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14.8 Using Nested Tags

343

There are two key new approaches for nested tags, however. First, nested

tags can use 

findAncestorWithClass

 to find the tag in which they are

nested. This method takes a reference to the current class (e.g., 

this

) and the

Class

 object of the enclosing class (e.g., 

EnclosingTag.class

) as arguments.

If no enclosing class is found, the method in the nested class can throw a 

Jsp 

TagException

 that reports the problem. Second, if one tag wants to store data

that a later tag will use, it can place that data in the instance of the enclosing

tag. The definition of the enclosing tag should provide methods for storing and

accessing this data. Listing 14.22 outlines this approach. 

Listing 14.22 Template for Nested Tags 

public class OuterTag extends TagSupport {

  public void setSomeValue(SomeClass arg) { ... }

  public SomeClass getSomeValue() { ... } 

}

public class FirstInnerTag extends BodyTagSupport {

  public int doStartTag() throws JspTagException {

    OuterTag parent =

      (OuterTag)findAncestorWithClass(this, OuterTag.class);

    if (parent == null) {

      throw new JspTagException("nesting error");

    } else {

      parent.setSomeValue(...);

    }

    return(EVAL_BODY_TAG);

  }

  ...

}

public class SecondInnerTag extends BodyTagSupport {

  public int doStartTag() throws JspTagException {

    OuterTag parent =

      (OuterTag)findAncestorWithClass(this, OuterTag.class);

    if (parent == null) {

      throw new JspTagException("nesting error");

    } else {

      SomeClass value = parent.getSomeValue();

      doSomethingWith(value);

    }

    return(EVAL_BODY_TAG);

  }

  ...

}

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