Jython Custom JSP Tags

I just tried to create a custom JSP Tag based on this sample. After compiling it with jythonc I relaized that the doStartTag() method had no throws in it's declaration which was a problem to javac. So I had to use an intermediate BaseTag class that provides a handleStartTag() method without throwing an explicit exception. I also added some sig-strings to the jython code.

The resulting JSP can access all implicit variables like this:

<jy:eval expr="request.getAttribute('now')"/>
<jy:eval expr="session"/>
<jy:eval expr="session.getAttribute('name')"/>
<jy:eval expr="pageContext"/>
<jy:eval expr="request.session"/>

Here are the sources:



========= JythonEvalTag.py =========

 from samples import BaseTag

 class JythonEvalTag(BaseTag):
     """Evaluates a jython expression.
     Call it like <jy:eval expr="1+3"/>
     or <jy:eval expr="request.getAttribute('blah')"/>"""

     def handleStartTag(self):
         "@sig public int handleStartTag()"
         self.out.print(
             eval(self.expr,
                 {'request':self.request,
                 'response':self.response,
                 'session':self.session,
                 'pageContext':self.pageContext}))
         return self.SKIP_BODY

     def setExpr(self, expr):
         "@sig public void setExpr(String expr)"
         self.expr = expr
       
 

========= BaseTag.java =========

package samples;

import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpSession;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.PageContext;
import javax.servlet.jsp.tagext.TagSupport;

public abstract class BaseTag extends TagSupport {

    /**
     * Returns the page context.
     *
     * @return JSP pageContext implicit variable
     */
    public PageContext getPageContext() {
        return pageContext;
    }

    /**
     * Returns the request.
     *
     * @return JSP request implicit variable
     */
    public ServletRequest getRequest() {
        return pageContext.getRequest();
    }

    /**
     * Returns the response.
     *
     * @return JSP response implicit variable
     */
    public ServletResponse getResponse() {
        return pageContext.getResponse();
    }

    /**
     * Returns the session.
     *
     * @return JSP session implicit variable
     */
    public HttpSession getSession() {
        return pageContext.getSession();
    }

    /**
     * Returns the output writer.
     *
     * @return JSP out implicit variable
     */
    public JspWriter getOut() {
        return pageContext.getOut();
    }

    /*
     * @see javax.servlet.jsp.tagext.Tag#doStartTag()
     */
    public int doStartTag() throws JspException {
        try {
            return handleStartTag();
        } catch (RuntimeException e) {
            throw new JspException(e);
        }
    }

    /**
     * Handles tag functionality. This method does not throw an exception and
     * thus can be overridden by jython classes using sig-strings.
     */
    abstract public int handleStartTag();
}