[FIXED] Java interfaces in jar archives

Issue

I'm studying servlets and I was looking at a fully working example created by my professor. The service() method of my servlet looks like this:

public void service (ServletRequest req, ServletResponse res) 
                        throws ServletException, IOException  {
    res.setContentType( "text/plain");
    PrintWriter out = res.getWriter();

    out.println( "Some response" );
    out.close();
}

In the above example service() uses a ServletResponse object, so I went into my servlet-api .jar, searched for javax.servlet.ServletResponse.class and found out that ServletResponse.class is an interface (public abstract interface javax.servlet.ServletResponse) . So in my code I use an object of type ServletResponse which is an interface, but I thought that you could not instantiate an interface in Java. I also noticed that this interface is implemented by ServletResponseWrapper.class.

So my questions are:

• Why does compiler let me instantiate an interface?
• why don't we use a ServletResponseWrapper object?

Solution

You aren't instantiating the interface ServletResponse. You are using a reference that was passed to you by the servlet container when it calls the service() method on your servlet. The container instantiates a class that implements ServletRepsonse interface. The instantiated (concrete) class could be ServletResponseWrapper or any other "internal" classes as long as it implements the ServletResponse interface.

Try printing res.getClass().getName() in your service() method. It should tell you the exact class name that's being instantiated by the servlet container.

Answered By - Bloodysock
Answer Checked By - David Goodson (JavaFixing Volunteer)