Servlet usage

summary

Is running on the web On the server java Program , Used to handle the front end （ client ） Responses and requests for

Life cycle

Servlet The object is tomcat Server created . By default, it is entered in the browser for the first time servlet Create this object when accessing , You can also create objects as soon as the server is started by configuring

Server down servlet It's destroyed

Use

Method

/**
 *  Run a project , When the server is turned on, it is not executed , Enter the address to access ,init（）service（） function ,
 *          Refresh once service Do it once , When shutting down the server tomcat When ,init（）service（） The destruction ,destroy perform 
 */
public class DemoServlet implements Servlet {

   //servlet When the object is created, execute ( Not at all tomcat When the server starts   Is the access entered in the browser servlet Class time )
    @Override
    public void init(ServletConfig servletConfig) throws ServletException {
        //ServletConfig Parameter function  1、 Get configuration information  2、 obtain xml Parameters in  3、 Get all parameter names  4、 obtain ServletContext object 
        servletConfig.getServletName();
        servletConfig.getInitParameter("xxx");
        servletConfig.getInitParameterNames();
        servletConfig.getServletContext();
    }

    @Override
    public ServletConfig getServletConfig() {
        return null;
    }

    //ServletRequest ： On behalf of request   Think ServletRequest  What's inside the package is http Requested information ,
    // ServletResponse ： For response   It is believed that the information to be encapsulated is the response information 
    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {

    }

    @Override
    public String getServletInfo() {
        return null;
    }

    //servlet Executed at the time of destruction 
    @Override
    public void destroy() {

    }
}

To configure

test

Browser input http://localhost:8080/web02---servlet practice /abc You can see it in the background servlet Content in method

Perform process analysis

Configuration instructions

url-pattern To configure

Default servlet

Can be url-pattern Configure as a /, On behalf of servlet It's the default servlet, When you access the resource address all servlet When none of them match , Default servlet Responsible for handling

overall situation web.xml

Static resource loading process

3.0 Time

Realization

from servlet3.0 From the beginning, you can directly use annotations to create servlet, direct new One servlet

// With this annotation   There is no need to write configuration files , The content in brackets is in the address bar path What follows 

@WebServlet("/myFirsetServlet")
public class MyFirsetServlet extends HttpServlet {

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

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

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

web.xml

metadata-complete Whether to scan annotations true Don't scan false scanning

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
                      http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
  version="3.0"
  metadata-complete="false">
  <absolute-ordering/>
  <display-name>market</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>

Calling procedure of internal method

Receive a servlet request ,tomcat Will find this servlet Corresponding service Method , without service Methods will go to the parent class

The following source code , The interior will ServletRequest Parameter conversion HttpServletRequest, After the conversion is completed, continue to call HttpServletRequest Of service Method , Call different methods according to different parameter types

    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        HttpServletRequest request;
        HttpServletResponse response;
        try {
            request = (HttpServletRequest)req;
            response = (HttpServletResponse)res;
        } catch (ClassCastException var6) {
            throw new ServletException(lStrings.getString("http.non_http"));
        }

        this.service(request, response);
    }

    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        long lastModified;
        if (method.equals("GET")) {
            lastModified = this.getLastModified(req);
            if (lastModified == -1L) {
                this.doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
                    ifModifiedSince = req.getDateHeader("If-Modified-Since");
                } catch (IllegalArgumentException var9) {
                    ifModifiedSince = -1L;
                }

                if (ifModifiedSince < lastModified / 1000L * 1000L) {
                    this.maybeSetLastModified(resp, lastModified);
                    this.doGet(req, resp);
                } else {
                    resp.setStatus(304);
                }
            }
        } else if (method.equals("HEAD")) {
            lastModified = this.getLastModified(req);
            this.maybeSetLastModified(resp, lastModified);
            this.doHead(req, resp);
        } else if (method.equals("POST")) {
            this.doPost(req, resp);
        } else if (method.equals("PUT")) {
            this.doPut(req, resp);
        } else if (method.equals("DELETE")) {
            this.doDelete(req, resp);
        } else if (method.equals("OPTIONS")) {
            this.doOptions(req, resp);
        } else if (method.equals("TRACE")) {
            this.doTrace(req, resp);
        } else {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[]{method};
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(501, errMsg);
        }

    }