JavaServer Pages, 3rd Edition

Using Explicit Parent-Child Cooperation

Let’s look at a possible implementation of the <sql:param> tag handler as one example of explicit parent-child cooperation. As you may recall from Chapter 12, this action can be nested within the body of either an <sql:query> or an <sql:update> action:

<sql:update sql="UPDATE Employee SET Salary = ? WHERE EmpId = ?">
  <sql:param value="${param.newSalary}" />
  <sql:param value="${param.empId}" />
</sql:update>

How does the <sql:param> action tell the enclosing <sql:update> action about the parameter it defines? The answer to this question lies in a couple of SimpleTag and Tag interface methods that I didn’t cover in Chapter 21, plus a utility method implemented by both the SimpleTagSupport class and the TagSupport class.

The interface methods are setParent( ) and getParent( ), implemented like this by the TagSupport class:

...
private Tag parent;
...
public void setParent(Tag t) {
    parent = t;
}

public Tag getParent(  ) {
    return parent;
}

These two methods are standard accessor methods for the parent instance variable. The SimpleTagSupport implementation differs only in that the parent’s type is JspTag—the common superclass for Tag and SimpleTag—instead of Tag.

For a nested action element, the setParent( ) method is always called on the tag handler with a reference to the enclosing tag handler as its value. This way a nested tag handler always knows its parent. So a tag handler at any nesting level can ask for its parent, using getParent( ), and ...