JSP自定义标签

1. 标签语言特点

<开始标签 属性=“属性值”>标签体</结束标签>

2.三大类标签

2.1 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
2.2 控制标签 if /foreach/c
特点是控制的对象是标签体
2.3 数据标签 set
特点是存储数据，没有任何的页面效果

3.了解标签的生命周期图

 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

4.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

5.自定义set、out、if、foreach和select标签

测试用的Test1

package com.wangcong;

public class Test1 {
	private String id;
	private String name;

	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 Test1() {
		super();
	}

	public Test1(String id, String name) {
		super();
		this.id = id;
		this.name = name;
	}

	@Override
	public String toString() {
		return "Demo [id=" + id + ", name=" + name + "]";
	}
}

SetTag

package wangcong.tag;

import javax.servlet.jsp.JspException;

import javax.servlet.jsp.tagext.BodyTagSupport;

public class SetTag extends BodyTagSupport {
	private static final long serialVersionUID = 3022286660316203872L;
	private String var;//取名
	private Object value;//数据
	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 {
		//将得到的数据value赋给var
		pageContext.setAttribute(var, value);
		
		return SKIP_BODY;
	}
}

OutTag

package wangcong.tag;

import java.io.IOException;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.tagext.BodyTagSupport;

public class OutTag extends BodyTagSupport{

	/**
	 * 
	 */
	private static final long serialVersionUID = 3825465757059185456L;
	private Object value;//任意值

	public Object getValue() {
		return value;
	}

	public void setValue(Object value) {
		this.value = value;
	}
	
	@Override
	public int doStartTag() throws JspException {
		//将value写到页面去
		JspWriter out = pageContext.getOut();
		
		try {
			out.print(value);
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		return super.doStartTag();
	}
}

IfTag

package wangcong.tag;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.BodyTagSupport;

public class IfTag extends BodyTagSupport{
	private static final long serialVersionUID = -27885651335235798L;
	private boolean flag;
	public boolean isFlag() {
		return flag;
	}
	public void setFlag(boolean flag) {
		this.flag = flag;
	}
	
	@Override
	public int doStartTag() throws JspException {
		return flag ? EVAL_BODY_INCLUDE : SKIP_BODY;
	}
}

ForerchTag

package wangcong.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 = 4208690989932419067L;
	private String var;//名字
	private List<Object> value = new ArrayList<>();//传过来的数据组
	public String getVar() {
		return var;
	}
	public void setVar(String var) {
		this.var = var;
	}
	public List<Object> getValue() {
		return value;
	}
	public void setValue(List<Object> value) {
		this.value = value;
	}
	
	@Override
	public int doStartTag() throws JspException {
		
			//迭代器遍历传过来的数组
			Iterator it = value.iterator();
			
			pageContext.setAttribute(var, it.next());
			pageContext.setAttribute("it", it);
			return EVAL_BODY_INCLUDE;
		
		
	}
	
	@Override
	public int doAfterBody() throws JspException {
		//获取原来的it
		Iterator it = (Iterator) pageContext.getAttribute("it");
		
		if(it.hasNext()) {
			pageContext.setAttribute(var, it.next());
			pageContext.setAttribute("it", it);
			return EVAL_BODY_AGAIN;
		}else {
			
			return EVAL_PAGE;
		}
		
		
	}
}

SelectTag

package wangcong.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
 * 
 * 分析：
 * 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 10570
 *
 */

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 {
			//对你想展现在页面上的html代码进行拼接
			out.print(toHTML());
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		
		return super.doStartTag();
	}
	
	//拼接标签代码
	private String toHTML() throws Exception, Exception {
		StringBuilder sb = new StringBuilder();//更快
		sb.append("<select id='"+id+"' name='"+name+"'>");
		//加载类似于下拉框默认显示的 请选择 
		//判定是否有值 
		if(!(headkey == null || "".equals(headkey) || headval == null || "".equals(headval))) {
			sb.append("<option selected value='"+headkey+"'>"+headval+"</option>");
		}
		
		
		String val = "";//写option value 的
		String html = "";//写在option标签体的 展示看
		
		//循环打印数据 items = value 
		for (Object obj : items) {
			//根据反射读写属性：
			//获取对象：obj.getClass()
			//获取对象的指定属性  textkey变量 → 对象的属性名(textval同理)
			//打开权限
			//接收对应的属性值 
			//底层代码写法:
			Field f = obj.getClass().getDeclaredField(textkey);
			f.setAccessible(true);
			val = (String) f.get(obj);
			f = obj.getClass().getDeclaredField(textval);
			f.setAccessible(true);
			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
	}
	
}

z.tld

<?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">
    
  <description>onlyK 1.1 core library</description>
  <display-name>onlyK core</display-name>
  <tlib-version>1.1</tlib-version>
  <short-name>c</short-name>
  <uri>/onlyK</uri>


  <!-- set -->
	<tag>
    
    <name>set</name>
    <tag-class>wangcong.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>wangcong.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>wangcong.tag.IfTag</tag-class>
    <body-content>JSP</body-content>
    <attribute>
        <name>flag</name>
        <required>true</required>
        <rtexprvalue>true</rtexprvalue>
    </attribute>
  </tag>
  
  <!-- foreach -->
   <tag>
    
    <name>foreach</name>
    <tag-class>wangcong.tag.ForeachTag</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>
  
  <!-- select -->
  
  <!--  以 select 为例  -->
   <tag>
    
    <name>select</name>
    <tag-class>wangcong.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>

index.jsp

<%@page import="com.wangcong.Test1"%>
<%@page import="java.util.ArrayList"%>
<%@page import="java.util.List"%>
<%@ page language="java" contentType="text/html; charset=utf-8"
    pageEncoding="utf-8"%>
    <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
    <%@ taglib uri="/onlyK" prefix="z" %>
<!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>

<z:set var="name" value="zs"></z:set>
<z:out value="${name}"></z:out>
<z:if flag="true">lx</z:if>
<z:if flag="false">ww</z:if>
<%
List list=new ArrayList();
list.add(new Test1("1", "王聪"));
list.add(new Test1("2", "王聪2"));
list.add(new Test1("3", "王聪3"));
pageContext.setAttribute("teas", list);
%>

<z:foreach var="t" value="${teas}">
${t.id},${t.name}<br>
</z:foreach>
<h2>新增查询下拉框</h2>
<z:select textkey="id" items="${teas }" textval="name"></z:select>
<z:select headkey="-1" headval="请选择" textkey="id" items="${teas }" textval="name"></z:select>
<h2>修改页面下拉框</h2>
<z:select  textkey="id" items="${teas }" textval="name" selectedval="王聪"></z:select>
</body>
</html>

测试结果如下：