceptions rather than having separate error-page elements for every exception:

<error-page>
    <exception-type>java.lang.Throwable</exception-type > 
   <location>/ErrorHandler</location>
</error-page>

Following are the points to be noted about above web.xml for Exception Handling:

• The servlet ErrorHandler is declared in same way as like other servlet and configured in web.xml.
• For the HTTP status code 404 (Not Found) or 403 (Forbidden) also ErrorHandler servlet is called.
• If web application throws either IOException or ServletException then web container invokes /ErrorHandler servlet.

Request Attributes - Errors/Exceptions

Below is list of request attributes for an error-handling servlet: