How to customize JSP Tags

Catalog

One 、 Characteristics of tag language

Two 、 The development of user-defined label and its application steps

3、 ... and 、 The life cycle of labels

Four 、 Practice custom labels ：if、set、out

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One 、 Characteristics of tag language

1.1  Before we want to customize our own labels , We must first understand

Label structure ：

We'll take  if  Tag as an example ：

Understand the label structure  
<c:if test="true">         The start tag
       true                      Tagging body
</c:if>                         End tag

Let's look at the classification of labels ： Control tags 、 Data labels 、UI label

Tags that can output content in a web page without a tag body are ：UI label

After testing, I found ： Click on c:if The tag can jump to c.tld file in , and c.tld file Namely c Tag library definition profile ！

	<c:if test="true">true</c:if>
	<c:if test="false">false</c:if>
	<c:set var="l" value="Leaf"></c:set>
	<c:out value="${l }"></c:out>

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Two 、 The development of user-defined label and its application steps

2.1 After understanding some basic structures and characteristics of labels , Let's explore further ,

Click  if  label Find out Custom tags are associated with tld file Relevant ;

We found that c.tld file Medium tag Elements The relevant tags cannot be used after deletion ;

So we get two conclusions ：

    <!-- 
        1、 Custom tags are associated with tld Document related
        2、 Tags in tag library and tld Medium tag The elements are related to , That is to say, with tag Element is related to the corresponding helper class .
    -->

  So we thought if we put this tld Make a copy of the document , Change what needs to be changed into your own , Can it also be used ？

So we put tld Make a copy of the document ：Leaf.tld, According to the introduced code ：

<%@ taglib prefix="l" uri="http://jsp.leaf.com" %>

We thought about tld Is there this in the file uri, therefore Ctrl+F Look for , As expected tld Found this in the file uri, So we changed it into our own definition uri：

  <description>JSTL 1.1 core library</description>
  <display-name>JSTL core</display-name>
  <tlib-version>1.1</tlib-version>
  <short-name>c</short-name>
  <uri>http://jsp.leaf.com</uri>

And then create a new one Jsp file , Introduce the customized tld file , It was found that there was ！

 

Then we use this self-defined tld File test those labels to see if they can be used ：

notes ： Copy of the tld The file must be saved in WEB-IFN In a directory or subdirectory

	<l:if test="true">true</l:if>
	<l:if test="false">false</l:if>
	<l:set var="l" value="Leaf"></l:set>
	<l:out value="${l }"></l:out>

  Run web page ：

  We found normal availability ！

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3、 ... and 、 The life cycle of labels

3.1 We also learned about custom tags tld file 、tag Elements ,

Next, let's take a look at its life cycle ：

Let's create a new class , Inherit tag The helper class corresponding to the element ：BodyTagSupport

DemoTag1 class ：

package com.leaf.tag;

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

/**
 *  Helper ： Must inherit bodytagSupport
 * @author Leaf
 *
 * 2022 year 6 month 17 Japan   Afternoon 7:32:58
 */
public class DemoTag1 extends BodyTagSupport {
	
	@Override
	public int doStartTag() throws JspException {
		System.out.println("========= The start tag doStartTag=========");
		//return SKIP_BODY;// Skip the label body 
		return EVAL_BODY_INCLUDE;
	}
	
	@Override
	public int doAfterBody() throws JspException {
		System.out.println("========= Tagging body doAfterBody=========");
		return super.doAfterBody();
		//return EVAL_BODY_AGAIN;// Loop label body 
	}
	
	@Override
	public int doEndTag() throws JspException {
		System.out.println("========= End tag doEndTag=========");
		return super.doEndTag();
		//return SKIP_PAGE;// Don't calculate the following content 
	}
	
}

After many running tests, we get some Conclusion ：

<!-- 
    1、 With label body , By default, the helper class will be called doStartTag、doAfterBody、doEndTag Method ;
    2、 If the start tag doStartTag The value returned is SKIP_BODY, Then label body doAfterBody There will be no call to execute ;
    3、 If the label will start doStartTag The return value of is changed to EVAL_BODY_INCLUDE, Then label body doAfterBody Will call to execute ;
     4、 If the label will start doStartTag The return value of is changed to EVAL_BODY_AGAIN, Then the tag body will be called all the time doAfterBody Method , Into the loop ;
     5、 If the end tag doEndTag The return value of is changed to SKIP_PAGE, Then the following content will not be calculated .
 -->

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Four 、 Practice custom labels ：if、set、out

Learned so much theoretical knowledge , Most of them are experimental tests , Not very good-looking running results ,

Next, we will use our theoretical knowledge to practice ：

4.1 Customize a few of your own tags ~

4.2 if Definition of label

We need to build one first if The tag class , Inherit helper class ： Define a if attribute test：boolean type

package com.leaf.tag;

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

/**
 * if label ：
 *  analysis 
 	*  If the conditions are met , Just print the label body --->doStartTag The return value of ：EVAL_BODY_INCLUDE
 	*  If you don't do that , The label body is not output --->doStartTag The return value of ：SKIP_BODY
 *  You need to get the result value of whether the condition is met , Then the tag has an attribute , The property value is boolean
 * @author Leaf
 *
 * 2022 year 6 month 17 Japan   Afternoon 8:29:39
 */
public class IfTag extends BodyTagSupport {
	
	private boolean test;
	
	public boolean isTest() {
		return test;
	}

	public void setTest(boolean test) {
		this.test = test;
	}

	@Override
	public int doStartTag() throws JspException {
		return test ? EVAL_BODY_INCLUDE : SKIP_BODY;
	}
	
}

  After we write this tag class, we will go Leaf.tld Write the relevant configuration in the file , We delete all other configurations ,

  Leave a template ：

4.3 To configure tld file ：<c:if test=""></c:if>

<?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>JSTL 1.1 core library</description>
  <display-name>JSTL core</display-name>
  <tlib-version>1.1</tlib-version>
  <short-name>l</short-name>
  <uri>http://jsp.leaf.com</uri>

  <validator>
    <description>
        Provides core validation features for JSTL tags.
    </description>
    <validator-class>
        org.apache.taglibs.standard.tlv.JstlCoreTLV
    </validator-class>
  </validator>

  <tag>
  	<!--  Represents the name of the tag library tag  -->
    <name>if</name>
    <!--  The helper class corresponding to this tag  -->
    <tag-class>com.leaf.tag.IfTag</tag-class>
    <!--  The representative is a JSP label  -->
    <body-content>JSP</body-content>
     <attribute>
    	<!--  This custom JSP The attribute name of the tag  -->
        <name>test</name>
        <!--  Whether this attribute is required  -->
        <required>true</required>
        <!--  Whether the attribute value supports the expression  -->
        <rtexprvalue>true</rtexprvalue>
     </attribute>
  </tag>

Let's use it to test ：

<l:if test="true">true</l:if>

<l:if test="false">false</l:if>

function ： 

  We found no problem ！

Then let's continue to try Set as well as Out The definition of label ：

Set The tag class ： This is where we get a place to store data ：pageContext

package com.leaf.tag;

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

/**
 *  Data labels ： Storing data 
 *  Scope ：pagecontext、request、session、application(servletContext)
 * 
 *  To store data , Store in the form of key value pairs , Analysis shows that the label has 2 Attributes 
 * @author Leaf
 *
 * 2022 year 6 month 18 Japan   In the morning 12:23:18
 */
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 {
		// To store data , Store in the form of key value pairs , Analysis shows that the label has 2 Attributes 
		pageContext.setAttribute(var, value);
		return super.doStartTag();
	}
	
}

Let's continue to define tag classes ：

Out The tag class ： We mainly get an output stream ：pageContext.getOut();

package com.leaf.tag;

import java.io.IOException;

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

/**
 * Out label 
 * servlet Background code   Foreground output ：out.print
 *  Output the data to the foreground , First get the output stream 
 * @author Leaf
 *
 * 2022 year 6 month 18 Japan   In the morning 12:32:10
 */
public class OutTag extends BodyTagSupport {
	
	private Object value;
	
	public Object getValue() {
		return value;
	}

	public void setValue(Object value) {
		this.value = value;
	}

	@Override
	public int doStartTag() throws JspException {
		// Get the output stream 
		JspWriter out = pageContext.getOut();
		try {
			// Output value
			out.print(value);
		} catch (IOException e) {
			e.printStackTrace();
		}
		return super.doStartTag();
	}
	
}

After writing the label class, we will go to Leaf.tld file Configure it ：

Leaf.tld File configuration ：

<tag>
    <name>set</name>
    <tag-class>com.leaf.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>
  
  <tag>
    <name>out</name>
    <tag-class>com.leaf.tag.OutTag</tag-class>
    <body-content>JSP</body-content>
     <attribute>
        <name>value</name>
        <required>true</required>
        <rtexprvalue>true</rtexprvalue>
     </attribute>
  </tag>

Finally, let's test it together ：

    <l:if test="true">true</l:if>
    <l:if test="false">false</l:if>
    <l:set var="l" value="Leaf"></l:set>
    <l:out value="${l }"></l:out>

Run web page ：

  We found that we all succeeded ~

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OK, such Leaf That's all for today's learning notes about custom tags ~