Custom evaluation of EL expressions - extend ExpressionEvaluator?

Question

I have a model class (corresponding to a database table) with some exclusion flags, where the flags indicated that a field should not be displayed on a page where we choose to apply exclusions. For example, a person may request that their username and date of birth not be listed in a directory search, yet we would still want to show the username/birthdate on a page where the user is looking at their own information. For example:
public class Person {
    private String name;
    private Date birthDate;
    private String username;
    private boolean excludeBirthDate;
    private boolean excludeUsername;
    // getters and setters
}I want to write a tag such that I can say in my JSP:
<page:applyExclusions>
    ${person.username} ${person.name} ${person.birthDate}
</page:applyExclusions>and have only the person's name display (if they have requested that the other fields be excluded).
Looking at the tags API, it seems like I would want to have my own ExpressionEvaluator whose evaluate method says something like
public Object evaluate(String expr, Class type, VariableResolver resolver, FunctionMapper mapper) {
    if (type.equals(Person.class) &&
         expr.equals(someExcludedField)) {
         // return empty String or something to say "don't display the actual value"
    else {
        // ok to display value, evaluate like normal
        return super.evaluate(expr, type, resolver, mapper);
}Not sure if I'm on the right track with my idea or if there's a better way to accomplish this?

Answer 1

An expression evaluator is exactly that - searches for some variables in scope and performs evaluation on them according to a set of rules.
IMO you shouldnt be changing this functionality to include your business logic there. You should be writing your own custom tags for such pruposes, something like
<myTag:applyExclusions username="${userName}" name="${name}" birthDate="${brithDate}/>And in your tag code, you would have
public int doEndTag() throws JspException {
        try {
            pageContext.getOut().println("UserName "+ userName);
            if(user is viewing the page)//your logic here{
                 pageContext.getOut().println("Name "+ name);
                 pageContext.getOut().println("Birth Date "+ birthDate);
        } catch (IOException exp) {
            throw new JspException(exp);
        return EVAL_PAGE;
}  where userName, name and birthDate are attribute properties of the tag with get and set methods as evnafets suggested.
Hope that helps.
cheers,
ram.