Basic lesson 11:- Controllers in jsp

Sometimes, some pre-processing needs to be done after the user has submitted a form. The result of this pre-processing decides where the control flow should go next. Such pre-processing code is frequently referred to as a "controller".
JSPs are very highly suited for writing simple controllers. For more complex controller logic, JSPs can easily call Java code in other classes. A sample controller is shown below:


<jsp:useBean id="user" class="user.UserData" scope="session"/>
<jsp:setProperty name="user" property="*"/>

<%
    String tgtPage = null;
    if ( user.choice.equals( "choice1" ))
        tgtPage = "tgt1.jsp";
    else if ( user.choice.equals( "choice2" ))
        tgtPage = "tgt2.jsp";
    else
        tgtPage = "tgtDefault.jsp";
     response.sendRedirect( tgtPage );
%>
Here we have a very simple code fragment, that is determining the next JSP to transfer control to. (Note that any output generated by this JSP would not actually be used, because the browser will be sent to a different location!)
If the logic were more complicated, it could have been put in a regular Java class, and the JSP could have made calls into that Java class.

Some frameworks use much more complicated mechanisms to do something as simple as this. For example, these complicated mechanisms may require configuration files which have to be changed for every change in call-flow, and may involve Java classes with particular complicated rules.

But an objective review reveals that there is no actual advantage to these over-complicated mechanisms. They merely introduce many more places where things can and do go wrong.

Sometimes, the desire to use these more complicated mechanisms arises from a misunderstanding - that JSPs are meant for display, therefore they can never be used when their output is not of interest. This is not correct.

It is true that JSPs are primarily HTML with embedded code. However, they have many advantages, such as requiring less (typically none) configuration, and being easy to modify and deploy. Therefore they can lend themselves to other usages, such as writing simple and straightforward controllers. Ultimately, JSPs are just one tool in your toolbox. You should look at and evaluate each tool objectively, and put them to best advantage. That means using JSPs as controllers, as long as they provide the simplest and most effective means of doing that.

This is even more important for large projects. Large projects tend to already have a lot of complexity. Adding more complexity via complicated frameworks is not recommended. As seasoned and successful veterans of large projects know, it is specially important in large projects to avoid complexity and find simple and elegant solutions.