Custom JSTL tag of JSP

Catalog

One , What is? JSP

Two , What is? JSTL（JSTL Tag library ）

3、 ... and , How to use JSTL 

How to use JSTL label ？ Used in development JSTL The tag library needs to perform the following two steps .

（1） To quote in a project JSTL Of the two jar File and tag library descriptor file ( extension .tld).

（2） In need of use JSTL Of JSP Page usage taglib Instruction to import label library description file . for example , To use JSTL Core tag library , Need to be in JSP Add the following at the top of the page taglib command .

Four , Why use JSTL

JSTL The life cycle of labels

  Custom tag

1、 Custom label class

 2、 Write tag library description file （tld） 

3、 stay JSP Use custom labels on （ test ）

summary

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One , What is? JSP

• JSP（ Full name JavaServer Pages）,JSP take Java Code and specific changes are embedded in static pages , The implementation takes the static page as the template , Part of it is generated dynamically .JSP At run time, files are converted by their compiler to more primitive Servlet Code .JSP The compiler can put JSP The file is compiled with Java It's written in code Servlet, And then by Java Compiler to compile binary machine code that can be executed quickly , It can also be directly compiled into binary code . 
• Simply speaking ：JSP It's a template engine , It simplifies development , Encapsulates the servlet Respond to html Cumbersome code of labels .JSP Technology to Java Language as a scripting language , For users HTTP Request for services , And can communicate with others on the server Java Programs work together to address complex business requirements .
  JSP It's a special kind of servlet.  

Two , What is? JSTL（JSTL Tag library ）

JSTL The full name is Java Server Pages Standard Tag Library , namely JSP Standard label library . It involves developing JSP A set of standard tags that are often used on a page , These tags provide a way to avoid embedding Java Code can develop complex JSP Page approach .JSTL The tag library contains a variety of tags , Such as general label , Condition judgment tag and iteration tag .

The concept of labels

• It's markup language (Mark Language), It is a language for annotating text , The computer can be operated easily . A lot to do with “ML” The ending language is markup language , such as ：HTML,XML,XHTML,VML wait
• Markup language is the same as other languages , You also need the environment in which they run , such as HTML Runtime browser ,XML We also need our own parsing and running environment

Label type

• UI label , Output page elements
• Control tags , Such as if label ,foreach Labels etc.
• Data labels , Used to enter data into a page

The basic structure

• < The start tag > Tagging body </ End tag >

Empty label ( Tags without tag body )

• < The start tag Property name =" Property value "/></ End tag >
• <br/><br/>
• < The start tag Property name =" Property value "/>

jstl Concept of tag library

It's a JSP Label set , It encapsulates the JSP Common core functionality for applications , be based on JSP The label can be understood as , yes JSP It should be a way of encapsulating common functions

  

3、 ... and , How to use JSTL 

How to use JSTL label ？ Used in development JSTL The tag library needs to perform the following two steps .

（1） To quote in a project JSTL Of the two jar File and tag library descriptor file ( extension .tld).

Just like using JDBC Connect to the database , Use JSTL The defined label library must also import two in the project jar file ：jstl.jar and standard.jar. besides , Tag class library descriptor files are also required , These resources can be downloaded online .
stay MyEclipse Integrated in JSTL, Therefore, this step can be realized by tools .

（2） In need of use JSTL Of JSP Page usage taglib Instruction to import label library description file . for example , To use JSTL Core tag library , Need to be in JSP Add the following at the top of the page taglib command .

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>

taglib Command passed uti Attribute refers to the configuration file of a tag library ,JSP Page through prefix Property to access a tag function in the tag library , Grammar is like <C: Tag name >
After completing the above two steps , You can use it JSTL Easy to develop JSP page , Without embedding Java Code. .

Four , Why use JSTL

We use EL expression User name information is easily output , Thus, it simplifies JSP The complexity of page development . But because of EL Expressions cannot achieve logical control , Loop traversal and other functions , If you're developing JSP When you encounter such a demand on the page , In addition to writing Java Script , How else can we solve ？ The answer is to use JSTL label .

JSTL The life cycle of labels

  Custom tag

1、 Custom label class

• out The tag class （ Inherit BodyTagSupport）

/**
*     out  Function of label ： towards JSP Write data in the page 
*/
public class OutTag extends BodyTagSupport{  
  
    private String val;
	private String defaultVal;
 
	public void setVal(String val) {
		this.val = val;
	}
    // Method to set the default value when it is empty 
	public void setDefaultVal(String defaultVal) {
		this.defaultVal = defaultVal;
	}
    
    // How to start labels ( What actions need to be added to the customized label , Rewrite the methods in the corresponding life cycle )
	@Override
	public int doStartTag() throws JspException {
        //this.pageContext： Get through the current class pageContext object 
        //pageContext One of the objects is getOut Write the way 
		JspWriter out = this.pageContext.getOut();		
		try {
			if(val==null&&val.equals("")) {
				out.println(this.defaultVal);
			} else {
				out.println(this.val);
			}
		} catch (Exception e) {
			e.printStackTrace();
		}		
        // Skip the label body ,out What is defined is an empty tag 
		return SKIP_BODY;
	}
}/**
*     out  Function of label ： towards JSP Write data in the page 
*/
public class OutTag extends BodyTagSupport{  
  
    private String val;
	private String defaultVal;
 
	public void setVal(String val) {
		this.val = val;
	}
    // Method to set the default value when it is empty 
	public void setDefaultVal(String defaultVal) {
		this.defaultVal = defaultVal;
	}
    
    // How to start labels ( What actions need to be added to the customized label , Rewrite the methods in the corresponding life cycle )
	@Override
	public int doStartTag() throws JspException {
        //this.pageContext： Get through the current class pageContext object 
        //pageContext One of the objects is getOut Write the way 
		JspWriter out = this.pageContext.getOut();		
		try {
			if(val==null&&val.equals("")) {
				out.println(this.defaultVal);
			} else {
				out.println(this.val);
			}
		} catch (Exception e) {
			e.printStackTrace();
		}		
        // Skip the label body ,out What is defined is an empty tag 
		return SKIP_BODY;
	}
}

•  if The tag class （ Inherit BodyTagSupport）

@SuppressWarnings("serial")
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() { 		
		if(this.isTest()) {
            // When judging the label body 
			return EVAL_BODY_INCLUDE;
		} 
		return SKIP_BODY;		
	}
}

• foreach The tag class

public class ForeachTag extends BodyTagSupport {
	private List<?> items;
	private String var;
	public void setItems(List<?> items) {
		this.items = items;
	}
	public void setVar(String var) {
		this.var = var;
	}	
	@Override
	public int doStartTag() { 
		
		if(Objects.isNull(this.items) || this.items.size() <= 0) {
			return SKIP_BODY;
		}
		
		Iterator<?> it = this.items.iterator();
		Object next = it.next();
		this.pageContext.setAttribute(var, next);
		this.pageContext.setAttribute("iterator", it);
		
		return EVAL_BODY_INCLUDE;
	}
		@Override
	public int doAfterBody() {
		Iterator<?> it= (Iterator<?>)this.pageContext.getAttribute("iterator");
		if(it.hasNext()) {
			this.pageContext.setAttribute(var, it.next());
			return EVAL_BODY_AGAIN;
		}
		
		return SKIP_BODY;
	}	
}

•  select The tag class

public class SelectTag extends BodyTagSupport {	
	private String id;	
	private String name;
	private List<?> items;
	private String cssClass;
	private String cssStyle;
	private String value;
	private String text;
	private String selectValue;
 
	public void setText(String text) {
		this.text = text;
	}
	public void setValue(String value) {
		this.value = value;
	}
	public void setSelectValue(String selectValue) {
		this.selectValue = selectValue;
	}
	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;
	}		
	@Override
	public int doStartTag()  {		
		JspWriter out = this.pageContext.getOut();
		try {
			String html = getSelectHtml();
			out.print(html);
		} catch (Exception e) {
			e.printStackTrace();
		}		
		return SKIP_BODY;
	}
	// By injecting attribute values , To construct the select String of element 
	private String  getSelectHtml() throws Exception {
		StringBuilder sb = new StringBuilder();		
		// What is needed to construct the page select Elements 
		sb.append("<select id='"+id+ "' name='"+name+"'");
		if(!StringUtils.isNullOrEmpty(this.cssClass)) {
			sb.append(" class='"+this.cssClass+"'");
		}
		if(!StringUtils.isNullOrEmpty(this.cssStyle)) {
			sb.append(" style='"+this.cssStyle+"'");
		}
		sb.append(">");		
		// Cycle generation options
		for(Object option: this.items) {
			String val = BeanUtils.getProperty(option, this.value);
			String txt = BeanUtils.getProperty(option, this.text);
			//value="id"  text="name" 
			if(val.equals(this.selectValue)) {
				sb.append("<option value='"+val+"' selected>" + txt + "</option>");
			} else {
				sb.append("<option value='"+val+"' >" + txt + "</option>");
			}
		}
		sb.append("</select>");
		return sb.toString();
	}
}

 2、 Write tag library description file （tld） 

Edit the label you defined with the corresponding format specification in tld In file

<!DOCTYPE taglib
  PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN"
   "http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd">
<!--  Tag library descriptor  -->
<taglib xmlns="http://java.sun.com/JSP/TagLibraryDescriptor">
	<tlib-version>1.0</tlib-version>
	<jsp-version>1.2</jsp-version>
	<short-name>Simple Tags</short-name>
	<uri>/zking</uri>
	
	<tag>
		<name>out</name>
		<tag-class>com.myjstl.OutTag</tag-class>
		<body-content>empty</body-content>
		<attribute>
			<name>val</name>
			<required>true</required>
			<rtexprvalue>true</rtexprvalue>
			<description>out label val attribute , For output val Value </description>
		</attribute>
		<attribute>
			<name>defaultVal</name>
			<required>false</required>
			<rtexprvalue>false</rtexprvalue>
			<description> Used to define default values </description>
		</attribute>
	</tag>
	
	<tag>
		<name>if</name>
		<tag-class>com.myjstl.IfTag</tag-class>
		<body-content>jsp</body-content>
		<attribute>
			<name>test</name>
			<required>true</required>
			<rtexprvalue>true</rtexprvalue>
			<description>if label </description>
		</attribute>
	</tag>
 
    <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>
	
	<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>
 
</taglib>

Detailed explanation

• <tag></tag>： Put all the format specifications of a custom label inside
• <name>out</name>： Represents a custom tag name
• <tag-class>com.zking.mymvc.tag.OutTag</tag-class>： Represents the full class name of the custom label class in the first step ( Here is how to find the custom tag class through the reflection mechanism )
•  <body-content>empty</body-content>： Represents no label body , fill jsp It means that there is

• <attribute>： Represents the attribute specification in the custom tag class  <name>val</name>： Custom attribute name
              <required>true</required>： The attributes that must be written out in the custom tag
              <rtexprvalue>true</rtexprvalue>： Whether attribute values can be used EL expression
              <description>out label val attribute , For output val Value </description>： Description of custom attributes
  </attribute>  

3、 stay JSP Use custom labels on （ test ）

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@taglib prefix="z" uri="/zking" %>
<!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>
 
	<%
		request.setAttribute("name", null);
	%>
	<!-- out  label  -->
	<z:out val="${name}" defaultVal=" You entered null keyword "/>
	
	
	<z:if test="${100 == 100}">
		 test if(100 == 100)
	</z:if>
	
	<z:if test="${100 != 200}">
		 test if(100 != 200)
	</z:if>
</body>
</html>

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summary

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