Add el support to your own taglib

[Rusty Enisin]

I am working on an older app server that does not have EL support. Is there a way to add EL support to my own taglib so I can do things like this?

<my:tag value="${param.myParam}"/>

I could parse the string ( ${param.myParam} ) and then get the value. Is there a class out there that I can use that already does this?

[Stefan Evans]

Certainly there is. I've never actually done it myself, but it should be simple enough (famous last words)

There are two approaches I can think of
1 - Jakarta Commons EL implementation. This is the implementation of the JSP2.0 package javax.servlet.jsp.el. You can then use those classes in your custom tag. Use the JSP2.0 api as a source for the base classes.

2 - Look at existing source code for the JSTL, and the struts-el implementations. You should be able to leverage off of the JSTL1.0 ExpressionEvaluatorManager class. It is in the package: org.apache.taglibs.standard.lang.support
The struts-el tags use that.

Good luck,
evnafets

[Rusty Enisin]

I finally got around to implementing this. Three steps to personal el bliss.

Step 1: Import these two packages from Apache:

import org.apache.taglibs.standard.tag.common.core.NullAttributeException;
import org.apache.taglibs.standard.tag.el.core.ExpressionUtil;

Step 2: Each taglib has different attributes that need to ba handled differently (null, not null are the two biggies). Create a method that, for each attribute for which you want to use el, calls ExpressionUtil.evalNotNull() like this.

private void evaluateExpressions() throws JspException {
    
try {
    param1 = (String)ExpressionUtil.evalNotNull("decimalFormat", "param1", param1, String.class, this, pageContext);
} 
catch (NullAttributeException ex) {
    // do not allow 'null' for value - if you want null do nothing
    throw new JspException("Attribute value cannot be null for DecimalFormatTag...");
}
}

Step 3: Then inside the doStartTag of your taglib call the method you created above before you do anything else in the method.

public int doStartTag() throws JspException {

// evaluates el expressions
evaluateExpressions();
 
// do whatever else....
}

Pretty snappy and works like a charm.

[Bear Bibeault]

Rusty, this is great information for those still stuck in the JSP 1.x world (kudos to Stefan for the pointer).

I have an idea... please contact me via email.