Sun Java System Application Server Platform Edition 8.1 2005Q2 Update 2 Developer's Guide

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## 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.

A sample caching application is in install-dir`/samples/webapps/apps/caching`.

For more information about JSP caching, see JSP Caching.

The rest of this section covers the following topics:

- Caching Features

- Default Cache Configuration

- Caching Example

- CacheKeyGenerator Interface

### Caching Features

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

- Caching is configurable based on the servlet name or the URI.

- When caching is based on the URI, this includes user specified parameters in the query string. For example, a response from `/garden/catalog?category=roses` is different from a response from `/garden/catalog?category=lilies`. These responses are stored under different keys in the cache.

- Cache size, entry timeout, and other caching behaviors are configurable.

- Entry timeout is measured from the time an entry is created or refreshed. To override this timeout for an individual cache mapping, specify the `cache-mapping` subelement `timeout`.

- To determine caching criteria programmatically, write a class that implements the `com.sun.appserv.web.cache.CacheHelper` interface. For example, if only a servlet knows when a back end data source was last modified, you can write a helper class to retrieve the last modified timestamp from the data source and decide whether to cache the response based on that timestamp.

- To determine cache key generation programmatically, write a class that implements the `com.sun.appserv.web.cache.CacheKeyGenerator` interface. See CacheKeyGenerator Interface.

- All non-ASCII request parameter values specified in cache key elements must be URL encoded. The caching subsystem attempts to match the raw parameter values in the request query string.

- Since newly updated classes impact what gets cached, the web container clears the cache during dynamic deployment or reloading of classes.

- The following `HttpServletRequest` request attributes are exposed:

  - `com.sun.appserv.web.cachedServletName`, the cached servlet target

  - `com.sun.appserv.web.cachedURLPattern`, the URL pattern being cached

- Results produced by resources that are the target of a `RequestDispatcher.include()` or `RequestDispatcher.forward()` call are cached if caching has been enabled for those resources. For details, see the descriptions of the cache-mapping and dispatcher elements in the `sun-web.xml` file.

### 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:

- The default cache timeout is 30 seconds.

- Only the HTTP GET method is eligible for caching.

- HTTP requests with cookies or sessions automatically disable caching.

- No special consideration is given to `Pragma:`, `Cache-control:`, or `Vary:` headers.

- The default key consists of the Servlet Path (minus `pathInfo` and the query string).

- A “least recently used” list is maintained to evict cache entries if the maximum cache size is exceeded.

- Key generation concatenates the servlet path with key field values, if any are specified.

- Results produced by resources that are the target of a `RequestDispatcher.include()` or `RequestDispatcher.forward()` call are never cached.

### 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"/>
    <cache-mapping>
    	<servlet-name>myservlet</servlet-name>
    	<timeout name="timefield">120</timeout>
    	<http-method>GET</http-method>
    	<http-method>POST</http-method>
    </cache-mapping>
    <cache-mapping>
    	<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>
    	</constraint-field>
    	 <!-- 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>
    	</constraint-field>
    	<!-- 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">
           bogus
    		</value>
    	</constraint-field>
    </cache-mapping>
    <cache-mapping>
    	<servlet-name> InfoServlet </servlet-name>
    	<cache-helper-ref>myHelper</cache-helper-ref>
    </cache-mapping>
    </cache>

For more information about the `sun-web.xml` caching settings, see cache.

### 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.

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