Servlet basic knowledge points

notes ： This article is only for personal learning records , If there is a mistake , Welcome to point out , thank you

Servlet

What is? Servlet?

Java Servlet  Is running on the  Web  A program on a server or application server , It's as coming from  Web  Browser or something  HTTP  Client requests and  HTTP  The middle tier between databases or applications on the server .
 Use  Servlet, You can collect user input from web forms , Render records from databases or other sources , You can also create web pages dynamically .

Sevlet characteristic

• Better performance .
• Servlet stay Web Execute in the address space of the server . So it doesn't have to create a separate process to handle every client request .
• Servlet It's platform independent , Because they use Java Compiling .
• On the server Java The security manager implements a series of restrictions , To protect resources on the server computer . therefore ,Servlet It's believable .
• Java All the functions of the class library are right Servlet It's all available . It can go through sockets and RMI Mechanism and applets、 Database or other software to interact .

Servlet framework

servlet be used for HTTP And the middle tier of the database , Used to connect to HTTP Servers and databases .

---

Servlet Mission

• Read the client （ browser ） Explicit data sent . This includes HTML Forms , Or it could be from applet Or custom HTTP Client forms .

• Read the client （ browser ） Sending implicit HTTP Request data . This includes cookies、 Media types and compression formats that browsers can understand .

• Process data and generate results . This process may require access to the database , perform RMI or CORBA call , call Web service , Or directly calculate the corresponding response .

• Send explicit data （ The document ） To client （ browser ）. The format of the document can be varied , Include text files （HTML or XML）、 Binary （GIF Images ）、Excel etc. .

• Send implicit HTTP Response to client （ browser ）. This includes telling the browser or other client the type of document returned （ for example HTML）, Set up cookies And cache parameters , And other similar tasks .

Servlet package

​ Java Servlet Is running with support Java Servlet The interpreter of the specification web On the server Java class .Servlet have access to javax.servlet and javax.servlet.http Package creation , It is Java Standard components of the Enterprise Edition ,Java The enterprise edition supports large-scale development projects Java An extended version of the class library . These classes implement Java Servlet and JSP standard . In writing this tutorial , The corresponding versions of the two are Java Servlet 2.5 and JSP 2.1.

Servlet Life cycle

Servlet Life cycle can be defined as the whole process from creation to destruction . Here are Servlet Follow the process ：

• Servlet Call after initialization init () Method .
• Servlet call service() Method to handle client requests .
• Servlet Call before destruction destroy() Method .
• Last ,Servlet By JVM Garbage collector for garbage collection .

init Method is designed to be called only once . It was created for the first time Servlet When called , It is not called at every subsequent user request .

service() Method is the main way to perform practical tasks .

service() Method is called by the container ,service Method is called when appropriate doGet、doPost、doPut、doDelete Other methods .

Architecture diagram

Servlet example

• newly build web project ( Deploy the project to web Server )
• New package , And create a new ordinary under the package java project
• Inherit HttpServer

Servlet The form data

get,post

servlet Common classes

HttpServletRequest

Cookie[] getCookies() Returns an array , Contains all of the  Cookie  object .
Enumeration getAttributeNames() Returns an enumeration , Contains the property names available for the request .
Enumeration getHeaderNames() Returns an enumeration , Include all headers included in the request .
Object getAttribute(String name) Returns the value of a named property as an object , If no attribute with the given name exists , Then return to  null.

Request to forward the small case (HttpServletRequest)

package com.wanglj.servlet;

import java.io.IOException;

import javax.print.attribute.standard.JobOriginatingUserName;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
@WebServlet("/ser05")
public class Servlet05 extends HttpServlet {
    
	
	private static final long serialVersionUID = 1L;

	@Override
	public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
    
//  Set encoding 
		req.setCharacterEncoding("utf-8");
		String userNameString = req.getParameter("name"); 
		String pwdString = req.getParameter("pwd");
// System.out.println(" user name ："+userNameString);
// System.out.println(" password ："+pwdString);
		try {
    
			// String comparison uses equals
			if(userNameString.equals("admin") && pwdString.equals("toor")) {
    
				System.out.println(" Login successful ");
				// Set scope 
// req.setAttribute("userName", userNameString);
//  Remove scope 
				req.removeAttribute("userName");
				req.getRequestDispatcher("success.jsp").forward(req, res);
			}
			else {
    
				System.out.println(" Login failed ");
				// Request forwarding ( Server jump , The address bar doesn't change , Request forwarding can share data , But redirection cannot share data )
// req.getRequestDispatcher(" route ").forward(req, res);
				req.setAttribute("msg", " Wrong user name or password ！");
				req.getRequestDispatcher("failed.jsp").forward(req, res);
			}
		} catch (Exception e) {
    
			e.printStackTrace();
		}
		
	}
}

<%@ page language="java" contentType="text/html; charset=utf-8"
    pageEncoding="utf-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Insert title here</title>
</head>
<body>
	<!--  Submitted to the ser05,name="para"  Can be in jsp Get the submitted parameters -->
	<form action="ser05">
		userName:<input type="text" name="name">
		password:<input type="password" name="pwd">
		<button type="submit"> Sign in </button>
		<!-- http://localhost:8080/ServletTest/ser05?name=wanglj&pwd=toor -->
	</form>
</body>
</body>
</html>

HttpServletResponse object

The response data

getWriter() Obtain byte stream

getOutputStream() Obtain byte stream , Able to respond to any type of data

！ Both cannot be used at the same time , Otherwise you may report an error

package com.wanglj.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.catalina.filters.SetCharacterEncodingFilter;
// When using a new servlet When , Reboot required tomcat The server 
// getWriter()  Response character 
// getOutputStream()  Respond to any type , Not with the above getWriter() share , Otherwise, an error will be reported 
@WebServlet("/ser06")
public class Servlet06 extends HttpServlet {
    
	private static final long serialVersionUID = 1L;
	@Override
	protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
// resp.setContentType("text/html");
//  Chinese and English garbled code solution 
// // Set the server coding format 
// resp.setCharacterEncoding("UTF-8");
// // Client code 
// resp.setHeader("content-type", "text/html;charset=UTF-8");
  Character input stream 
		resp.setContentType("text/html;charset=UTF-8");
// PrintWriter writer = res.getWriter(); 
// writer.write("<h2>hello getWriter</h2>");
// // Refresh 
// writer.flush();
// // close 
// writer.close();
		// Byte input stream 
		// When the character encoding of the server is different from that of the client , Or when Chinese is not supported , It's going to be messy 
		ServletOutputStream out = resp.getOutputStream();
		//getBytes("utf-8") Also write the type , Otherwise, it may also cause garbled code 
		out.write("<h2> China </h2>".getBytes("UTF-8"));
		out.flush();
		out.close();
	}
}

//  Chinese and English garbled code solution 
//  The first one is 
// // Set the server coding format 
		resp.setCharacterEncoding("UTF-8");
// // Client code 
		resp.setHeader("content-type", "text/html;charset=UTF-8");
  The second kind ：
		resp.setContentType("text/html;charset=UTF-8");
		out.write("<h2> China </h2>".getBytes("UTF-8"));

The difference between request forwarding and redirection

Request forwarding is a server jump , Redirection is a client jump

Request forwarding is a request , Redirection is two requests

Request forwarding can be shared request data , Redirection cannot

The request forwarding address bar does not change , The redirection address will change

Request forwarding can only jump to the resources under the current project , Redirection can be any resource （ It can span servers ）

package com.wanglj.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/ser07")
public class Servlet07 extends HttpServlet {
    
	private static final long serialVersionUID = 1L;
	@Override
	protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
		// Redirect , You can redirect to jsp,servlet, And cross server forwarding 
// resp.sendRedirect("sign.jsp");
		// Request forwarding 、
// req.getRequestDispatcher("sign.jsp").forward(req, resp);
		resp.sendRedirect("https://www.baidu.com/");
	}
}

cookie object

meaning ：

Cookie, Sometimes in the plural Cookies. The type is “ Small text files ”, It's some websites to identify users , Conduct Session Tracking data stored on the user's local terminal ）, By the user client Information temporarily or permanently stored by a computer .

Create and send Cookie

// establish cookie object 
Cookie cookie = new Cookie("wanglj", "123456");
// send out cookie
resp.addCookie(cookie);
// obtain cookie
req.getCookies()
private String comment; // describe 
private String domain; //  You can visit the Cookie Domain name of 
private int maxAge = -1; // The Cookie Expiration time , The unit is in seconds , And often Expires Use it together , It can be used to calculate its effective time .Max Age If it's a positive number , Then Cookie stay Max Age Seconds later . If it's negative , When the browser is closed Cookie I.e. failure , The browser will not save the Cookie.
private boolean secure; // Whether to use SSL

package com.wanglj.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/ser07")
public class Servlet07 extends HttpServlet {
    
	private static final long serialVersionUID = 1L;
	@Override
	protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
		// establish cookie object 
		Cookie cookie = new Cookie("demo", "6473456");
		// send out cookie
		resp.addCookie(cookie);
		// obtain cookies
		Cookie[] cookies = req.getCookies();
        // Traverse cookie
        if(cookies != null){
    
            for(Cookie cok:cookies) {
    
				System.out.println(cok.getName());
				System.out.println(cok.getValue());
			}
        }
	}
}

Refer to the rookie tutorial :https://www.runoob.com/servlet/servlet-writing-filters.html