= Using AspectJ in servlets

_Last updated: 2003-09-27 by wisberg_

This contains short notes on using AspectJ with various J2EE servers and
deployment tools.

== Generally

AspectJ programs work if run in the same namespace and with
aspectjrt.jar. Servlet runners and J2EE web containers should run
AspectJ programs fine if the classes and required libraries are deployed
as usual. As with any shared library, if more than one application is
using AspectJ, then the aspectjrt.jar should be deployed where it will
be loaded by a common classloader. The same is true of any shared
aspects.

== Running AspectJ servlets in Tomcat 4.x

In Tomcat, you can deploy application servlets in WAR's or in exploded
web directories and share code across applications.

. Use `ajc` to compile the servlets, and deploy the classes as usual
  into `{WebRoot}/WEB-INF/classes`.
. If your web applications or aspects do not interact, deploy
  `aspectjrt.jar` into `{WebRoot}/WEB-INF/lib`.
. If your web applications or aspects might interact, deploy them to
`${CATALINA_BASE}/shared/lib`.

Tomcat 4.x uses the Jasper engine based on Ant to compile JSP's. To set
up ajc as the compiler, do the following before starting Tomcat:

. Put `aspectjtools.jar` in `${CATALINA_HOME}/common/lib` so that it can
  be loaded by Jasper.

. Update Jasper servlet parameters in `${CATALINA_HOME}/conf/web.xml` to
  tell Ant to use `ajc` by setting the compiler property to the AspectJ
  compiler adapter:
+
[source, xml]
....
<servlet>
    <servlet-name>jsp</servlet-name>
    <servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
    ...
    <init-param>
        <param-name>compiler</param-name>
        <param-value>org.aspectj.tools.ant.taskdefs.Ajc11CompilerAdapter</param-value>
    </init-param>
....

. The classpath is dynamically generated from the webapp deployment, so
  `aspectjrt.jar` should be in `{webapp}/WEB-INF/lib` or some shared or
  common directory supported by the server.

. Alternatively, you can precompile JSP's using
  xref:#j2ee-tomcat4-precompileJsp[this Ant script]. That involves
  manually updating the `web.xml` file with the `Jasper`-generated servlet
  mappings.