OCWCD Chapter 2 : Web App architecture

Containers

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A container runs and controls the servlets.

A full J2EE application server must have both a web container and an EJB container.

When a request is received by the webserver and needs to access a servlet, the webserver hands the

request to the servlet-helper app : the container. Then the container hands the request to the servlet

itself.

Role

•  Match the request URL to a specific servlet;
• Creates request and response objects used to get informations from the client and send back informations to him;
• Creates the thread for the servlet;
• Calls the servlet’s service() method, passing request and response objects as parameters;
• service() method will call the doXxx() method of the servlet depending on request type;
•  Get the response object from the finished service() method and converts it to an HTTP response;
•  Destroys request and response objects;
•  Sends back response to the webserver

Capabilities of containers

Containers provide :

• Communications support : handles communication between the servlets and the webserver
• Lifecycle management : controls life and death of servlets, class loading, instantiation, initialization, invocation the servlets’ methods, and making servlet instances eligible for GC
• Multithreading support : Automatically creates a new thread for every servlet request received. When the Servlet service() method completes, the thread dies.
• Declarative security : manages the security inside the XML deployment descriptor file. Security can be configured and changed only by modifying this file.
• JSP support : Manages the JSPs.

URL to servlet mapping

The servlet container uses the deployment descriptor to map URL to servlets. Two DD elements

are used to achieve this :

<servlet> : maps internal name to fully qualified class name;

<servlet-mapping> : maps internal name to public URL name.

Model-View-Controller (MVC) Design

MVC does not only separates the Business Logic from the presentation, the Business Logic doesn’t

even know there is a presentation.

MVC : separate Business Logic (model) and the presentation, then put something between them toconnect them (controler), so the business can become a real re-usable code, and the presentation layer can be modified/changed at will.