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Custom Tag with Body (addition of body content)

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posted 11 years ago

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I was trying the example in the sun site 'http://java.sun.com/developer/technicalArticles/xml/WebAppDev3/' I have some problem trying to understand the last example 'Sample 13: LoopTag.java' which prints the tag's body to the times given in the tag's attribute 'times'.

The example seems to work fine, but I am not able to understand the flow of it

public class LoopTag extends BodyTagSupport {

int times = 0;
   BodyContent bodyContent;

public void setTimes(int times) {
      this.times = times;
   }

public int doStartTag() throws JspException {
      if (times>0) {
        return EVAL_BODY_TAG;
      } else {
         return SKIP_BODY;
      }
   }

public void setBodyContent(BodyContent 
   bodyContent) {
      this.bodyContent = bodyContent;
   }

public int doAfterBody() throws JspException {
      System.out.println(getBodyContent().getString()); // Here is where I print the BodyContent
      if (times >1) {
        times--;
        return EVAL_BODY_TAG;
      } else {
         return SKIP_BODY;
      }
   }

public int doEndTag() throws JspException {
      try {
         if(bodyContent != null) {
           bodyContent.writeOut(
           bodyContent.getEnclosingWriter());
         }
      } catch(IOException e) {
        throw new JspException(
        "Error: "+e.getMessage());
      }
      return EVAL_PAGE;
   }
}

The corresponding JSP page

I am sure that the setBodyContent(BodyContent bodyContent) method is called only once and doAfterBody() is called 4 times, but I am not sure where the addition to the BodyContent is made. I just include a sysout in the doAfterBody() to examine the contents of BodyContent

i.e.,

The first call to doAfterBody() prints

"Welcome to Custom Tags Programming.<BR>"

and the second call to doAfterBody() prints

"Welcome to Custom Tags Programming.<BR>"
"Welcome to Custom Tags Programming.<BR>"

and so on..

How does this occur in the background without the BodyContent variable being altered any where in the code and setBodyContent() method being called??

Thanks for the help

Stefan Evans

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Posts: 1845

posted 11 years ago

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The addition to the body happens in the standard part of the tag lifecycle.
I would suggest you go and read the API Docs on the body tag to find out about its lifecycle. Basically the tag is designed so that it will do work behind the scenes, and let you have control at certain points in the lifecycle.

The tag executes the body (as part of its lifecycle) which writes to the BodyContent object, and then calls the doAfterBody() method do find out what you want to do next.
The bodyContent variable itself doesn't change, but its not immutable. Its contents can be written to - which is what the "DO BODY" part of the lifecycle would do.

Also note that you are using constants that don't exactly fit with the Body tag.
EVAL_BODY_TAG has been deprecated. You should instead use EVAL_BODY_BUFFERED (has the same constant value, so it currently works).

The doAfterBody method should return EVAL_BODY_AGAIN rather than EVAL_BODY_TAG.
I guess thats just a really old article?

Kumar Jaya

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posted 11 years ago

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I went through the API docs of the BodyTag and BodyContent, The lifecycle of BodyTag is as follows "doStartTag(), setBodyContent(), doInitBody(), BODY, doAfterBody()". BodyContent is a subclass of JspWriter, so when a BODY lifecycle occurs can I assume the contents of the action are appended to the previous content?

The API for the BodyContent says "An encapsulation of the evaluation of the body of an action so it is available to a tag handler.", So I think evaluation of a Body results in content addition(also).

Stefan Evans wrote:The bodyContent variable itself doesn't change, but its not immutable. Its contents can be written to - which is what the "DO BODY" part of the lifecycle would do.

Also note that you are using constants that don't exactly fit with the Body tag.
EVAL_BODY_TAG has been deprecated. You should instead use EVAL_BODY_BUFFERED (has the same constant value, so it currently works).

The doAfterBody method should return EVAL_BODY_AGAIN rather than EVAL_BODY_TAG.
I guess thats just a really old article?

Yes the bodyContent variable itself doesn,t change, but how does the framework have access to my bodyContent variable (which can private also) if it has to change the contents? does it use some sort of reflection mechanism?

Its an old code from sun site. I am just trying to figure out the mechanism behind the scenes. I appreciate the reply, thanks.

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