The content of this article is mainly to use customization JSP Labels are more skilled

One 、 Customize foreach label

The first step is to create a label processing class
ForeachTag.java

package com.zking.mymvc.tag;

import java.util.Iterator;
import java.util.List;
import java.util.Objects;

import javax.servlet.jsp.tagext.BodyTagSupport;

/** *  Helper , Tag handling class  * @author zjjt * */
public class ForeachTag extends BodyTagSupport {
    
	//  I want to make a legal label processing class for him , Then we must extends BodyTagSupport
	
	// Store data source 
	private List<?> items;// because foreach Labels need to be cycled , So you need a cyclic result set 
	
	// The objects obtained in each cycle are put into pageContext in , And var The value of the property is key Preservation 
	// Example ：  Label on page var Property specified as item, Then the objects taken out in each cycle （obj） This will be done as follows ：
	//pageContext.setAttribute("item", obj); 
	// You can use EL Expression gets the attribute in the object ,  Such as ： ${item.name}
	private String var;

	// need get（ Don't write ） and set Method 

	public void setItems(List<?> items) {
    
		this.items = items;
	}

	public void setVar(String var) {
    
		this.var = var;
	}
	
	@Override
	public int doStartTag() {
    
		// The next step is judgment  
		if(Objects.isNull(this.items)||this.items.size()<=0) {
    // It is empty or the set is empty 
			return SKIP_BODY;// There is no data to process , Just skip 
		}
		Iterator<?> it = this.items.iterator();// iterator 
		Object next = it.next();// Got an object 
		this.pageContext.setAttribute(var, next);// Get page object , Put objects in pageContext Scope （ Another page is available ）
		this.pageContext.setAttribute("iterator", it);// Put the iterator in pageContext
		
		return EVAL_BODY_INCLUDE;// Return to this to continue processing the label body , call doAfterBody
	}
	
	@Override
	public int doAfterBody() {
    
		// What we need to do is to judge whether this method has value , If there is value, continue to return the call 
		Iterator<?> it = (Iterator<?>)this.pageContext.getAttribute("iterator");// This is the iterator obtained 
		if(it.hasNext()) {
    // If you find that there is still value in it 
			this.pageContext.setAttribute(var, it.next());
			return EVAL_BODY_AGAIN;
		}
		return SKIP_BODY;
	}
}

The second step is to establish a test data auxiliary class
Book.java

package com.zking.mymvc.model;

public class Book {
    
	
	private int id;
	
	private String name;

	public int getId() {
    
		return id;
	}

	public void setId(int id) {
    
		this.id = id;
	}

	public String getName() {
    
		return name;
	}

	public void setName(String name) {
    
		this.name = name;
	}

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

}

TestData.java

package com.zking.mymvc.model;

import java.util.ArrayList;
import java.util.List;

public class TestData {
    
	
	public static List<Book> getBooks(){
    
		
		List<Book> books = new ArrayList<>();
		
		Book b1 =  new Book();
		b1.setId(1);
		b1.setName(" Water margin ");
		Book b2 =  new Book();
		b2.setId(2);
		b2.setName(" A dream of red mansions ");
		Book b3 =  new Book();
		b3.setId(3);
		b3.setName(" Journey to the west ");
		
		books.add(b1);
		books.add(b2);
		books.add(b3);
		
		return books;
	}

}

The third step is to write in the tag description library foreach

<tag>
		<name>foreach</name>
		<tag-class>com.zking.mymvc.tag.ForeachTag</tag-class>
		<body-content>jsp</body-content>
		<description>foreach label </description>
		<attribute>
			<name>items</name>
			<required>true</required>
			<rtexprvalue>true</rtexprvalue>
		</attribute>
		<attribute>
			<name>var</name>
			<required>true</required>
			<rtexprvalue>false</rtexprvalue>
		</attribute>
	</tag>

We will test it at the end
The next step is to customize select label

Two 、 Customize select label

It's the same , The first step is to establish a label processor
SelectTag.java

package com.zking.mymvc.tag;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.util.List;

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

import org.apache.commons.beanutils.BeanUtils;

import com.mysql.jdbc.StringUtils;

public class SelectTag extends BodyTagSupport {
    
	
	// The attributes defined below are select The properties of the tag 
	private String id;
	
	private String name;
	
	private List<?> items;
	
	private String cssClass;
	
	private String cssStyle;
	
	private String value;
	
	private String text;
	
	private String selectValue;

	// Below is set Method ,get The method is not written 
	public void setId(String id) {
    
		this.id = id;
	}

	public void setName(String name) {
    
		this.name = name;
	}

	public void setItems(List<?> items) {
    
		this.items = items;
	}

	public void setCssClass(String cssClass) {
    
		this.cssClass = cssClass;
	}

	public void setCssStyle(String cssStyle) {
    
		this.cssStyle = cssStyle;
	}
	
	
	public void setValue(String value) {
    
		this.value = value;
	}

	public void setText(String text) {
    
		this.text = text;
	}

	public void setSelectValue(String selectValue) {
    
		this.selectValue = selectValue;
	}

	// Override the doStartTag Method 
	@Override
	public int doStartTag() {
    
		
		JspWriter out = this.pageContext.getOut();
		
		
		try {
    
			String html = getSelectHtml();
			out.print(html);
		} catch (Exception e) {
    
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		return SKIP_BODY;
	}
	
	private String getSelectHtml() throws IllegalAccessException, InvocationTargetException, NoSuchMethodException {
    
		StringBuilder sb = new StringBuilder();// Is to put index.jsp That page is dead select Put the label here 
		
		// This piece of construction page needs select Elements , After the construction is completed, it will be doStartTag() Write this output method this.pageContext.getOut();
		// The main function of this block is to write dead on the page select Elements , Generate by string splicing , And then through this.pageContext.getOut() Throw it on the page 
		// Functions that can also be done , Is whether the tag has a default value , With default values, we can directly make the default values into the display status 
		
		sb.append("<select id='"+id+"' name='"+name+"'");
		
		// then cssClass It's optional , Then we have to judge 
		if(StringUtils.isNullOrEmpty(this.cssClass)) {
    // If this.cssClass Not empty words , We are going to add （append It means adding ）,class=null
			sb.append("class='"+this.cssClass+"'");
		}
		
		if(StringUtils.isNullOrEmpty(this.cssStyle)) {
    
			sb.append("style='"+this.cssStyle+"'");
		}
		
		sb.append(">");
		
		// Next is to generate circularly select Labeled option Elements 
		for(Object option:this.items) {
    
			String val = BeanUtils.getProperty(option, this.value);
			String txt = BeanUtils.getProperty(option, this.text);
			// Will give me a result set , Get what the role is value and text
			// And then concatenate the string 
			if(val.equals(this.selectValue)) {
    // Write default 
				sb.append("<option value='"+val+"' selected>" + txt + "</option>");
			} else {
    //
				sb.append("<option value='"+val+"' >" + txt + "</option>");
			}
		}
		
		sb.append("</select>");
		
		return sb.toString();
	}

}

The third step is to write in the tag description library select

<tag>
		<name>select</name>
		<tag-class>com.zking.mymvc.tag.SelectTag</tag-class>
		<body-content>empty</body-content>
		<attribute>
			<name>id</name>
			<required>true</required>
			<rtexprvalue>false</rtexprvalue>
		</attribute>
		<attribute>
			<name>name</name>
			<required>true</required>
			<rtexprvalue>false</rtexprvalue>
		</attribute>
		<attribute>
			<name>items</name>
			<required>true</required>
			<rtexprvalue>true</rtexprvalue>
		</attribute>
		<attribute>
			<name>cssClass</name>
			<required>false</required>
			<rtexprvalue>false</rtexprvalue>
		</attribute>
		<attribute>
			<name>cssStyle</name>
			<required>false</required>
			<rtexprvalue>false</rtexprvalue>
		</attribute>
		<attribute>
			<name>value</name>
			<required>true</required>
			<rtexprvalue>false</rtexprvalue>
		</attribute>
		<attribute>
			<name>text</name>
			<required>true</required>
			<rtexprvalue>false</rtexprvalue>
		</attribute>
		<attribute>
			<name>selectValue</name>
			<required>false</required>
			<rtexprvalue>false</rtexprvalue>
		</attribute>
	</tag>

Another is to test whether the label is available

<%@page import="com.zking.mymvc.model.*,java.util.List" %>
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
    <!--  The first step is to introduce the tag library  -->
    <%@taglib prefix="z" uri="/zking" %><!--  Notice the uri With the tag library uri equally  -->
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>

<!-- foreach -->
	<%
	// Get test data 
		List<Book> books = TestData.getBooks(); 
		// Put in request In the object 
		request.setAttribute("books", books);
		%>
		
		<z:foreach items="${books}" var="book">
			${
    book.id} -- ${
    book.name} <br>
		</z:foreach>
	
	<!--  By the following select The label looks like , We can know which properties we need to define in the helper class  -->
<!-- 	<select id="test" name="test" class="" style="width:100px">
		<option value="1"> Paid </option>
		<option value="1" selected > Shipped </option>
		<option value="1"> Signed for </option>
	</select> -->
	
	
	<select id="test" name="test" class="" style="width:100px;">
		<option value=0> Paid </option>
		<option value= selected> Shipped </option>
		<option value=2> Signed for </option>
	</select>
	
	<z:select id="test" 
	   name="test" items="${books}" value="id"
	   text="name" selectValue="3"/>
	
	
</body>
</html>