Servlet-02 lifecycle

Servlet The life cycle of Servlet From creation to destruction .Servlet The life cycle of Servlet Container management , Mainly divided into the following 5 Stages ：

1. Loading phase
2. Instantiation phase
3. Initialization phase （ call init() Method ）
4. Service stage （ call service() Method ）
5. The destruction （ call destroy( Method )）

When the server starts ,Servlet The container first reads web.xml Configuration information in , And load 、 initialization load-on-startup The value is less than 0 Or no specified Servlet class （ Other Servlet Load and initialize when first requested ）. Initialize it Servlet Object will handle all client requests , Until the last time the server shuts down , Will destroy Servlet object , perform destroy() Method .

1、 load 、 Instantiation and initialization Servlet

When the server starts , Load and read web.xml file

<?xml version="1.0" encoding="UTF-8"?>
<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" metadata-complete="true">

    <servlet>
        <servlet-name>myServlet-01</servlet-name>
        <servlet-class>com.chen.servlet.myServlet01</servlet-class>
        <!--Web Server Startup time ,Servlet It also loads 、 Instantiation -->
        <load-on-startup>0</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>myServlet-01</servlet-name>
        <url-pattern>/s1</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>myServlet-02</servlet-name>
        <servlet-class>com.chen.servlet.myServlet02</servlet-class>
        <!-- Only when the first request occurs ,Servlet Before loading 、 Instantiation -->
        <load-on-startup>-1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>myServlet-02</servlet-name>
        <url-pattern>/s2</url-pattern>
    </servlet-mapping>
</web-app >

• because load-on-startup The value of the element ≥ 0, therefore myServlet01 It is loaded when the server starts 、 Instantiation .
• because load-on-startup The value of the element < 0, therefore myServlet02 It will not be loaded until it is first requested 、 Instantiation .

Servlet Once the class is loaded 、 Instantiation , The container will call init(ServletConfig conf) Method is used to initialize the Servlet object .

@Override
public void init() throws ServletException {
    
    System.out.println("myServlet01 initialization ！");
}

2、 Service stage

Servlet When the container receives a request from a client , A separate... Will be created for the request ServletRequest object req and ServletResponse object resp, Pass them in as parameters service() In the way , And call this method to process the request .

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    System.out.println(" Called myServlet02 Of service() Method ");
}

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

• When Servlet When the container returns the response information to the client ,ServletRequest Objects and ServletResponse The object will be destroyed ;
• about Servlet Every request of ,Servlet The container will call once service() Method , namely service() Method in Servlet Will be called many times in the life cycle of

3、 Destruction phase

When the server is down ,Servlet The container will call destory() Method , The destruction Serlvet example , Release the resources used by the station .

@Override
public void destroy() {
    
    System.out.println("myServlet01 Be destroyed ");
}

4、 Test case

1. web.xml The configuration file

   <?xml version="1.0" encoding="UTF-8"?>
   <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" metadata-complete="true">
   
       <servlet>
           <servlet-name>myServlet-01</servlet-name>
           <servlet-class>com.chen.servlet.myServlet01</servlet-class>
           <!--Web Server Startup time ,Servlet It also loads 、 Instantiation -->
           <load-on-startup>0</load-on-startup>
       </servlet>
       <servlet-mapping>
           <servlet-name>myServlet-01</servlet-name>
           <url-pattern>/s1</url-pattern>
       </servlet-mapping>
   
       <servlet>
           <servlet-name>myServlet-02</servlet-name>
           <servlet-class>com.chen.servlet.myServlet02</servlet-class>
           <!-- Only when the first request occurs ,Servlet Before loading 、 Instantiation -->
           <load-on-startup>-1</load-on-startup>
       </servlet>
       <servlet-mapping>
           <servlet-name>myServlet-02</servlet-name>
           <url-pattern>/s2</url-pattern>
       </servlet-mapping>
   </web-app >
   

2. myServlet01 class

   public class myServlet01 extends HttpServlet {
         
       @Override
       protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         
           System.out.println(" Called myServlet02 Of service() Method ");
       }
   
       @Override
       protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         
           doGet(req, resp);
       }
   
       @Override
       public void init() throws ServletException {
         
           System.out.println("myServlet01 initialization ！");
       }
   
       @Override
       public void destroy() {
         
           System.out.println("myServlet01 Be destroyed ");
       }
   }
   

3. myServlet02 class

   public class myServlet02 extends HttpServlet {
         
       @Override
       protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         
           System.out.println(" Called myServlet02 Of service() Method ");
       }
   
       @Override
       protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         
           doGet(req, resp);
       }
   
       @Override
       public void init() throws ServletException {
         
           System.out.println("myServlet02 initialization ！");
       }
   
       @Override
       public void destroy() {
         
           System.out.println("myServlet02 Be destroyed ！");
       }
   }
   

4. test

   function Tomcat The server , You can find myServlet01 During the server startup phase , The initialization is complete .
   
   visit myServlet01 Registered URL：http://localhost:8080/servlet08/s1, Get the picture below ：
   
   visit myServlet02 Registered URL：http://localhost:8080/servlet08/s2,Servlet The container is initialized on the first access myServlet02, And call service() Method .
   
   close Tomcat The server ,myServlet01 and myServlet02 Were destroyed .
   

5、 summary

Servlet The life cycle of is divided into ： Loading phase 、 Instantiation phase 、 Initialization phase and destruction phase . The first three phases are bound together and occur continuously .

• if Servlet Of load-on-startup Element value ≥ 0, The first three phases occur when the server starts ;
• if Servlet Of load-on-startup Element value < 0, The first three phases occur on the first request .