Tag Handler Does Not Manipulate the Body

If a tag handler needs simply to evaluate the body, it gets the body using the getJspBody method of SimpleTag and then evaluates the body using the invoke method.

The following tag handler accepts a test parameter and evaluates the body of the tag if the test evaluates to true. The body of the tag is encapsulated in a JSP fragment. If the test is true, the handler retrieves the fragment using the getJspBody method. The invoke method directs all output to a supplied writer or, if the writer is null, to the JspWriter returned by the getOut method of the JspContext associated with the tag handler.

public class IfSimpleTag extends SimpleTagSupport {
    private boolean test;
    public void setTest(boolean test) {
        this.test = test;
    }
    public void doTag() throws JspException, IOException {
        if(test){
            getJspBody().invoke(null);
        }
    }
}

Tag Handler Manipulates the Body

If the tag handler needs to manipulate the body, the tag handler must capture the body in a StringWriter. The invoke method directs all output to a supplied writer. Then the modified body is written to the JspWriter returned by the getOut method of the JspContext. Thus, a tag that converts its body to uppercase could be written as follows:

public class SimpleWriter extends SimpleTagSupport {
    public void doTag() throws JspException, IOException {
        StringWriter sw = new StringWriter();
        jspBody.invoke(sw);
        jspContext().
            getOut().println(sw.toString().toUpperCase());
    }
}