Java Server Pages

Letting Actions Cooperate

Now that you’ve seen how to develop basic tag handlers, let’s discuss some more advanced features. In this section, we look at tag handlers that let a page author use custom actions that cooperate with each other.

You have seen examples of this throughout this book. For instance, in Chapter 9, various types of value actions are nested within the body of an <ora:sqlQuery> action to set the values of place holders in the SQL statement. Another example is the <ora:encodeURL> action with nested <ora:param> actions, which are used in Chapter 8:

<ora:encodeURL url="product1.jsp">
  <ora:param name="id" value="<%= product.getId( )%>" />
</ora:encodeURL>

How does the <ora:param> action tell the enclosing <ora:encodeURL> action about the parameter it defines? The answer to this question lies in a couple of Tag interface methods and a utility method implemented by the TagSupport class that I skipped earlier.

The Tag 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. For a nested action element, the setParent( ) method is always called on the tag handler with the value of the enclosing Tag as its value. This way, a nested tag handler always has a reference to its parent. So a tag handler at any nesting level can ask ...