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JSP learning part

2022-06-29 07:52:00 【Joy's sister is a rookie】

JSP Introduction to

jsp The full name is java server pages.

Java The server page of .

jsp Its main function is to replace Servlet Program returns html Page data .

because Servlet Program returns html Page data is a very complicated thing . Development costs and maintenance costs are very high .（Servlet The code is jsp It's very complicated ）

eg：

public class PringHtml extends HttpServlet {
     
    @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
     
        //  Through the return flow of the response  html  The page data  
        resp.setContentType("text/html; charset=UTF-8"); 
        PrintWriter writer = resp.getWriter(); 
        writer.write("<!DOCTYPE html>\r\n"); 
        writer.write(" <html lang=\"en\">\r\n"); 
        writer.write(" <head>\r\n"); 
        writer.write(" <meta charset=\"UTF-8\">\r\n"); 
        writer.write(" <title>Title</title>\r\n"); 
        writer.write(" </head>\r\n"); 
        writer.write(" <body>\r\n"); 
        writer.write("  This is a  html  The page data  \r\n"); 
        writer.write(" </body>\r\n"); 
        writer.write("</html>\r\n"); 
        writer.write("\r\n"); 
    } 
}

<%@ page contentType="text/html;charset=UTF-8" language="java" %> 
<html> 
    <head> 
        <title>Title</title> 
    </head> 
    <body> 
         This is a  html  The page data  
    </body> 
</html>

JSP The essence of

jsp A page is essentially a Servlet Program .

When we first visited jsp On the page .Tomcat The server will help us jsp Page translation becomes a java Source file . And compile it into .class Bytecode program .

From the source code, you can find ,HttpJspBase class . It directly inherits HttpServlet class . in other words .jsp Translated java class , It indirectly inherited HttpServlet class . in other words , The translation is a Servlet Program

Summary ：

From the generated files, it is not difficult to find a rule .

a.jsp Translate into java The full name after the file is ：a_jsp.java file

b.jsp Translate into java The full name after the file is ：b_jsp.java file

that When we visit One xxx.jsp After the document

Translate into java The full name of the file is ：xxx_jsp.java file

xxx_jsp.java The file is a Servlet Program . original jsp Medium html The content is translated into Servlet Class service Output as is in the method .

JSP The grammar of

JSP The head of the page Instructions

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" %>

jsp The header of the file declares ：

attribute	explain
language	Express jsp What language files are translated . Only for now java.
contentType	Express jsp What is the data type returned . Also in the source code response.setContentType() Parameter values
pageEncoding	At present jsp The character set of the page file itself
import	To current jsp The class package that needs to be used for page import
autoFlush	Set when out When the output stream buffer is full , Whether to refresh the impulse area automatically . The default value is true
buffer	Set up out Size of buffer （ The default is 8kb）
errorPage	Set when jsp An error occurred while the page was running , Automatically jump to the wrong page path
isErrorPage	Set up current jsp Whether the page is an error message page . The default is false. If it is true have access to exception Exception object
session	Set up current jsp Whether the page gets session object , The default is true
extends	Set up jsp Translated java Class inherits by default

JSP Frequently used scripts

Declaration script

Format ：

<%! Statement java Code %>

effect ：

1. We can define global variables .

2. Definition static Static code block

3. Define methods

4. Define inner classes

Code that can be written almost inside a class , Can be implemented through declaration scripts

<%--1、 Declare class properties --%> 
<%! 
    private Integer id; 
	private String name; 
	private static Map<String,Object> map; %> 
<%--2、 Statement  static  Static code block --%> 
<%! 
    static {
    	map = new HashMap<String,Object>(); 
    	map.put("key1", "value1"); 
    	map.put("key2", "value2"); 
    	map.put("key3", "value3"); 
	} 
%> 
<%--3、 Declare class methods --%> 
<%! 
    public int abc(){ 
    	return 12; 
	} 
%>
<%--4、 Declare inner classes --%> 
<%! 
    public static class A { 
        private Integer id = 12; 
        private String abc = "abc"; 
    } 
%>

Expression script

Format ：

<%= expression  %>

effect ：

Expression script Used to output content to the page .

Expression script Translate to Servlet programmatic service In the method With out.print() Printout

out yes jsp A built-in object of , Used to generate html Source code

Be careful ： Don't end the expression with a semicolon , Otherwise, an error will be reported

The expression script can output any type .

<%=12 %> <br> 
<%=12.12 %> <br> 
<%=" I'm a string " %> <br> 
<%=map%> <br> 
<%=request.getParameter("username")%>

Code script

Format ：

<% java  sentence  %>

effect ：

Can be in jsp On the page , Write the functions we need （ Is written java sentence ）

The characteristics of code scripts are ：

1、 Code scripts are translated in _jspService In the method

2、 Code script due to translation to _jspService() In the method , So in _jspService() All existing objects in the method can be used directly .

3、 You can also combine multiple code script blocks to complete a complete java sentence .

4、 Code scripts can also be combined with expression scripts , stay jsp Output data on page

JSP Three notes

java notes

// A single  java  notes 

/*  Multiple lines  java  code annotation  */

Single line comments and multi line comments can be used in the translated java See in the source code

html notes

<!-- html  notes  -->

html Your comments will be translated into java Output the code to html Page view

jsp notes

<%--  This is a  jsp  notes  --%>

jsp Comments will be directly ignored in translation

JSP Nine built-in objects

Nine built-in objects , It's all we can do in 【 Code script 】 Medium or 【 Expression script 】 Objects that are used directly in

object	effect
request object	Request object , Request information can be obtained
response object	The response object , Response information can be set
pageContext object	The current page context object . You can save attribute information in the current context
session object	Conversation object . You can get session information
exception object	Exception object . Only in jsp Page page Set in command isErrorPage=“true” Will exist when
application object	ServletContext Object instances , You can get some information about the whole project
config object	ServletConfig Object instances , Can get Servlet Configuration information
out object	jsp Output stream object
page object	Point to the present jsp The object of （ It's useless , It's better to use this object ）

JSP Four domain objects

Four domain objects are often used to store data information .

Domain objects can be like Map The same object that accesses data . The four domain objects function the same . What's different is their access range to the data .

Although all four domain objects can access data . They have priority in use .

When the four domains are in use , The order of priority is , The order of their range from small to large .

pageContext ——> request ——> session ——> application

Domain object	effect	Scope of action
pageContext	You can save data in the same jsp Page usage	At present jsp Valid within page range
request	You can save data in the same jsp Page usage	Valid within one request
session	It can be saved in a session and used	Valid within a session scope （ Open the browser to access the server , Until the browser is closed ）
application(ServletContext)	ServletContext object	Whole web Valid within the scope of work （ as long as web The project doesn't stop , The data are all there ）

JSP Of out Output and respon.getWriter Similarities and differences of output

response In response to , We often use it to set the content returned to the client （ Output ）

out It's also used for user output .

$[ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-XKLnzpI4-1654616042020)(C:\Users\pon18\AppData\Roaming\Typora\typora-user-images\image-20220607231441018.png)]$

Example ：

$[ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-gXcUimBJ-1654616042022)(C:\Users\pon18\AppData\Roaming\Typora\typora-user-images\image-20220607231530605.png)]$

$[ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-Xyqgnqqc-1654616042023)(C:\Users\pon18\AppData\Roaming\Typora\typora-user-images\image-20220607231602344.png)]$

JSP Commonly used labels

In the work , Almost all use static inclusion .

Static inclusion

Format ：

<%@ include file=""%>
file  Property specifies what you want to include  jsp  The path of the page 
 First slash in address  /  Expressed as  http://ip:port/ Project path /  Mapping to code  web  Catalog 

eg:
<%@ include file="/include/footer.jsp"%>

characteristic ：

1、 Static inclusion does not translate the included jsp page .

2、 Static inclusion is actually the inclusion of jsp Copy the code of the page to the included location to execute the output .

Dynamic inclusion

Format ：

<jsp:include page=""></jsp:include>
page  Property specifies what you want to include  jsp  The path of the page   Dynamic inclusion can also be like static inclusion . Execute and output the included content to the included location 

eg:
<jsp:include page="/include/footer.jsp"> 
    <jsp:param name="username" value="bbj"/> 
    <jsp:param name="password" value="root"/> 
</jsp:include>

characteristic ：

1、 Dynamic inclusion will include jsp The page is also translated into java Code

2、 The underlying code of dynamic inclusion uses the following code to call the included jsp Page execution output .

JspRuntimeLibrary.include(request, response, "/include/footer.jsp", out, false);

3、 Dynamic inclusion , You can also pass parameters

Request forwarding

Format ：

<jsp:forward page=""></jsp:forward>  Is the request forwarding tag , Its function is request forwarding  page  Property to set the path for request forwarding 

eg:
<jsp:forward page="/scope2.jsp"></jsp:forward>

<jsp:forward The forwarding function is equivalent to request.getRequestDispatcher(“/xxxx.jsp”).forward(request, response); The function of .

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