Instantiation of servlet classes

[alex seba]

Hi,
how can I be sure that only one instance of a servlet class is created? I thought the web container just creates a new instance when there is no one before. In my application several servlet instances of the same class ( 5 with Weblogic, 2 with JBoss-Tomcat4.0.3) are created within the first POST-Method. But I only want one instance of this class. When exactly does a web container create new instances of a servlet class? Can I configure this behavior in the deployment descriptors?
Thanks in advance
Alex

[William Brogden]

According to the servlet API, the servlet engine is supposed to create only one instance for a given web application (unless the servlet class implements the SingleThreadModel interface.)
The servlet engine is supposed to create an instance and execute the init method before any request is processed. This might be when the engine starts up or when the first request is received.
What is leading you to believe that multiple instances are being created?
Bill

[alex seba]

Hi Bill,
thanks for your response!
My Servlet actually implements the SingleThreadModel.
I'm quite sure that multiple instances are being created, because I put a control message in the servlet's init-method (System.out.println("Servlet initialised...")), as well as one in the servlet's doGet and one in the doPost method( System.out.println(this.hashcode()). While running the application I'm getting multiple messages from the init-method, when I invoke the servlet's methods for the first time. And after that I'm also getting multiple hashcodes when the servlet's methods are being used. So there is definitely more than one instance.
Is there any possibility that a servlet engine creates more than one instance of the same servlet class for just one request?
Best regards
Alex

[Tim Holloway]

 

Originally posted by William Brogden:
According to the servlet API, the servlet engine is supposed to create only one instance for a given web application (unless the servlet class implements the SingleThreadModel interface.)

There's your answer. Were it not so, the server would be forced to block for each request, impacting performance.
If you want a singleton object, don't make it a SingleThreadModel object, but make sure you guard class-scope data in accordance with good multi-threading practices.

[alex seba]

Hi,
now I got it! Thank you both for your help!
Alex