Introduction of the Servlet

Servlets need to run in a specific container. The container here refers to the runtime environment required by the Servlet. Generally, the common Java Web Server in the market can support servlets. For example, Tomcat, Resin, Weblogic, WebSphere, etc., Tomcat is used as the container of Servlet in this book, and Tomcat provides the basic running environment for Servlet.

The Servlet container environment implements an abstraction layer between HTTP communication and the Web server platform. The Servlet container is responsible for passing the request to the Servlet and returning the result to the client. The container environment also provides a simple way to configure Servlet applications and also provides a way to configure servlets using XML files. When the Servlet container receives a request from the user to the Servlet, the Servlet engine determines whether the Servlet has been accessed for the first time. If so, the Servlet engine initializes the Servlet. That is, the init () method in the Servlet is called to do the necessary initialization. When a subsequent client requests the Servlet service, the init () method is not called, but the service () method is directly called. In other words, each Servlet is initialized only once. The subsequent request simply creates a new thread and calls the service () method in the Servlet.

In the process of using Servlet, the problem of concurrent access is handled by the Servlet container. When multiple users request the same Servlet, the Servlet container is responsible for starting a thread for each user, and the operation and destruction of these threads are responsible for the Servlet container. In traditional CGI programs, which start a process for each user, servlets are much more efficient than CGI programs.

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet("/helloServlet") // Request path, this is annotation development, so that you don't need to write the servlet-mapping in web.xml
public class HelloServlet extends HttpServlet {
    @Override
    public void init(a) throws ServletException {
        System.out.println("HelloServlet initialization");
    }
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("HelloServlet doGet handles GET requests");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("HelloServlet doPost Handles POST requests");
    }
    @Override
    public void destroy(a) {
        System.out.println("HelloServlet destroyed"); }}Copy the code

The most common operation with servlets is that the page passes values to the background, the background receives and processes the background, and returns the execution results to the front end. The method of receiving the value requested by the front end is req.getParameter(” The name of the input box name “); Example: Let’s pass the username and password from the front end to the back end, verify that they are correct, and output the result to the console. The front-end input corresponding values, you can test the results.

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet("/login")
public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // Get the user name
        String userName = req.getParameter("userName");
        // Get the password
        String userPassword = req.getParameter("userPassword");
        if (userName.equals("admin") && userPassword.equals("123456")) {
            System.out.println("Verified");
        } else {
            System.out.println("Verification failed"); }}}Copy the code

<%@ page contentType="text/html; charset=UTF-8" language="java" %>
<html>
<head>
    <title>index</title>
</head>
<body>
<form action="/login" method="post"> User name: <input type="text" name="userName"/><br> Password: <input type="password" name="userPassword" /><br>
    <input type="submit" value="Submit" />
</form>
</body>
</html>
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In servlets, there are two ways to jump to a page, redirection and forwarding.

The difference between a

When redirected, the url on the browser changes. Forwarding is the same url on the browser

The difference between the two

A redirection actually generates two requests and forwards only one request

The difference between the three

The redirected url can be any url. The redirected URL must be the url of this site

redirect

Send a request – > The server runs – > Responds to the request and returns a new address and response code to the browser. – > The browser determines that the response is redirected based on the response code and automatically sends a new request to the server. – > The server runs – > Responds to the request

Forwarding:

Send a request – > Run the server – > reconfigure the request, for example, using request.setAttribute(name,value) – > Obtain the web page based on the forwarded address – > respond to the request to the browser

Break down

Redirect: All variables stored in the previous request are invalidated and a new request scope is entered. Forwarding: Variables stored in previous requests are not invalidated, like putting two pages together.

Write the difference between

req.getRequestDispatcher("index.jsp").forward(req, resp); / / forwarding
resp.sendRedirect("index.jsp"); / / redirection
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In JavaWeb development, each time you write a Servlet, you need to configure it in a web.xml file, as follows:

<servlet>
    <servlet-name>LoginServlet</servlet-name>
    <servlet-class>com.sponge.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>LoginServlet</servlet-name>
    <url-pattern>/login</url-pattern>
</servlet-mapping>
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Servlet3.0 provides annotations that eliminate the need to describe Servlet deployment in a web.xml file. Simplify the development process.