jsp自定义标签

一、标签语言特点

<开始标签 属性=“属性值”>标签体</结束标签>

二、标签类别

1、ui标签 c:out
特点是显示数据，并且数据不是来源于标签体的，而是来源于jsp标签本身

2、 控制标签 if /foreach/c
特点是控制的对象是标签体

3、 数据标签 set
特点是存储数据，没有任何的页面效果

三、自定义标签的开发及使用步骤

1 、创建一个标签助手类(继承BodyTagSupport)
标签属性必须助手类的属性对应、且要提供对应get/set方法
rtexprvalue

2、 创建标签库描述文件(tld)，添加自定义标签的配置
注：tld文件必须保存到WEB-INF目录或其子目录

3 、在JSP通过taglib指令导入标签库，并通过指定后缀
访问自定义标签

四、 标签生命周期图

SKIP_BODY：跳过主体
EVAL_BODY_INCLUDE：计算标签主体内容并[输出]
EVAL_BODY_BUFFERED：计算标签主体内容并[缓存]
EVAL_PAGE:计算页面的后续部分
SKIP_PAGE:跳过页面的后续部分
EVAL_BODY_AGAIN:再计算主体一次

五、自定义 c:out/c:foreach/c:if/c:set标签

1.out标签

package com.cjq.jsp.tag;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.tagext.BodyTagSupport;
/**
 * out属于UI（需要展示效果，是依靠标签升序展示页面效果）
 * JspWriter
 * 
 * @author cjq
 *
 */public class OutTag extends BodyTagSupport{ private static final long serialVersionUID = 2163790218700691365L;
 private Object value;
 public Object getValue() {
  return value;
 }
 public void setValue(Object value) {
  this.value = value;
 }
 @Override
 public int doStartTag() throws JspException {
  JspWriter out=pageContext.getOut();
  try {
   out.print(value);
  } catch (Exception e) {
   e.printStackTrace();
  }
  return super.doStartTag();
 }}

2.set标签

package com.cjq.jsp.tag;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.BodyTagSupport;
/**
 * 数据标签（不需要展示内容）
 * 作用：
 *    是将value值赋给变量var
 * 
 * @author cjq
 *
 */public class SetTag extends BodyTagSupport{
 private String var;
 private Object value;
 
 public String getVar() {
  return var;
 }
 public void setVar(String var) {
  this.var = var;
 }
 public Object getValue() {
  return value;
 }
 public void setValue(Object value) {
  this.value = value;
 }
 
 @Override
 public int doStartTag() throws JspException {
  pageContext.setAttribute(var, value);
  return SKIP_BODY;
 }}

3.if标签

package com.cjq.jsp.tag;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.BodyTagSupport;
/**
 * if属于控制标签(页面展示效果依赖的是标签体)
 * @author cjq
 *
 */
public class IfTag extends BodyTagSupport {
 private static final long serialVersionUID = -8065316329441275709L;
 private boolean test; public boolean isTest() {
  return test;
 } 
 public void setTest(boolean test) {
  this.test = test;
 }
  public int doStartTag() throws JspException {
  return test ? EVAL_BODY_INCLUDE : SKIP_BODY;
 }}

4.foreach标签

package com.cjq.jsp.tag;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.BodyTagSupport;
public class ForeachTag extends BodyTagSupport {
 private static final long serialVersionUID = -2888366640462823432L;
 private String var;
 private List<Object> items = new ArrayList<>(); public String getVar() {
  return var;
 } 
 public void setVar(String var) {
  this.var = var;
 } 
 public List<Object> getItems() {
  return items;
 }
  public void setItems(List<Object> items) {
  this.items = items;
 } 
 public int doStartTag() throws JspException {
  Iterator<Object> it = items.iterator();
  pageContext.setAttribute(var, it.next());
  pageContext.setAttribute("it", it);
  return EVAL_BODY_INCLUDE;
 }
  public int doAfterBody() throws JspException {
//  获取原来状态的迭代器，而非创建一个迭代器
     Iterator<Object> it=(Iterator<Object>) pageContext.getAttribute("it");
        if(it.hasNext()) {
         pageContext.setAttribute(var, it.next());
      pageContext.setAttribute("it", it);
         return EVAL_BODY_AGAIN;
        }else {
         return EVAL_PAGE; //EVAL_PAGE 计算页面后续的部分
        }
 }}

5.select标签

package com.cjq.jsp.tag;
import java.io.IOException;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.tagext.BodyTagSupport;import org.apache.commons.beanutils.PropertyUtils;/**
 * select 的功能：需求
 * 1、新增查询页面，只要通过一个标签就可以完成数据的绑定，而不用使用c:foreach去循环绑定数据
 * 2、修改页面，同样通过这个标签完成数据的遍历展示以及默认选中指定向
 * 
 * 需要：
 * 1、需要往后台传值  id，name
 * 2、定义数据库存储的对应的标签属性、前台展示的标签属性  textkey   textval
 * 3、定义下拉框的默认值 headkey  headval
 * 4、下拉框加载数据 items
 * 5、属性值接受数据库中保存的value值  被选中的值  selectedval
 * 
 * 属于展示的ui标签，要展示内容
 * @author cjq
 *
 */
 public class SelectTag extends BodyTagSupport{
 private static final long serialVersionUID = 8680576328606908002L;
 private String id;
 private String name;
 private List<Object> items = new ArrayList<>();
 private String textkey;
 private String textval;
 private String headkey;
 private String headval;
 private String selectedval;
 public String getId() {
  return id;
 }
 public void setId(String id) {
  this.id = id;
 }
 public String getName() {
  return name;
 }
 public void setName(String name) {
  this.name = name;
 }
 public List<Object> getItems() {
  return items;
 }
 public void setItems(List<Object> items) {
  this.items = items;
 }
 public String getTextkey() {
  return textkey;
 }
 public void setTextkey(String textkey) {
  this.textkey = textkey;
 }
 public String getTextval() {
  return textval;
 }
 public void setTextval(String textval) {
  this.textval = textval;
 }
 public String getHeadkey() {
  return headkey;
 }
 public void setHeadkey(String headkey) {
  this.headkey = headkey;
 }
 public String getHeadval() {
  return headval;
 }
 public void setHeadval(String headval) {
  this.headval = headval;
 }
 public String getSelectedval() {
  return selectedval;
 }
 public void setSelectedval(String selectedval) {
  this.selectedval = selectedval;
 }
 
 @Override
 public int doStartTag() throws JspException {
  JspWriter out = pageContext.getOut();
  
  try {
   //能够在也面上看到select下拉框的效果，原因在于后台做了html代码的拼接
   out.print(toHTML());
  } catch (Exception e) {
   e.printStackTrace();
  }
  return super.doStartTag();
 }
 
 private String toHTML() throws Exception, Exception {
  StringBuilder sb = new StringBuilder();
  sb.append("<select id='"+id+"' name='"+name+"'>");
  if(!(headkey == null || "".equals(headkey) || headval == null || "".equals(headval))) {
   sb.append("<option selected value='"+headkey+"'>"+headval+"</option>");
  }
  String val = "";//获取对象ID
  String html = "";
  
  for (Object obj : items) {
   //底层代码
   Field f = obj.getClass().getDeclaredField(textkey);
   f.setAccessible(true);
   val = (String) f.get(obj);
   f = obj.getClass().getDeclaredField(textval);
   f.setAccessible(true);//打开权限
   html = (String) f.get(obj);
   
   //第三方工具包写法：（commons-beanutils-1.8.0 和 commons-logging）
//   val = (String) PropertyUtils.getProperty(obj, textkey);
//   html = (String) PropertyUtils.getProperty(obj, textval);
   
   //考虑如果是修改页面时，下拉框回选数据库所存储的值 也就是默认选中数据库所存储的值
   if(val.equals(selectedval)) {
    sb.append("<option selected value='"+val+"'>"+html+"</option>");
   }else {
    sb.append("<option value='"+val+"'>"+html+"</option>");
   }
  }
  
  sb.append("</select>");
  return sb.toString();//返回拼接的值
 }
 
}

6.配置描述文件tld

<?xml version="1.0" encoding="UTF-8" ?>
<taglib xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
 version="2.0"> <description>cjq 1.1 core library</description>
 <display-name>cjq core</display-name>
 <tlib-version>1.1</tlib-version>
 <short-name>c</short-name>
 <uri>/cjq</uri>
 
    <!-- demo  -->
 <tag>
  <!-- 填写的是标签库中的标签名 -->
  <name>demo</name>
  <!-- 标签对应后台助手类 -->
  <tag-class>com.cjq.jsp.tag.DemoTag</tag-class>
  <!-- 标签类别 -->
  <body-content>JSP</body-content>
  <attribute>
  <!-- 自定义标签中属性 -->
   <name>test</name>
   <!-- 属性值是否必填 -->
   <required>true</required>
   <!-- 是否支持表达式EL表达式/ognl表达式 -->
   <rtexprvalue>false</rtexprvalue>
  </attribute>
 </tag>
 
 <!-- set  -->
 <tag>
  <name>set</name>
  <tag-class>com.cjq.jsp.tag.SetTag</tag-class>
  <body-content>JSP</body-content>
  <attribute>
   <name>var</name>
   <required>true</required>
   <rtexprvalue>false</rtexprvalue>
  </attribute>
  <attribute>
   <name>value</name>
   <required>true</required>
   <rtexprvalue>true</rtexprvalue>
  </attribute>
 </tag>
 
 <!-- out -->
 <tag>
  <name>out</name>
  <tag-class>com.cjq.jsp.tag.OutTag</tag-class>
  <body-content>JSP</body-content>
  <attribute>
   <name>value</name>
   <required>true</required>
   <rtexprvalue>true</rtexprvalue>
  </attribute>
 </tag>
 
 <!-- if -->
 <tag>
  <name>if</name>
  <tag-class>com.cjq.jsp.tag.IfTag</tag-class>
  <body-content>JSP</body-content>
  <attribute>
   <name>test</name>
   <required>true</required>
   <rtexprvalue>true</rtexprvalue>
  </attribute>
 </tag>
 
 <!-- foreach -->
 <tag>
  <name>foreach</name>
  <tag-class>com.cjq.jsp.tag.ForeachTag</tag-class>
  <body-content>JSP</body-content>
  <attribute>
   <name>var</name>
   <required>true</required>
   <rtexprvalue>false</rtexprvalue>
  </attribute>
  <attribute>
   <name>items</name>
   <required>true</required>
   <rtexprvalue>true</rtexprvalue>
  </attribute>
 </tag>
 
 <!-- select -->
 <tag>
    <name>select</name>
    <tag-class>com.cjq.jsp.tag.SelectTag</tag-class>
    <body-content>JSP</body-content>
    <attribute>
        <name>id</name> 
        <required>false</required>
        <rtexprvalue>false</rtexprvalue>
    </attribute>
    <attribute>
        <name>name</name>
        <required>false</required>
        <rtexprvalue>false</rtexprvalue>
    </attribute>
    <attribute>
        <name>items</name>
        <required>true</required>
        <rtexprvalue>true</rtexprvalue>
    </attribute>
    <attribute>
        <name>textkey</name>
        <required>true</required>
        <rtexprvalue>false</rtexprvalue>
    </attribute>
    <attribute>
        <name>textval</name>
        <required>true</required>
        <rtexprvalue>false</rtexprvalue>
    </attribute>
    <attribute>
        <name>headkey</name>
        <required>false</required>
        <rtexprvalue>false</rtexprvalue>
    </attribute><attribute>
        <name>headval</name>
        <required>false</required>
        <rtexprvalue>false</rtexprvalue>
    </attribute>
    <attribute>
        <name>selectedval</name>
        <required>false</required>
        <rtexprvalue>true</rtexprvalue>
    </attribute>
  </tag></taglib>

7.加入jsp动态网页

<%@page import="com.cjq.jsp.entity.Teacher"%>
<%@page import="java.util.List"%>
<%@page import="java.util.ArrayList"%>
<%@ page language="java" contentType="text/html; charset=utf-8"
    pageEncoding="utf-8"%>
    <%@ taglib prefix="z" uri="/cjq" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Insert title here</title>
</head>
<body>
<h2>set、out标签</h2>
<z:set var="name" value="zs"></z:set>
<z:out value="${name }"></z:out>
<h2>if标签</h2>
<z:if test="true">ls</z:if>
<z:if test="false">ww</z:if><h2>list标签</h2>
<%
List list=new ArrayList();
list.add(new Teacher("1", "cjq1"));
list.add(new Teacher("2", "cjq2"));
list.add(new Teacher("3", "cjq3"));
pageContext.setAttribute("teas", list);
%>
<z:foreach items="${teas }" var="t">
</z:foreach><h2>新增查询下拉框</h2>
<z:select textkey="tid" items="${teas }" textval="tname"></z:select>
<z:select headkey="-1" headval="--请选择--" textkey="tid" items="${teas }" textval="tname"></z:select><h2>修改页面下拉框</h2>
<z:select  textkey="tid" items="${teas }" textval="tname" selectedval="cjq"></z:select></body>
</html>

测试结果如下：