JavaWeb Study ( Nine )—JSP principle /JSP Basic grammar /JSP Instructions /JSP label

01 JSP principle

JSP The full name is Java Server Pages, It and servle The technology is the same , All are SUN A company defined one that is used to develop dynamic web Technology of resources .

JSP The biggest thing about this technology characteristic lie in , Write jsp It’s like writing html, But it compares html for ,html Only static data can be provided to users , and Jsp Technology allows you to nesting java Code , For the user Provide dynamic data .

JSP principle ： The browser sends a request to the server , No matter what resources are being accessed , In fact, they are all visiting Servlet, So when you visit a jsp When the page is , Actually, I’m also visiting a Servlet, Server executing jsp When , First turn on the jsp Translate into a Servlet, So we visit jsp when , It’s not actually an interview jsp, It’s a visit jsp The translated one Servlet. give an example ： When we access through the browser index.jsp when , The server will first index.jsp Translate into a index_jsp.class, stay Tomcat Server’s work\Catalina\localhost\ Project name \org\apache\jsp You can see index_jsp.class Source code file for index_jsp.java,index_jsp.java. We can see ,index_jsp This class is Inherit org.apache.jasper.runtime.HttpJspBase This class of , By looking at Tomcat The source code of the server , It can be seen in apache-tomcat-6.0.20-src\java\org\apache\jasper\runtime In the directory HttpJspBase The source code file for this class .HttpJspBase Class is inheritance HttpServlet Of , therefore HttpJspBase Class is a Servlet, and index_jsp It’s inheritance again HttpJspBase Class , therefore index_jsp Class is also a Servlet, therefore When the browser accesses the server index.jsp When the page is , It’s actually a visit to index_jsp This Servlet,index_jsp This Servlet Use _jspService This method handles the request .

**JSP How to output the page to the client ？** stay jsp Writing in the java Code and html The code will be translated into _jspService In the method, go through Out Object output .

JSP Eight people ： see _jspService And the way you can see ,Web Server is calling jsp when , Will give Jsp Provide the following 8 individual java object

PageContext pageContext;// Scope
HttpSession session;// conversation
ServletContext application;// application , In fact, that is servletContext
ServletConfig config;// To configure
JspWriter out;// Output
Object page = this;// page
HttpServletRequest request;// request
HttpServletResponse response// Respond to

among page object ,request and response It’s done Instantiation , Others 5 Objects that are not instantiated are instantiated in the following way

pageContext = _jspxFactory.getPageContext(this, request, response,null, true, 8192, true);
application = pageContext.getServletContext();
config = pageContext.getServletConfig();
session = pageContext.getSession();
out = pageContext.getOut();

02 JSP Basic grammar

JSP yes JAVA On the application of , stay JSP in , be-all JAVA All statements can be used . its Extended grammar by ：

<%-- assignment --%>
<%= Variable or expression %>
<%-- stay <% %> You can define variables in 、 Write sentences , Can't define method --%>
<%
Multiple lines java Code
%>
<%--
Statement ：JSP A declaration can be used to define JSP Page converted to Servlet Static code block of a program 、 Member variables and methods .
--%>
<%！
java Code
%>
<!--HTML Style notes -->
<%
//JAVA A single line comment in
/*
JAVA Multi line comments in
*/
%>
<%--JSP My own notes --%>

matters needing attention ：

• JSP In the script fragment Can only appear java Code , No other template elements can appear , JSP The engine is translating JSP On the page , Will JSP In the script fragment Java The code will be placed intact in Servlet Of _jspService In the method .
• JSP In the script fragment Java Code must be Follow strictly Java grammar , for example , After each execution statement Must use semicolon （;） end .
• In a JSP On the page There can be multiple script fragments , Between two or more script fragments You can embed text 、HTML Marks and other JSP Elements .
• HTML When viewing the source file in the browser, it is You can see it Of , and JAVA Annotation and JSP notes When you view the source file in the browser, it is Out of sight Of the content of the note , This is the difference between these three annotations .

03 JSP Instructions

JSP There are only three instructions , Namely page include taglib

• Commonly used Page Instructions

<%-- Set page text type --%>
<%@ page contentType="text/html;charset=UTF-8" %>
<%-- Set the page to refresh automatically , The default value is false--%>
<%@ page autoFlush="true" %>
<%-- Set up JSP programing language --%>
<%@ page language="java" %>
<%-- Jump to error page （ It is not recommended to use , Recommended in the XML Middle configuration ）--%>
<%@ page errorPage="/error/404.jsp" %>
<%-- Guide pack --%>
<%@ page import="com.hooi.pojo.Student" %>
<%-- Set page encoding --%>
<%@ page pageEncoding="utf-8" %>

stay xml Middle configuration error page

<!-- If the error code is 404, Then go to error Under the document 404 Error page -->
<error-page>
<error-code>404</error-code>
<location>/error/404.jsp</location>
</error-page>
<!-- If the error code is 500, Then go to error Under the document 500 Error page -->
<error-page>
<error-code>500</error-code>
<location>/error/500.jsp</location>
</error-page>

• Commonly used include Instructions

<%@ include file="{ Pages to include }"%>

Usually , There will be some common locations in the website that can be extracted , Like the head and tail of the site .

For testing purposes , We design the head and tail of the page as ：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<head>
<style>
div h2 {
margin: 0 auto;
text-align: center;
}
</style>
</head>
<div>
<% int i = 1; // Define a global variable %>
<h2>Header_Page</h2>
<h3>i=<%=i%></h3>
</div>

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<head>
<style>
div h2 {
margin: 0 auto;
text-align: center;
}
</style>
</head>
<div>
<% int i = 1;// Define a global variable with the same name in the header page %>
<h2>Footer_Page</h2>
<h3>i=<%=i%></h3>
</div>

How to use include Instructions contain pages ？

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title> home page </title>
</head>
<body>
<%-- Insert page --%>
<h1>@include test </h1>
<%@ include file="common/header.jsp"%>
<h1>@include test </h1>
<%@ include file="common/footer.jsp"%>
</body>
</html>

The operation effect is as shown in the figure ：（ Here we will 500 The error page is passed through xml Configure for your own defined page .） because header.jsp and footer.jsp The variable with the same name is defined , So in use include When the command contains the page , There will be 500 error . From this we can know that ： Use @include Instructions contain pages , Will be included in the page and their own page fusion into a servlet, In the same servlet Defining a variable with the same name will prompt 500 error .

• taglib

  Use jstl Or other tag libraries need to download the corresponding jar package , Then import .

  Reuse taglib Instructions will be jar Package added as dependency

  <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
  <%--
  Attribute specification ：
  prefix: Name of tag library to be used
  tagdir： The local directory of the tag library
  uri : The network address of the corresponding library
  --%>
  

  need Be careful Yes. ：

  1. The project should be imported into the corresponding tag library jar package .
  2. Tomcat Also need to be placed in jar package
  3. In the code , Some special symbols require the use of escape characters ;

04 JSP label

grammar ：jsp:xxx

JSP There are three common tags :

<jsp:include>: Used to insert the output of another resource into the current JSP In the output of the page , This kind of JSP The introduction method of page execution is called dynamic introduction .

<jsp:forward>: Used to forward a request to another resource .

<jsp:param>： When using <jsp:include> and <jsp:forward> When tags introduce or forward requests to other resources , have access to jsp:param Tag passes parameters to this resource .

Test code ：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>Title</title>
</head>
<body>
<h1>jsp:include test </h1>
<jsp:include page="common/header.jsp"/>
<jsp:forward page="index.jsp">
<jsp:param name="username" value="DOGIOOH"/>
<jsp:param name="age" value="24"/>
</jsp:forward>
<h1>jsp:include test </h1>
<jsp:include page="common/footer.jsp"/>
</body>
</html>

<html>
<head>
<title> home page </title>
</head>
<body>
<div>
test jsp:forward adopt request Get the input parameters ( Use request Object needs to be guided )
<h1>jsp:forward test </h1>
<h2>User：<%=request.getParameter("username")%></h2>
<h2>Age：<%=request.getParameter("age")%></h2>
</div>
</body>
</html>

test result ：

First of all jsp:include Test results ： because jsp:include Include… For dynamic , Will not include the page and their own page fusion into a servlet, Therefore, no error will be reported because the variable with the same name appears in the header and footstep page .

jsp:forward and jsp:param The test results are as follows :

jsp:useBean label

First we need to understand JavaBean It’s a specific way of writing Java class , It usually has the following characteristics ：

• This Java Class must have a parameterless constructor
• Property must be privatized .
• The property of privatization must pass public Type of method exposed to other programs , And the naming of methods must also comply with certain naming norms .

Example ：

public class Student {

private String name;
private int age;
private String id;
private String sex;
private boolean graduated;
public Student() {

}
public Student(String name, int age, String id, String sex, boolean graduated) {

this.name = name;
this.age = age;
this.id = id;
this.sex = sex;
this.graduated = graduated;
}
public String getName() {

return name;
}
public void setName(String name) {

this.name = name;
}
public int getAge() {

return age;
}
public void setAge(int age) {

this.age = age;
}
public String getId() {

return id;
}
public void setId(String id) {

this.id = id;
}
public String getSex() {

return sex;
}
public void setSex(String sex) {

this.sex = sex;
}
public boolean isGraduated() {

return graduated;
}
public void setGraduated(boolean graduated) {

this.graduated = graduated;
}
@Override
public String toString() {

return "Student{" +
"name='" + name + '\'' +
", age=" + age +
", id='" + id + '\'' +
", sex='" + sex + '\'' +
", graduated=" + graduated +
'}';
}
}

<jsp:useBean> The label is used to find the JavaBean object , If it exists, it will be returned directly JavaBean References to objects , If it doesn’t exist, instantiate a new JavaBean Object and store it in the specified domain scope with the specified name .

<jsp:useBean> Examples of tag usage ：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ page import="com.hooi.pojo.Student" %>
<html>
<head>
<title>Title</title>
</head>
<body>
<%--Java Code implementation ： Premise , Need to guide package --%>
<% Student stu1 = new Student(); %>
<% stu1.setName("Jackson"); %>
<% stu1.setAge(18); %>
<% stu1.setSex("male"); %>
<% stu1.setId("0000001"); %>
<% stu1.setGraduated(false); %>
<%--jsp Tag implementation --%>
<jsp:useBean id="stu2" class="com.hooi.pojo.Student" scope="page"/>
<jsp:setProperty name="stu2" property="name" value="Curry"/>
<jsp:setProperty name="stu2" property="age" value="31"/>
<jsp:setProperty name="stu2" property="sex" value="male"/>
<jsp:setProperty name="stu2" property="id" value="0000002"/>
<jsp:setProperty name="stu2" property="graduated" value="true"/>
<%--=============================================================--%>
<%--Java Code to get properties --%>
<div>
<h1>Java Realization </h1>
<h2>User:<%=stu1.getName()%></h2>
<h2>Age:<%=stu1.getAge()%></h2>
<h2>Sex:<%=stu1.getSex()%></h2>
<h2>ID:<%=stu1.getId()%></h2>
<h2>Graduated:<%=stu1.isGraduated()%></h2>
</div>
<%--EL expression : Can't express objects --%>
<div>
<h1>EL Expression implementation </h1>
<h2>User:${stu2.name}</h2>
<h2>Age:${stu2.age}</h2>
<h2>Sex:${stu2.sex}</h2>
<h2>ID:${stu2.id}</h2>
<h2>Graduated:${stu2.graduated}</h2>
</div>
<%--jsp Tag implementation gets properties --%>
<div>
<h1>JSP Realization </h1>
<h2>
User:<jsp:getProperty name="stu2" property="name"/>
</h2>
<h2>
Age:<jsp:getProperty name="stu2" property="age"/>
</h2>
<h2>
Sex:<jsp:getProperty name="stu2" property="sex"/>
</h2>
<h2>
ID:<jsp:getProperty name="stu2" property="id"/>
</h2>
<h2>
Graduated:<jsp:getProperty name="stu2" property="graduated"/>
</h2>
</div>
</body>
</html>

test result ：