Sun Java System Application Server Platform Edition 9 Developer's Guide

Using Servlets

Application Server supports the Java Servlet Specification version 2.5.

Note –

Servlet API version 2.5 is fully backward compatible with versions 2.3 and 2.4, so all existing servlets should work without modification or recompilation.

To develop servlets, use Sun Microsystems’ Java Servlet API. For information about using the Java Servlet API, see the documentation provided by Sun Microsystems at

The Application Server provides the wscompile and wsdeploy tools to help you implement a web service endpoint as a servlet. For more information about these tools, see the Sun Java System Application Server Platform Edition 9 Reference Manual.

This section describes how to create effective servlets to control application interactions running on an Application Server, including standard-based servlets. In addition, this section describes the Application Server features to use to augment the standards.

This section contains the following topics:

Invoking a Servlet With a URL

You can call a servlet deployed to the Application Server by using a URL in a browser or embedded as a link in an HTML or JSP file. The format of a servlet invocation URL is as follows:


The following table describes each URL section.

Table 8–1 URL Fields for Servlets Within an Application

URL element  



The IP address (or host name) and optional port number. 

To access the default web module for a virtual server, specify only this URL section. You do not need to specify the context-root or servlet-name unless you also wish to specify name-value parameters.


For an application, the context root is defined in the context-root element of the application.xml or sun-application.xml file. For an individually deployed web module, the context root is specified during deployment.

For both applications and individually deployed web modules, the default context root is the name of the WAR file minus the .war suffix.


The servlet-mapping as configured in the web.xml file.


Optional request parameters. 

In this example, localhost is the host name, MortPages is the context root, and calcMortgage is the servlet mapping:


When invoking a servlet from within a JSP file, you can use a relative path. For example:

<jsp:forward page="TestServlet"/>
<jsp:include page="TestServlet"/>

Servlet Output

ServletContext.log messages are sent to the server log.

By default, the System.out and System.err output of servlets are sent to the server log, and during startup, server log messages are echoed to the System.err output. Also by default, there is no Windows-only console for the System.err output.

To change these defaults using the Admin Console, select the Application Server component, and the Logging tab, then check or uncheck this box:

For more information, click the Help button in the Admin Console from the Logging page.

Caching Servlet Results

The Application Server can cache the results of invoking a servlet, a JSP, or any URL pattern to make subsequent invocations of the same servlet, JSP, or URL pattern faster. The Application Server caches the request results for a specific amount of time. In this way, if another data call occurs, the Application Server can return the cached data instead of performing the operation again. For example, if your servlet returns a stock quote that updates every 5 minutes, you set the cache to expire after 300 seconds.

Whether to cache results and how to cache them depends on the data involved. For example, it makes no sense to cache the results of a quiz submission, because the input to the servlet is different each time. However, it makes sense to cache a high level report showing demographic data taken from quiz results that is updated once an hour.

To define how an Application Server web application handles response caching, you edit specific fields in the sun-web.xml file.

Note –

A servlet that uses caching is not portable.

For Javadoc tool pages relevant to caching servlet results, go to and click on the com.sun.appserv.web.cache package.

For information about JSP caching, see JSP Caching.

The rest of this section covers the following topics:

Caching Features

The Application Server has the following web application response caching capabilities:

Default Cache Configuration

If you enable caching but do not provide any special configuration for a servlet or JSP, the default cache configuration is as follows:

Caching Example

Here is an example cache element in the sun-web.xml file:

<cache max-capacity="8192" timeout="60">
<cache-helper name="myHelper" class-name="MyCacheHelper"/>
	<timeout name="timefield">120</timeout>
	<url-pattern> /catalog/* </url-pattern>
	<!-- cache the best selling category; cache the responses to
	   -- this resource only when the given parameters exist. Cache
	   -- only when the catalog parameter has 'lilies' or 'roses'
	   -- but no other catalog varieties:
	  -- /orchard/catalog?best&category='lilies'
	  -- /orchard/catalog?best&category='roses'
	  -- but not the result of
	   -- /orchard/catalog?best&category='wild'
	<constraint-field name='best' scope='request.parameter'/>
	<constraint-field name='category' scope='request.parameter'>
		<value> roses </value>
		<value> lilies </value>
	 <!-- Specify that a particular field is of given range but the
	   -- field doesn't need to be present in all the requests -->
	<constraint-field name='SKUnum' scope='request.parameter'>
		<value match-expr='in-range'> 1000 - 2000 </value>
	<!-- cache when the category matches with any value other than
	   -- a specific value -->
	<constraint-field name="category" scope="request.parameter>
		<value match-expr="equals" cache-on-match-failure="true">
	<servlet-name> InfoServlet </servlet-name>

For more information about the sun-web.xml caching settings, see cache in Sun Java System Application Server Platform Edition 9 Application Deployment Guide.

The CacheKeyGenerator Interface

The built-in default CacheHelper implementation allows web applications to customize the key generation. An application component (in a servlet or JSP) can set up a custom CacheKeyGenerator implementation as an attribute in the ServletContext.

The name of the context attribute is configurable as the value of the cacheKeyGeneratorAttrName property in the default-helper element of the sun-web.xml deployment descriptor. For more information, see default-helper in Sun Java System Application Server Platform Edition 9 Application Deployment Guide.

About the Servlet Engine

Servlets exist in and are managed by the servlet engine in the Application Server. The servlet engine is an internal object that handles all servlet meta functions. These functions include instantiation, initialization, destruction, access from other components, and configuration management. This section covers the following topics:

Instantiating and Removing Servlets

After the servlet engine instantiates the servlet, the servlet engine calls the servlet’s init() method to perform any necessary initialization. You can override this method to perform an initialization function for the servlet’s life, such as initializing a counter.

When a servlet is removed from service, the servlet engine calls the destroy() method in the servlet so that the servlet can perform any final tasks and deallocate resources. You can override this method to write log messages or clean up any lingering connections that won’t be caught in garbage collection.

Request Handling

When a request is made, the Application Server hands the incoming data to the servlet engine. The servlet engine processes the request’s input data, such as form data, cookies, session information, and URL name-value pairs, into an HttpServletRequest request object type.

The servlet engine also creates an HttpServletResponse response object type. The engine then passes both as parameters to the servlet’s service() method.

In an HTTP servlet, the default service() method routes requests to another method based on the HTTP transfer method: POST, GET, DELETE, HEAD, OPTIONS, PUT, or TRACE. For example, HTTP POST requests are sent to the doPost() method, HTTP GET requests are sent to the doGet() method, and so on. This enables the servlet to process request data differently, depending on which transfer method is used. Since the routing takes place in the service method, you generally do not override service() in an HTTP servlet. Instead, override doGet(), doPost(), and so on, depending on the request type you expect.

To perform the tasks to answer a request, override the service() method for generic servlets, and the doGet() or doPost() methods for HTTP servlets. Very often, this means accessing EJB components to perform business transactions, then collating the information in the request object or in a JDBC ResultSet object.