Share some information about the servlet life cycle in Java

This article mainly introduces the relevant information of the servlet life cycle in detail, which has certain reference value. Interested friends can refer to it

This article shares the relevant information of the servlet life cycle with you. Information for your reference, the specific content is as follows

1.Servlet life cycle: Servlet loading--->instantiation--->service--->destruction.
2.init(): In the life cycle of the Servlet, the init() method is only executed once. It is executed when the server loads the Servlet and is responsible for initializing the Servlet object. The server can be configured to load the servlet when the server is started or when a client accesses the servlet for the first time. No matter how many clients access the Servlet, init() will not be executed repeatedly.
3.service(): It is the core of Servlet and is responsible for responding to customer requests. Whenever a client requests an HttpServlet object, the object's Service() method is called, and a "request" (ServletRequest) object and a "response" (ServletResponse) object are passed to this method as parameters. The Service() method already exists in HttpServlet. The default service function is to call the do function corresponding to the HTTP request method.
4.destroy(): Executed only once, this method is executed when the server stops and the Servlet is uninstalled. When the Servlet object exits the life cycle, it is responsible for releasing the occupied resources. A Servlet may generate other threads when running the service() method, so you need to confirm that these threads have been terminated or completed when the destroy() method is called.

How Tomcat and Servlet work:

Steps:

1 .Web Client sends an Http request to the Servlet container (Tomcat)
2. The Servlet container receives the request from the Web Client
3. The Servlet container creates a Request object and encapsulates the information requested by the Web Client into this object.
4. The Servlet container creates a Response object
5. The Servlet container calls the service method of the HttpServlet object and passes the Request object and Response object as parameters to the Servlet object.
6.Servlet calls the relevant methods of the Request object to obtain Http request information.
7.HttpServlet calls the relevant methods of the Response object to generate response data.
8. The Servlet container passes the response result of HttpServlet to the Web Client.

Servlet working principle:

1. First, briefly explain the process of Servlet receiving and responding to customer requests. First, the customer sends a request, and the Servlet The service() method is called to respond to the request. It can be seen from the source code that the service() method matches the request method, chooses to call doGet, doPost and other methods, and then enters the corresponding method to call the logic layer. Methods to implement responses to customers. There are no doGet(), doPost(), etc. methods in the Servlet interface and GenericServlet. These methods are defined in HttpServlet, but they all return error information. Therefore, every time we define a Servlet, we must implement doGet. Or doPost and other methods.
2. Every customized Servlet must implement the Servlet interface. The Servlet interface defines five methods. Three of the more important methods involve the life cycle of the Servlet, namely the init( mentioned above). ), service(), destroy() methods. GenericServlet is a general Servlet that is not specific to any protocol and implements the Servlet interface. HttpServlet inherits from GenericServlet, so HttpServlet also implements the Servlet interface. So when we define Servlet, we only need to inherit HttpServlet.
3. The Servlet interface and GenericServlet are not specific to any protocol, while HttpServlet is a class specific to the HTTP protocol, so the service() method is implemented in HttpServlet and the requests ServletRequest and ServletResponse are forced to HttpRequest and HttpResponse.

Timing to create a Servlet object:

1. When the Servlet container is started: read the information in the web.xml configuration file and construct the specified Servlet object, create a ServletConfig object, and use the ServletConfig object as a parameter to call the init method of the Servlet object.
2. After the Servlet container is started: the customer makes a request to the Servlet for the first time. The Servlet container will determine whether the specified Servlet object exists in the memory. If not, create it, and then create the HttpRequest and HttpResponse objects according to the customer's request, thereby calling The service method of the Servlet object.
3.Servlet The Servlet container automatically creates a Servlet when it starts. This is determined by the attribute set for the Servlet in the web.xml file. From this we can also see that Servlet objects of the same type exist as singletons in the Servlet container.

<servlet>
  <servlet-name>Init</servlet-name>
  <servlet-class>org.xl.servlet.InitServlet</servlet-class>
  <load-on-startup>1</load-on-startup>
</servlet>

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