JavaServer Pages, 3rd Edition

Preparing for Distributed Deployment

As I described in the previous section, some web containers can distribute the requests for a web application’s resources over multiple servers, each server running its own Java Virtual Machines (JVM). Of course, this has implications for how you develop your application. So, by default, a web container must use only one JVM for an application.

If you want to take advantage of web-container controlled load balancing, you must do two things: mark the application as distributable and follow the rules for a distributed application defined by the servlet specification.

To mark an application as distributable means adding a <distributable/> element in the deployment descriptor for the application:

<web-app>
  <description>A distributable application</description>
  
  <distributable/>
  
  <context-param>
    ...
  
</web-app>

By doing so, you’re telling the web container that your application adheres to the rules for distributed applications. According to the servlet specification, a distributed application must be able to work within the following constraints:

• Each JVM has its own unique servlet instance for each servlet declaration. If a servlet implements the javax.servlet.SingleThreadModel interface, each JVM may maintain multiple instances of the servlet class.

• Each JVM has its own unique javax.servlet.ServletContext instance. Objects placed in the context are not distributed between JVMs.

• Each JVM has its own unique listener class instances. Event notification is ...