我们可以在servlet中创建非参数化的构造函数吗？

Question

What I know till now:

• Instance of Servlet is first created by container via reflection and no argument constructor gets used.
• Then parameterized init method gets called.

Also it is suggested that we should not create a constructor in servlet class as it is of no use. And I agree with that.

Lets say, I have created a no argument constructor in servlet class and from within that I am calling a parameterized constructor. My question is, will it be called by the container?

public class DemoServlet extends HttpServlet{  
   public DemoServlet() {
      this(1);
   }
   public DemoServlet(int someParam) {
      //Do something with parameter
   }
}

Will DemoServlet() be called by the container and if we put some initializing stuff inside it, it will be executed? My guess is yes but it's just a guess based on my understanding.

This might be pretty useless, I am asking out of curiosity.

Answer 1

DemoServlet() will be called (as you are overriding the defined no-arg constructor in HttpServlet (which is a no-op constructor).

However the other DemoServlet(int arg) will not be called.

Answer 2

You are correct with your guess. DemoServlet() would be called by the container and any initialization code within it would be executed - even if that initialization is done through constructor-chaining And as a matter of fact this is a good way to have dependency injection and create a thread-safe servlet which is testable Typically it would be written this way

public class DemoServlet extends HttpServlet
{
   private final someParam; //someParam is final once set cannot be changed

   //default constructor called by the runtime.
   public DemoServlet()
   {
       //constructor-chained to the paramaterized constructor
       this(1);
   }

   //observe carefully that this paramaterized constructor has only
  //package-level visibility. This is useful for being invoked through your
  //  unit and functional tests which would typically reside within the same 
  //package. Would also allow your test code to inject required values to 
 //verify behavior while testing.
   DemoServlet(int someParam)
   {
      this.param = param
   }

   //... Other class code...
}