Servlet life cycle

Servlet life cycle

The life cycle of a servlet, as its name suggests, is the whole process from the appearance of a servlet to its demise (destruction).

Mainly divided into the following stages:
Loading class—>Instantiation (Allocating space for objects)—>Initialization(assignment to the properties of the object)—>request response (service phase)—>destroy

Three methods of the servlet life cycle:

init() initialization phase

service() handles client request phase

destroy() termination phase

​ The container (tomcat, etc.) loads the servlet

Instantiation Phase

1.1 When the client sends the first request for the first time, the servlet container parses the request and finds whether there is the corresponding servlet according to the request.
1.2 Determine whether there is an object of the Servlet implementation class?If it exists, use it directly. If it does not exist, first create an object of the servlet implementation class.

Initialization Phase

Servlet initialization is the first phase of its life cycle and the basis for other phases.Only after initialization is complete, the servlet can process requests from clients.

Servlet initialization phase is divided into 2 steps:

1. Load and instantiate the servlet;
2. Call the init() method to initialize

1. Loading and instantiation

The servlet container is responsible for loading and instantiating servlets.

When the container starts or requests a servlet for the first time, the container will read the configuration information in web.xml (configure load-on-startup=1, the default is 0) or @WebServlet, and load the specified servlet.After the loading is successful, the container will instantiate the servlet through reflection.

2.Call the init() method to initialize

After loading and instantiation is completed, the servlet container will create a servlet object and call the servlet's init method (the init method can only be called once in the servlet life cycle) to initialize the servlet instance.

Request Response Phase

After the initialization is completed, the service() method is called, and the service() determines the client's request method.
3.1 If it is a get request, execute the doGet() method.
3.2 If it is a post request, execute doPost().
3.3 After the processing method is completed, the corresponding result will be returned to the client, and the processing of a single request is completed.

When the user sends the second and subsequent requests, it will determine whether the object exists, but no longer execute init(), and directly execute the service method to call the doGet() / doPost() method.

Service Termination Phase

When the server shuts down, restarts or removes the servlet instance, the servlet calls the destroy() method to destroy it, announcing the end of the life cycle.

public class EmpServlet extends HttpServlet{
    //初始化servlet,调用init方法
  
    public void init() throws ServletException {
        System.out.println("初始化时调用");
    }
    //开启服务
  
    protected void service(HttpServletRequest arg0, HttpServletResponse arg1)throws ServletException, IOException {
System.out.println("Called when the service is started");
}

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
     
  }
  // call destroy when destroyed

public void destroy() {
System.out.println("call when destroyed");
} }
}