The Servlet,

1. Servlets run in Java-enabled application servers to interactively browse and modify data and generate dynamic Web content.

• We are going to customize a Servlet and we have to implement the Servlet interface broadly
• The Servlet interface has two implementation classes, and HttpServlet in turn inherits the GenericServlet class, both of which are abstract classes
  • GenericServlet
  • HttpServlet
• We typically create JavaWeb projects based on the Http protocol, so creating servlets inherits httpservlets

2. Servlet working mode

• The client sends the request to the server
• The server starts and accepts the request sent by the client, invokes the corresponding Servlet to process the request, and generates the response content and passes it to the server
• The server returns the response content to the client

3. Servlet lifecycle

public interface Servlet {
    void init(ServletConfig var1) throws ServletException;
 
    void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;
 
    void destroy(a);
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• When a Servlet is first requested, the server calls the init() method to initialize a Servlet object, but this method is not called again in subsequent requests, only once. When this method is called, the Servlet container passes in a ServletConfig object to initialize the Servlet object.
• Each time a client requests this Servlet, the Servlet container calls the service() method to handle the request; On subsequent requests, the Servlet container will only call this method.
• When the server shuts down or stops, the Servlet container calls Destroy () to destroy the Servlet.
• Each Servlet has only one Servlet object in the container, which is a singleton

4. ServletContext object

When the Web container is started, it creates a unique ServletContext object for each Web application, which represents the current Web application

Shared data

@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // Use ServletContext to share data
        ServletContext servletContext = this.getServletContext();
        servletContext.setAttribute("username"."Vecal");
    }

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

        resp.setContentType("text/html; charset=utf-8");
        resp.setCharacterEncoding("utf-8");

        resp.getWriter().print("Name:" + username);
    }
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<! -- Configure Servlet mapping -->
<servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.vecal.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>get</servlet-name>
        <servlet-class>com.vecal.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>get</servlet-name>
        <url-pattern>/get</url-pattern>
    </servlet-mapping>
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Gets initialization parameters

<! -- Custom initialization parameters -->
<context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
</context-param>
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@Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);
    }
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Forward requests

// Request forwarding
ServletContext context = this.getServletContext();
context.getRequestDispatcher("/initParam").forward(req,resp);
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Reading resource files

• Create a new resource file in the Java directory

• Create a new resource file under the Resources directory

All are packaged into the same directory: classes, collectively known as classpath: path

If you find that the resource files in the Java directory are not packaged in the classes directory, you need to do the following configuration in pom.xml

<build>
      <resources>
        <resource>
            <directory>src/main/resources</directory>
            <excludes>
                <exclude>**/*.properties</exclude>
                <exclude>**/*.xml</exclude>
             </excludes>
            <filtering>false</filtering>
        </resource>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>false</filtering>
        </resource>
    </resources>
</build>
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@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // Read the configuration file
        ServletContext context = this.getServletContext();
        InputStream in = context.getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop = new Properties();
        prop.load(in);

        String username = prop.getProperty("username");
        String password = prop.getProperty("password");

        resp.getWriter().print(username + ":" + password);
    }
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5, HttpServletResponse

When the Web server receives the HTTP request from the client, it creates an HttpServletRequest object representing the request and an HttpServletResponse object representing the response

• To get some of the data parameters of a client request, use HttpServletRequest
• To respond with some data to the client, use HttpServletResponse

1. Send data to the browser

ServletOutputStream getOutputStream(a) throws IOException;

PrintWriter getWriter(a) throws IOException;
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2. Set the response header

void setCharacterEncoding(String var1);

void setContentLength(int var1);

void setContentLengthLong(long var1);

void setContentType(String var1);
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3. Response status code

int SC_OK = 200;
int SC_FOUND = 302;
int SC_BAD_REQUEST = 400;
int SC_INTERNAL_SERVER_ERROR = 500;
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4. Download the file

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 1. Obtain the image path
        String realPath = "D:\\How2JTest\\project\\ Javaweb-02 -servlet\\ Response \\target\\ Response \ web-INF \classes\\;
        // 2. Obtain the file name
        String fileName = realPath.substring(realPath.lastIndexOf("/") + 1);
        System.out.println("File download path:" + realPath);
        // 3. Set the response header,URLEncoder. Encode () to handle the filename Chinese garble problem
        response.setHeader("Content-Disposition"."attachment; filename=" + URLEncoder.encode(fileName,"UTF-8"));
        // 4. Get the file stream
        FileInputStream in = new FileInputStream(realPath);
        // 5. Set the buffer
        int len = 0;
        byte[] buffer = new byte[1024];

        // 6. Write the file
        ServletOutputStream os = response.getOutputStream();
        while((len = in.read(buffer)) ! = -1){
            os.write(buffer, 0, len);
        }

        os.close();
        in.close();
    }
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5. Implement redirection

• The URL address bar will change

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse response) throws ServletException, IOException {
    // The request is redirected and the URL bar is changed
    response.sendRedirect("/r/down");
}
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6, it

HttpServletRequest represents a client request. The user accesses the server through THE HTTP protocol, and the HTTP server encapsulates all the request information into this object.

1. Obtain parameters passed by the front-end

// Gets an argument with the specified name
String getParameter(String var1);
// Get multiple arguments with the specified name
String[] getParameterValues(String var1);
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7. The difference between request and forward

Interview question: Can you talk about the difference between forwarding and redirecting?

Similarities:

• The page jumps

Difference:

• The URL address bar does not change when a request is forwarded
• The URL address bar changes when the request is redirected