The servlet life cycle is divided into several stages

Servlet life cycle refers to the entire process of servlet from creation to destruction, which can be divided into 3 stages: 1. Initialization stage, calling the init() method to implement the initialization work of Servlet; 2. Running stage (processing requests ), the container will create a ServletRequest object representing the HTTP request and a ServletResponse object representing the HTTP response for the specified request, and then pass them as parameters to the service() method of the Servlet; 3. Destruction phase.

The operating environment of this tutorial: windows7 system, java8 version, DELL G3 computer.

1. What is Servlet and its function?

1. What is Servlet?

Servlet (Server Applet) is the abbreviation of Java Servlet, is called a small service program or service connector, a server-side program written in Java, which has independent Platform and protocol characteristics, is a program running on a Web server or application server.

2. The role of Servlet

It is used as a server from a web browser or other HTTP client The middle layer between the request and the database or application on the HTTP server. The main function is to collect user input from web forms, interactively browse and generate data, and also generate dynamic web content.

2. Servlet life cycle

The Servlet life cycle can be defined as the entire process from creation to destruction .

The three steps are: initialization------> processing request------> destruction

Initialization (call init () method)--------->Process/respond to customers End request (call service() method) --------->Destroy (call destroy() method, finally Garbage collection is performed by the JVM's garbage collector)

1) Initialization phase

2) Running phase (processing request)

3) Destruction phase

3. Detailed explanation of life cycle methods

Create a Servlet object and create a Servlet object through the server reflection mechanism. First It will be created upon request. (Default)

1. init() method

Call Servlet The object's init() method initializes Servlet information. The init() method will only be called once after creation (initialization phase) (1)
The init method is designed to be called only once. It is called when the Servlet is first created and will no longer be called on each subsequent user request. Therefore, It is used for one-time initialization, Created when the user calls the URL of the Servlet for the first time, or you can specify that the Servlet is loaded when the server starts for the first time.

(2)When the user calls a Servlet, a Servlet instance will be created, and each user request will generate a new thread, handed over to the doGet or doPost method when appropriate.

(3) The init() method simply creates or loads some data, These data will be used throughout the entire life cycle of the Servlet.

The init method is defined as follows:

public void init() throws ServletException {
  // 初始化代码...
}

2. service() method

In response to the request, call service() or doGet(), doPost() method to process the request. These methods run in a multi-threaded state. (Response to client request phase)

(1) The service() method is the main method to perform actual tasks. It handles requests from the client (browser) and formats them. The response is written back to the client.

(2) Each time the server receives a Servlet request, the server will generate a new thread and call the service.

(3) The process of Servlet receiving and responding to customer requests. First, the customer sends a request. The Servlet calls the service() method to respond to the request. It can be seen from the source code that in the service() method Match the request method, choose to call doGet, doPost and other methods, and then enter the corresponding method to call the logic layer method to implement the response to the customer. Since there are no doGet, doPost, etc. methods in the Servlet interface and GenericServlet, these methods are defined in HttpServlet. Therefore, Every time we define a Servlet, we must implement doGet or doPost and other methods.

(4) service() method Check the HTTP request type (GET, POST, PUT, DELETE, etc.), and call doGet, doPost, doPut, doDelete when appropriate and other methods to process.

The service method is defined as follows:

public void service(ServletRequest request,ServletResponse response) 
      throws ServletException, IOException{
}

Since the service() method is called by the container, the service method will call doGet, doPost, doPut, doDelete, etc. when appropriate. method. Therefore, We don’t need to do anything with the service() method. We only need to rewrite doGet() or doPost() according to the client’s request type.

3. doGet() method

is used to process requests, The method is run in multi-threaded state. (Response to customer request phase)

doGet() is the most commonly used method in service requests. It is used to process requests. When the request type of the URL is a GET request, Or if it is an unspecified request typeHTML form, then the request will be processed by the doGet() method.

The doGet method is defined as follows:

public void doGet(HttpServletRequest request,HttpServletResponse response)
    throws ServletException, IOException {
    // Servlet 代码
}

##4. doPost() method

is used to process requests, and the method is run in a multi-threaded state. (Response to customer request phase)

request type as POST HTML form, then the request will be processed by the doPost() method.

The doPost method is defined as follows:

public void doPost(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
    // Servlet 代码
}

##5. destroy() method

The destroy() method will be called to destroy the Servlet object when it has not been called for a long time or the server is shut down. (Termination phase) (1) The

(2) The destroy() method allows the

(3) After calling the destroy() method, the

public void destroy() {
    // 终止化代码...
  }

4. Life cycle diagram It will be much easier to look at the period chart after understanding the above content.

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