标签getBodyContent()输出为NULL，为什么

package com.tags;

import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;
import java.io.*;

public class Body
extends BodyTagSupport {

public int doStartTag() throws JspException {
System.out.println("f==" + getBodyContent());
return EVAL_BODY_TAG;
}

public int doEndTag() throws JspException {
return EVAL_PAGE;
}
}

输出f==null

test.jsp
...

eeeeeeeeeeeeeeeee

*.tld

example2
com.tags.Body
JSP