21.2 Java4535: introduction to servlet code  (Page 5/7)

The servlet life cycle

Servlets have a prescribed life cycle. Servers load and run servlets. Servers accept requests from clients, and may use their servlets to return data to the clients.

Servers can also remove servlets. So, the stages of a servlet's life cycle are:

• The servlet is loaded and initialized.
• The servlet is used to satisfy client requests.
• The servlet is removed or unloaded.

A server runs the servlet's init method when it loads the servlet. Most servlets are run in multi-threaded servers. However, there are no concurrency issues during servlet initialization. The server calls the init method when it loads the servlet, and does not call it again unless it is reloading the servlet.

The server cannot reload a servlet until after it has removed the servlet by calling the destroy method. Initialization is allowed to complete before client requests are handled or the servlet is destroyed.

A practical problem

This can be a problem when you are developing a servlet and repeatedly testing it with a server. Once the servlet is initially loaded from the class file, for some servers simply providing a new class file does not cause the server to remove the old version of the servlet and load the new version. (This appears to be the case with the Apache Tomcat server that we will be using for some of these modules.)

With the Apache Tomcat server , you must stop and restart the server to cause it to load the new version of a servlet. (It may be possible to use some feature of the administration tool to force the server to remove an old version and load a new version of a servlet without stopping and restarting the server but I have never investigated thatpossibility.)

The second stage

The second stage in the life of the servlet begins after the servlet has been loaded and initialized. At that point, the server may call upon the servlet to respond to client requests. In so doing, the servlet handles client requests by processing them in its service method, which is called by the server. Normally, the service request from each client is run in a separate thread by the servlet.

Generic versus HTTP servlets

If you are writing a generic servlet, you will probably override the service method. However, if you are writing servlets to be used with HTTP servers, you probably won't need to override service . The service method of the HttpServlet class handles the setup and dispatches the request to methods such as doGet and doPost . When writing HTTP servlets, you will normally extend the HttpServlet class and override the doXXX methods instead of overriding the service method.

Multithreaded operation

Servlets can process requests from multiple clients concurrently in a multithreaded manner. This means that the service methods should be written to be thread safe. One description that I have seen for how to write a thread-safe method is to:

Use the fewest possible number of instance variables, and synchronize access to them if you use them.

Single-threaded operation

In some cases and for some reason known only to you, you may decide to prevent your servlet from processing concurrent client requests. In this case, you should cause your servlet to implement the SingleThreadModel interface. This interface guarantees that no two threads will execute the servlet's service methods concurrently. Implementing the interface does not require writing any extra methods. Merely declaring that the servlet implements the interface is sufficient to prevent the server from making concurrent calls to the service method.