Monday, October 8, 2012

Scriptlets Are Not (Inherently) Evil

JSP scriptlets have a bad reputation; everyone can point to their favorite example of scriptlet abuse. In my case, it was a 12,000 line (yes, three zeros) monstrosity that held a “mere” 1,000 lines or so of markup. The rest was a big if-else construct with plenty of embedded logic. But that example, and ones like it, are examples of bad programming, not indictments of scriptlets as a programming medium. They're no different than Java classes that have a single method with thousands of lines.

Monstrosities aside, I don't think there's a valid argument against scriptlets. Actually, when I Googled for “why are scriptlets bad,” most of the top-ranked pages defended them. As far as I can tell, the main arguments are that scriptlets encourage bad programmers to put business logic in the page, that they make the page untestable, that they limit reuse, and that web developers won't understand pages with embedded Java code. All of which seem to me like red, rotted herrings.

Don't get me wrong, I believe in the separation of concerns that underlies the MVC model. Actually, I believe in the separation of concerns found in what I call the “CSV” model: a lightweight Controller that interacts with business logic via a Service layer, and passes any returned data to a View for rendering. But after working with several alternative technologies and languages, I'm convinced that scriptlets are the best way to implement any programmatic constructs involved in view rendering.

And some amount of programmatic rendering resides in almost every view. One common example is populating a <select> element. On the surface this is an easy task: iterate over a list of values, and emit <option> elements for each. In the real world, it's more complex: the option value will come from a different field than the option text, you probably have a (possibly null) value that should be selected, and maybe you'll decorate different options with different classes or IDs. To handle this, you need a language that, if not Turing-complete, is very close.

I'm going to work through just such an example, comparing scriptlets and JSTL. Both examples use the same Spring controller, which stores two values in the request context: a list of employees in allEmployees, and the desired selection in selectedEmployee (which may be null). If you want to walk through the code, it's available here, and builds with Maven.

First up is the JSTL version. Nothing complex here: a forEach over the list of employees, and some EL to extract the bean values. Perhaps the worst thing about this code is that the HTML it generates looks ugly: line breaks carry through from the source JSP, so the <option> element gets split over three lines. You can always tell a JSP-generated page by the amount of incongruous whitespace it contains (if you're security-conscious, that might be an issue).

<select name="employeeSelector">
<c:forEach var="emp" items="${employees}">
    <option value="${}"
        <c:if test="${!(empty selectedEmployee) and (selectedEmployee eq emp)}"> selected </c:if>

Now for the scriptlet version. It's a few lines longer than the JSTL version, partly because I have to retrieve the two variables from the request context (something EL does for me automatically). I also chose to create a temporary variable to hold the “selected” flag; I could have used an inline “if” that matched the JSTL version, or I could have changed the JSTL version to use a temporary variable; I find that I have different programming styles depending on language.

<select name="employeeSelector">
    List<Employee> allEmployees = (List<Employee>)request.getAttribute(Constants.PARAM_ALL_EMPLOYEES);
    Employee selectedEmployee  = (Employee)request.getAttribute(Constants.PARAM_SELECTED_EMPLOYEE);
    for (Employee employee : allEmployees)
        String selected = ObjectUtils.equals(employee, selectedEmployee) ? "selected" : "";
        <option value="<%=employee.getId()%>" <%=selected%>> <%=employee%> </option>

Even though the scriptlet version has more lines of code, it seems cleaner to me: the code and markup are clearly delineated. Perhaps there's a web developer who will be confused by a Java “for” statement in the middle of the page, but is the JSTL “forEach” any better? Assuming that your company separates the roles of web developer and Java developer, it seems like a better approach to say “anything with angle brackets is yours, everything else is mine.”

Putting subjective measures aside, I think there are a few objective reasons that the scriptlet is better than JSTL. The first of these is how I retrieve data from the request context: I use constants to identify the objects, the same constants that I use in the controller to store them. While I have no inherent opposition to duck typing, there's a certain comfort level from knowing that my page won't compile if I misspell a parameter. Sometimes those misspellings are obvious, but if you happen to write your JSTL with “selectedEmploye” it will take rigorous testing (either manual or automated) to find the problem.

Another benefit to using scriptlets is that you can transparently call out to Java. Here, for example, I use Jakarta Commons to do a null-safe equality check.

A better example would be formatting: here I rely on Employee.toString() to produce something reasonable (and if you look at the example you may question my choice). But let's say the users want to see a different format, like “Last, First”; or they want different formats in different places. In JSTL, you'll be required to manually extract and format the fields, and that code will be copy/pasted everywhere that you want to display the employee name. Then you'll have to track down all of that copy/paste code when the users inevitably change their mind about what they want to see. Or you could add different formatting functions to the Employee object itself, breaking the separation of model and view. With a scriptlet, you can call a method like EmployeeFormattingUtils.selectionName().

OK, before I get comments, I should note that there are easier ways to implement the JSTL version, using 3rd-party tag libraries. The Spring Form Tags, for example, would reduce my code to a single line, assuming that my controller followed the Spring “command” pattern; But those libraries limit you to common cases, restricting your ability to customize your HTML. And although you can write your own taglibs, I haven't seen many projects that do that (in fact, the only ones I've seen are ones where I did it).

I want to finish with a punchline: everything I've just written is moot, because the web-app world is transitioning from server-side view rendering to client-side, using technologies such as Backbone and Spine. That change will come with its own perils, and history tends to repeat itself. I worry that five years from now we'll be dealing with multi-thousand-line JavaScript monstrosities, mixing business logic with view rendering.