Servlet knowledge points

1. brief introduction

servlet Interface sun The company has two default implementation classes ：HttpServlet,GenericServlet

• Namely sun Company development trends web A technology of
• sun In these API Provides an interface called ：Servlet, If you want to develop a servlet Program , Only two small steps need to be completed ：
  • Write a class , Realization servlet Interface
  • Deploy to web Server

To achieve Servlet Interface Java The program is called ,Servlet

2.HelloServlet

1. Construct a Maven project , Delete the inside src Catalog , Build... In the project module;

2. About Maven Understanding of father son project ：

   There will be

   <modules>
       <module>servlet-01</module>
   </modules>
   

   The subproject will have

   <parent>
       <artifactId>javaweb-02-servlet</artifactId>
       <groupId>org.example</groupId>
       <version>1.0-SNAPSHOT</version>
   </parent>
   

   In the parent project java Subprojects can directly use

   son extends father
   

3. Maven Environmental optimization

   1. modify web.xml For the latest

webapp

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" version="4.0">
</web-app>

3. take Maven The structure of the building is complete

4. web Project dependence

   servlet and jsp

   pom.xml

   <!-- https://mvnrepository.com/artifact/javax.servlet.jsp/javax.servlet.jsp-api -->
   <dependency>
       <groupId>javax.servlet.jsp</groupId>
       <artifactId>javax.servlet.jsp-api</artifactId>
       <version>2.3.3</version>
   </dependency>
   <!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
   <dependency>
       <groupId>javax.servlet</groupId>
       <artifactId>javax.servlet-api</artifactId>
       <version>4.0.1</version>
   </dependency>
   

5. Write a Servlet Program

   1. Write a common class

   2. Realization Servlet Interface , Direct inheritance HttpServlet

      public class HelloServlet extends HttpServlet {
              
      //  because get perhaps post It's just the different ways that requests are implemented , Can call each other , The business logic is the same 
          @Override
          protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
              
              PrintWriter writer = resp.getWriter(); // Response flow 
              writer.println("hello,Servlet");
          }
      
          @Override
          protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
              
              super.doPost(req, resp);
          }
      }
      

6. To write Servlet Mapping

   Why do I need to map ： What we're writing is java Program , But access through a browser , The browser needs to connect web The server , So we need to web Registration in the service we wrote Servlet, Also need to give him a browser can access the path ;

   <!--  register servlet-->
       <servlet>
           <servlet-name>hello</servlet-name>
           <servlet-class>com.wan.servlet.HelloServlet</servlet-class>
       </servlet>
   <!-- servlet The request path for -->
       <servlet-mapping>
           <servlet-name>hello</servlet-name>
           <url-pattern>hello</url-pattern>
       </servlet-mapping>
   

7. To configure Tomcat

​ Be careful ： Just configure the project publishing path

3.Servlet principle

servlet Yes, there is web Server calls ,web The server will provide a response after receiving the browser request ;

4.Mapping problem

1. One Servlet You can specify a mapping path

   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>/hello</url-pattern>
   </servlet-mapping>
   

2. One Servlet Multiple mapping paths can be specified

   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>/hello1</url-pattern>
   </servlet-mapping>
   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>/hello2</url-pattern>
   </servlet-mapping>
   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>/hello3</url-pattern>
   </servlet-mapping>
   

3. One Servlet You can specify a generic mapping path

   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>/hello/*</url-pattern>
   </servlet-mapping>
   

4. Specify some suffixes or prefixes and so on ...

   <!--* The path of project mapping cannot be added in the front  -->
   <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>*.do</url-pattern>
   </servlet-mapping>
   

   Priority questions

   The inherent mapping path is specified with the highest priority , If it cannot be found, it will follow the default request path ;

5.ServletContext

web When the container starts , It will web Programs create a corresponding servletContext object , It represents the present web application ;

1. Shared data

eg:

Add attribute

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        ServletContext context = this.getServletContext();
        String username = " Zhang San ";
        context.setAttribute("username", username); // Key value pair form input 
    }

get attribute ：

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    ServletContext context = this.getServletContext();
    String name = (String) context.getAttribute("username");

    resp.setCharacterEncoding("utf-8");
    resp.setContentType("text/html");
    resp.getWriter().println(" full name "+name);
}

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    super.doGet(req, resp);
}

To configure

<servlet>
    <servlet-name>hello</servlet-name>
    <servlet-class>com.wan.servlet.HelloServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello</url-pattern>
</servlet-mapping>

<servlet>
    <servlet-name>getc</servlet-name>
    <servlet-class>com.wan.servlet.GetServletContext</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>getc</servlet-name>
    <url-pattern>/getc</url-pattern>
</servlet-mapping>

2. Get initialization parameters

xml Set initialization parameters in
eg： Database parameters

<context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql:http://localhost:3306/mybatis</param-value>
</context-param>

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    String url = this.getServletContext().getInitParameter("url");
    resp.getWriter().println(url);
}

3. Request forwarding

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    ServletContext context = this.getServletContext();
    System.out.println(" Into the RequestD");
    context.getRequestDispatcher("/getp").forward(req, resp); // call forward Implement request forwarding ;
}

4. Read resource file

Properties

• stay java New under the directory properties( Pay attention to the configuration pom.xml file , Or maybe maven Cannot export properties file )

  pom.xml The configuration file

<!-- stay build Middle configuration resources, To prevent our resource export failure -->
<build>
    <resources>
        <resource>
            <directory>src/main/resources</directory>
            <includes>
                <includes>**/*.properties</includes>
                <includes>**/*.xml</includes>
            </includes>
            <filtering>false</filtering>
        </resource>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>false</filtering>
        </resource>
    </resources>
</build>

• stay resource Under the new properties

Need a file stream ;

username = root
password = 123456

java Code

public class PropertiesServlet extends HelloServlet{
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        InputStream is = getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop = new Properties();
        prop.load(is);
        String usr = prop.getProperty("username");
        String pwd = prop.getProperty("password");
        resp.getWriter().println(usr+":"+pwd);

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        super.doGet(req,resp);
    }
}

web.xml

<servlet>
    <servlet-name>ps</servlet-name>
    <servlet-class>com.wan.servlet.PropertiesServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>ps</servlet-name>
    <url-pattern>/ps</url-pattern>
</servlet-mapping>

Access test

6.HttpServletRequest

HttpServletRequest Requests on behalf of clients , User pass Http Protocol access server ,HTTP All information in the request will be encapsulated in HttpServletRequest, adopt HttpServletRequest The method in , Get all the information from the client ;

1. Get the parameters passed by the front end

index.jsp Home page

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@page isELIgnored="false"%>
<html>
<head>
    <title> Sign in </title>
</head>
<body>
<div style="text-align: center">
    <form action="${pageContext.request.contextPath}/login" method="post">

        <p> user name :<input type="text" name="username"></p>
        <p> password ：<input type="password" name="password"></p>
        <p>
             hobby ：
            <input type="checkbox" name="hobbys" value=" Code "> Code 
            <input type="checkbox" name="hobbys" value=" The movie "> The movie 
            <input type="checkbox" name="hobbys" value=" Sing a song "> Sing a song 
        </p>
        <input type="submit" value=" Sign in ">
    </form>
</div>

</body>
</html>

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    System.out.println("hello");
    req.setCharacterEncoding("utf-8");
    resp.setCharacterEncoding("utf-8");
    String password = req.getParameter("password");
    String username = req.getParameter("username");
    String[] hobbys = req.getParameterValues("hobbys");
    System.out.println("==========================");
    System.out.println(" user name ："+username);
    System.out.println(" password :"+password);
    System.out.println(" hobby ："+ Arrays.toString(hobbys));
    System.out.println("==========================");
    // Request forwarding 
    req.getRequestDispatcher("success.jsp").forward(req,resp);
}

7.HttpServletResponse

web The server received... From the client http request , In response to this request , Create one for each request HttpServletRequest object , Represents a response HttpServletResponse;

• If you want to get the parameters requested by the client ： look for HttpServletRequest
• If you want to respond to some information to the client ： look for HttpServletResponse

1. Simple classification

Responsible for sending data to the browser

• ServletOutputStream getOutputStream() throws IOException;
  PrintWriter getWriter() throws IOException;
  

The method responsible for sending the response header to the browser

void setCharacterEncoding(String var1);
void setContentLength(int var1);
void setContentLengthLong(long var1);
void setContentType(String var1);
void setDateHeader(String var1, long var2);
void addDateHeader(String var1, long var2);
void setHeader(String var1, String var2);
void addHeader(String var1, String var2);
void setIntHeader(String var1, int var2);
void addIntHeader(String var1, int var2);
void setStatus(int var1);

The status code of the response

int SC_CONTINUE = 100;
int SC_SWITCHING_PROTOCOLS = 101;
int SC_OK = 200;
int SC_CREATED = 201;
int SC_ACCEPTED = 202;
int SC_NON_AUTHORITATIVE_INFORMATION = 203;
int SC_NO_CONTENT = 204;
int SC_RESET_CONTENT = 205;
int SC_PARTIAL_CONTENT = 206;
int SC_MULTIPLE_CHOICES = 300;
int SC_MOVED_PERMANENTLY = 301;
int SC_MOVED_TEMPORARILY = 302;
int SC_FOUND = 302;
int SC_SEE_OTHER = 303;
int SC_NOT_MODIFIED = 304;
int SC_USE_PROXY = 305;
int SC_TEMPORARY_REDIRECT = 307;
int SC_BAD_REQUEST = 400;
int SC_UNAUTHORIZED = 401;
int SC_PAYMENT_REQUIRED = 402;
int SC_FORBIDDEN = 403;
int SC_NOT_FOUND = 404;
int SC_METHOD_NOT_ALLOWED = 405;
int SC_NOT_ACCEPTABLE = 406;
int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
int SC_REQUEST_TIMEOUT = 408;
int SC_CONFLICT = 409;
int SC_GONE = 410;
int SC_LENGTH_REQUIRED = 411;
int SC_PRECONDITION_FAILED = 412;
int SC_REQUEST_ENTITY_TOO_LARGE = 413;
int SC_REQUEST_URI_TOO_LONG = 414;
int SC_UNSUPPORTED_MEDIA_TYPE = 415;
int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
int SC_EXPECTATION_FAILED = 417;
int SC_INTERNAL_SERVER_ERROR = 500;
int SC_NOT_IMPLEMENTED = 501;
int SC_BAD_GATEWAY = 502;
int SC_SERVICE_UNAVAILABLE = 503;
int SC_GATEWAY_TIMEOUT = 504;
int SC_HTTP_VERSION_NOT_SUPPORTED = 505;

It's enough to remember the general

200： Request response successful

3**： request redirections

404： Resource not found

• Resource does not exist

5XX： Server code error 500 502： Gateway error

2. Download the file

1. Output message to browser

2. Download the file

   1. To get the path of the downloaded file

   2. Download file name

   3. The setting method enables the browser to support downloading what we need

   4. Get the input stream of the download file

   5. Create buffer

   6. obtain outputstream object

   7. take FileOut Ordinary Stream Stream write to buffer buffer

   8. Use OutPutStream Output the data in the buffer to the client

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    // 1. To get the path of the downloaded file 
    String realPath = "D:\\code\\javaweb\\javaweb-02-servlet\\response\\src\\main\\resources\\ tencent qqicon.png";
    //String realPath = this.getServletContext().getRealPath("/ tencent qqicon.png");  This is the wrong path to find 
    System.out.println(" Download file path ："+realPath);
    // 2. Download file name 
    String filename = realPath.substring(realPath.lastIndexOf("\\") + 1);
    System.out.println("filename="+filename);
    // 3. The setting method enables the browser to support downloading what we need , Chinese file name URLEncoder.encode code , Otherwise, it may be garbled 
    resp.setHeader("Content-disposition","attachment;filename="+ URLEncoder.encode(filename,"utf-8"));
    // 4. Get the input stream of the download file 
    FileInputStream in = new FileInputStream(realPath);
    // 5. Create buffer 
    int len = 0;
    byte[] buffer = new byte[1024];
    // 6. obtain outputstream object 
    ServletOutputStream out = resp.getOutputStream();
    // 7. take FileOutStream Stream write to buffer buffer 
    while((len = in.read(buffer))!=-1){
    
    // 8. Use OutPutStream Output the data in the buffer to the client 
        out.write(buffer,0,len);
    }
    in.close();
    out.close();
}

3. Verification code function

How to get the verification code ？

• The front-end implementation
• The backend implementation , use java Picture class of , Produce a picture

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    // browser 3 Refresh every second 
    resp.setHeader("refresh","3");
    // Create an image in memory 
    BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
    Graphics2D g = (Graphics2D) image.getGraphics();
    // Set the picture background color 
    g.setColor(Color.white);
    g.fillRect(0,0,80,20);
    g.setColor(Color.BLUE);
    g.setFont(new Font(null,Font.BOLD,20));
    g.drawString(makeNum(),0,20);
    // Tell the browser , This request is opened in the form of a picture 
    resp.setContentType("image/jpeg");
    // There is a cache on the site , Don't let the browser cache 
    resp.setDateHeader("expires",-1);
    resp.setHeader("Cache-control","no-cache");
    resp.setHeader("Pragma","no-cache");
    // Return the image to the browser 
    ImageIO.write(image, "jpg", resp.getOutputStream());
}

/** *  Generate random number  * @return */
private String makeNum(){
    
    Random random = new Random();
    String num = random.nextInt(9999999) + "";
    StringBuffer sb = new StringBuffer();
    for (int i = 0; i < 7-num.length(); i++) {
    
        sb.append("0");
    }
    num = sb.toString()+num;
    return num;
}

4. Implement redirection

One web After the resource receives a client request , He will tell the client to access another web resources C, This process is called redirection

void sendRedirect(String var1) throws IOException;

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    /* resp.setHeader("Location","/response_Web_exploded/image"); resp.setStatus(302); */
    resp.sendRedirect("/response_Web_exploded/image");

}

The difference between redirection and forwarding ：

The same thing ：

• Page will jump

Difference ：

• When the request is forwarded ,url Will change 307
• When redirecting ,url And then change 302

Case study

demand ： Redirect after login

RequestTest.java

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    System.out.println(" Get into requestTest class ！");
    String username = req.getParameter("username");
    String password = req.getParameter("password");
    System.out.println(username+":"+password);
    resp.sendRedirect("/response_Web_exploded/success.jsp");
}

web.xml

<servlet>
    <servlet-name>request</servlet-name>
    <servlet-class>RequestTest</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>request</servlet-name>
    <url-pattern>/login</url-pattern>
</servlet-mapping>

index.jsp

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
</head>
<body>
<h2>Hello World!</h2>
<%@ page pageEncoding="UTF-8" %>
<form action="${pageContext.request.contextPath}/login" method="get">
    <p> user name ：<input type="text" name="username"></p>
    <p> password ：<input type="password" name="password"><br></p>
    <input type="submit">
</form>

</body>
</html>

success.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>success</title>
</head>
<body>
<h1>congratulation</h1>
</body>
</html>