About Question enthuware.jwpv6.2.1179 :

[ETS User]

The servlet container may maintain multiple instances of the servlet class for a servlet. - isn`t it a valid answer for a SingleThreadModel servlets and servlets defined with different names and the same class?

[admin]

Yes, but the question is asking, "Which is true for ***all*** servlets?"

[sebastien]

Hi

Even without SingleThreadModel, container may use multiple instance of a servlet ...

Tell me if I'm wrong ?

[admin]

No, a container is not allowed to create multiple instances of the Servlet class if it doesn't implement SingleThreadModel (which is deprecated, btw).

-Paul.

[kumarkhiani]

The following method setInitParameter of ServletContext can change the Context Parameters since Servlet 3.0.

Given this method, are the context parameters still thread safe?

Code: Select all

setInitParameter

boolean setInitParameter(java.lang.String name, java.lang.String value)
Sets the context initialization parameter with the given name and value on this ServletContext.
Parameters:
name - the name of the context initialization parameter to set
value - the value of the context initialization parameter to set
Returns:
true if the context initialization parameter with the given name and value was set successfully on this ServletContext, and false if it was not set because this ServletContext already contains a context initialization parameter with a matching name
Throws:
IllegalStateException - if this ServletContext has already been initialized
UnsupportedOperationException - if this ServletContext was passed to the ServletContextListener#contextInitialized method of a ServletContextListener that was neither declared in web.xml or web-fragment.xml, nor annotated with WebListener
Since:
Servlet 3.0

[kumarkhiani]

If the Servlet name in annotations and in <servlet-name> tag are different, then container can make 2 different instances of the servlet-class. According to this, can't the container maintain multiple instances of the same servlet?

No, a container is not allowed to create multiple instances of the Servlet class if it doesn't implement SingleThreadModel (which is deprecated, btw).

-Paul.

[admin]

That statement of mine that you have quoted was meant for simple case where there is only one name. If you have configured multiple servlet names with same class, you may have multiple instances. Even so, all requests for one name will go to the same instance.

HTH,
Paul.

[niallcol]

Hi Paul,

Does this question need to revised given that:

1. It is possible for the container to maintain multiple instances of a servlet class for a servlet.
2. Context parameters are not inherently thread safe because of the ServletContext.setInitParameter() method.

Kind Regards

[admin]

1. No, it is not possible for the container to maintain multiple instances of a servlet class for the same servlet unless it implements SingleThreadModel, which is deprecated and is not on the exam.
2. The setInitParameter works only once i.e. once a parameter is set, you cannot change it. Also, it works on until the servlet is initialized. After that you cannot use this method to modify the parameters. So effectively, servlet context parameters are still thread safe.

HTH,
Paul.

[niallcol]

Hi Paul,

Thanks for the quick reply. Just on the issue of the container maintaining multiple instances of a servlet class. The spec says this for the case where a servlet is defined using annotations and the DD:

If the same servlet class is declared in the deployment
descriptor under a different name, a new instance of the servlet MUST be
instantiated.

Does this not imply that the container can maintain multiple instances of the same servlet class?

Thanks

[admin]

Yes, but I said for the same servlet. You can use the same servlet class to define two servlets and in that case two instances will be created. But that is not the case here. The option is talking about a servlet class for a particular "servlet".

[himaiMinh]

There is no <context-param> in dd.

Code: Select all

<% 
       getServletContext().setInitParameter("myValue", "20");
   %>

java.lang.IllegalStateException: Initialization parameters can not be set after the context has been initialized

The setInitParameter method cannot return successfully for the first time because context has been initialized.

[admin]

Check this out: http://stackoverflow.com/questions/2221 ... one-use-it