Extending Servlet Code
Servlet Filters in the VSM
Contents
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The
Virtual Shopping Mall (VSM) sample application implements servlet
filters. These filters add features to the servlet-based application,
and they can be customized and extended without touching the existing
servlet code. This article describes how the VSM uses servlet filters
to implement the following features:
- Hit Counter
- File Upload
- Access Control
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About Servlet
Filters
The Java
Servlet 2.3 specification introduced the concept of filters,
components that can intercept and modify requests to and responses from
servlets and JSPs (JavaServer Pages). You can extend and enhance an application
by adding one or more filters, and you can apply filters individually
or in a series called a filter chain.
From a developer's point of view, a filter is a Java
class that implements the interface defined in javax.servlet.Filter.
The key method is doFilter, which has the following signature:
public void doFilter(ServletRequest request, ServletResponse
response, FilterChain chain)
As you can see, doFilter provides access
to the request and response objects, and to the filter chain. This method
does the actual filtering, and can be called any number of times. The
Filter interface also defines an init method, called once
before a filter goes into service, and a destroy method,
called once before the filter is taken out of service.
The following figure shows the request/response flow
in three scenarios: an application with a client (C) and a servlet (S)
but no filters, the same application using one filter (F), and again using
two filters (F1 and F2) in a filter chain. Note that while a filter can
modify a request or a response object, it doesn't have to. A filter could
pass the object along unmodified. Also, you don't need a servlet at the
back end to use filters—you can configure an application server (or
other container) to apply filters to any request.
| No Filters |
One Filter |
Two-Filter Chain |
 |
 |
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In addition to implementing
the filter, you must edit the Web application deployment descriptor file
(web.xml) so the container can find and invoke the filter.
Following is a portion of a web.xml file that declares a
simple filter, the Java class that implements it, and a URL pattern to
which the filter will be applied. The filter name, defined by the developer,
maps the Java class to the URL pattern. In this example, the filter named
FilterAllRequests is implemented in mypackage1.FilterOne.java,
and the container will apply it to all requests it receives (an asterisk
is a wildcard in a URL pattern). You can also apply filters to specific
directories or resources (files). Other tags are available for declaring
filter attributes, but those shown below are essential.
...
<filter>
<filter-name>FilterAllRequests</filter-name>
<filter-class>mypackage1.FilterOne</filter-class>
</filter>
<filter-mapping>
<filter-name>FilterAllRequests</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
...
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Here is XML code that defines another filter. Because
of the value specified in the <url-pattern> element,
the container only applies this filter when it receives a request for
a resource in the /mydocs directory.
...
<filter>
<filter-name>FilterMyDocs</filter-name>
<filter-class>mypackage1.FilterTwo</filter-class>
</filter>
<filter-mapping>
<filter-name>FilterMyDocs</filter-name>
<url-pattern>/mydocs/*</url-pattern>
</filter-mapping>
...
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To define a filter chain, put two or more filter declarations
into the configuration file and supply appropriate values for the <url-pattern>
elements . For example, given the code below, the container will apply
FilterAllRequests and FilterMyDocs when it receives
a request for a resource like http://127.0.0.1:8989/mydocs/foo.html.
When two or more filters apply to the same resource, they are invoked
in the order that they appear in the configuration file.
...
<filter>
<filter-name>FilterAllRequests</filter-name>
<filter-class>mypackage1.FilterOne</filter-class>
</filter>
<filter-mapping>
<filter-name>FilterAllRequests</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter>
<filter-name>FilterMyDocs</filter-name>
<filter-class>mypackage1.FilterTwo</filter-class>
</filter>
<filter-mapping>
<filter-name>FilterMyDocs</filter-name>
<url-pattern>/mydocs/*</url-pattern>
</filter-mapping>
...
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The rest of this article describes how the VSM filters
were implemented. The next section
describes a filter that counts people who visit the mall.
Questions or comments? Post a message in the OTN
Sample Code discussion forum or send email to the author.
Extending Servlet Code: Servlet Filters in the VSM
Author: Robert Hall, Oracle
Corporation
Date: June 2002
This document is provided for information purposes only
and the information herein is subject to change without notice. Please
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