public class MyReferer extends BodyTagSupport {
  private String site;
  private String back;
  public String getSite() {
    return site;
  }
  public void setSite(String site) {
    this.site = site;
  }
  public String getBack() {
    return back;
  }
  public void setBack(String back) {
    this.back = back;
  }
  public int doEndTag() throws JspException {
    // 获取JSP上下文环境对象
    PageContext pageContext = this.pageContext;
    // 获取到request对象
    HttpServletRequest request = (HttpServletRequest)pageContext.getRequest();
    // 判断
    String header = request.getHeader("referer");
    if(header != null && header.startsWith(getSite())){
      // 执行后续的页面
      return Tag.EVAL_PAGE;
    }else{
      // 页面的重定向
      HttpServletResponse response = (HttpServletResponse)pageContext.getResponse();
      try {
        response.sendRedirect(getBack());
      } catch (IOException e) {
        e.printStackTrace();
      }
      // 不执行
      return Tag.SKIP_PAGE;
    }
  }
}

<?xml version="1.0" encoding="UTF-8"?>
<taglib 
  xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
  version="2.1">
 <!-- 2. 编写标签库描述文件 --> 
 <tlib-version>1.0</tlib-version>
 <short-name>jnb</short-name>
  <tag> 
  <name>referer</name>
  <tag-class>cn.itcast.custom.MyReferer</tag-class>
  <body-content>empty</body-content>
   <attribute>
    <name>site</name>
    <required>true</required>
    <rtexprvalue>true</rtexprvalue>
  </attribute>
   <attribute>
    <name>back</name>
    <required>true</required>
    <rtexprvalue>true</rtexprvalue>
  </attribute>
 </tag>
</taglib> 

<%@taglib uri="/WEB-INF/referer.tld" prefix="my"%>
  <my:referer site=http://localhost:8080/day11/list.jsp
 back="/day11/list.jsp"/>

 <%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
  <c:choose>
   <c:when test="<%= 12>1 %>">
      大于
   </c:when>
   <c:otherwise>
      小于
   </c:otherwise>
  </c:choose>

public class ChooseTag extends SimpleTagSupport {
  private boolean tag = true;
  public boolean isTag() {
    return tag;
  }
  public void setTag(boolean tag) {
    this.tag = tag;
  }
  // 遇到标签自动执行
  public void doTag() throws JspException, IOException {
    // 获取标签体对象
    JspFragment body = this.getJspBody();
    // 执行标签体
    body.invoke(null);
    super.doTag();
  }
}

public class WhenTag extends SimpleTagSupport {
  private boolean test;
  public boolean isTest() {
    return test;
  }
  public void setTest(boolean test) {
    this.test = test;
  }
  // 遇到标签自动执行
  public void doTag() throws JspException, IOException {
    // 获取父元素
    ChooseTag choose = (ChooseTag)this.getParent();
    // 获取父元素的标记变量值
    boolean parent = choose.isTag();
    // 判断
    if( parent && this.isTest() ){
      // 执行标签体
      JspFragment body = this.getJspBody();
      body.invoke(null);
    }
    super.doTag();
  }
}

public class OtherwiseTag extends SimpleTagSupport {
  
  // 遇到标签自动执行
  public void doTag() throws JspException, IOException {
    // 获取父元素
    ChooseTag choose = (ChooseTag)this.getParent();
    // 获取父元素的标记变量值
    boolean parent = choose.isTag();
    // 判断
    if(parent){
      // 执行标签体
      JspFragment body = this.getJspBody();
      body.invoke(null);
    }
    super.doTag();
  }
}

<?xml version="1.0" encoding="UTF-8"?>
<taglib 
  xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd"
  version="2.1">
 <!-- 2. 编写标签库描述文件 --> 
 <tlib-version>1.0</tlib-version>
 <short-name>jnb</short-name>
  <tag> 
  <name>choose</name>
  <tag-class>cn.itcast.tags.ChooseTag</tag-class>
  <body-content>scriptless</body-content>   JSP2.0方式
 </tag>
 <tag> 
  <name>when</name>
  <tag-class>cn.itcast.tags.WhenTag</tag-class>
  <body-content>scriptless</body-content>
  <attribute>
    <name>test</name>
    <required>true</required>
    <rtexprvalue>true</rtexprvalue>
  </attribute>
 </tag>
  
  <tag> 
  <name>otherwise</name>
  <tag-class>cn.itcast.tags.OtherwiseTag</tag-class>
  <body-content>scriptless</body-content>
 </tag>
</taglib>

<%@taglib uri="/WEB-INF/ifelse.tld" prefix="jnb"%>
   <jnb:choose>
    <jnb:when test="<%= 1>2 %>">
        小于
    </jnb:when>
    <jnb:otherwise>
         大于
    </jnb:otherwise>
   </jnb:choose>