Difference between _jspService() and other life cycle methods.

JSP contains three life cycle methods namely jspInit( ), _jspService() and jspDestroy(). In these, jspInit() and jspDestroy() can be overridden and we cannot override _jspService().

The contents what we write in the JSP will go into _jspService() because we are implicitly implementing it. If we again try to override it explicitly, JSP compliler will giver error as ‘the method is already implemented and cannot override’. But in the case of other two methods we are not implementing it in the JSP. Therefore the JSP compiler will allow to override jspInit() and jspDestroy().

To show this difference _jspService() method is prefixed with ‘_’ by the JSP container and the other two methods jspInit() and jspDestroy() has no special prefixes.

What happens if you call destroy() from init() in java servlet?

destroy() gets executed and the initialization process continues. It is a trick question in servlets interview.

In java servlet, destroy() is not supposed to be called by the programmer. But, if it is invoked, it gets executed. The implicit question is, will the servlet get destroyed? No, it will not. destroy() method is not supposed to and will not destroy a java servlet. Don’t get confused by the name. It should have been better, if it was named onDestroy().

The meaning of destroy() in java servlet is, the content gets executed just before when the container decides to destroy the servlet. But if you invoke the destroy() method yourself, the content just gets executed and then the respective process continues. With respective to this question, the destroy() gets executed and then the servlet initialization gets completed.

Why not declare a constructor in servlet?

Technically you can define constructors in servlet. But, the declared constructor cannot access the ServletConfig object or throw a ServletException.

Then why is it not customary to declare a constructor in a servlet? Because the init() method is used to perform servlet initialization. In JDK 1.0 (servlet were written in this version), constructors for dynamically loaded Java classes such as servlets cannot accept arguments. Therefore init() was used to initialize by passing the implemented object of ServletConfig interface and other needed parameters.

Also, Java constructors cannot be declared in interfaces. So, javax.servlet.Servlet interface cannot have a constructor that accepts a ServletConfig parameter. To overcome this, init() method is used for initialization instead of declaring a constructor.