Servlets will know and will know

The Servlet is briefly

A Servlet is a Java class that usually plays the role of Controler in the MVC pattern of Web applications. Its job is to receive a request from a customer and send back a response. After receiving the request, the model is invoked to process the request data and set the processed data as the request properties. And send it to the JSP of the control page. Let’s take a look at how the Servlet works with a complete HTTP request.

The arrival of an HTTP request

<servlet>
	<servlet-name>ActionServletName</servlet-name>
	<servlet-class>com.gyf.web.ActionServlet</servlet-class>
</servlet>
<servlet-mapping>
	<servlet-name>ActionServletName</servlet-name>
	<url-pattern>/action.do</url-pattern>
</servlet-mapping>Copy the code

The life cycle of a Servlet

Through the above process, the container finds the Servlet to call, and if the Servlet class has not been loaded, the container calls the lifecycle of the Servlet from scratch:

1. First load the target class (actionservlet.class), then call the Servlet’s default no-argument constructor (note that we don’t need to override the Servlet’s constructor).

1. The init() method is then called, which is only called once in the lifetime of the Servlet if you have other initialization code (such as getting a database connection).
2. The Service() method is then called. If the container finds that the Servlet class has been loaded, it skips the previous two steps and goes directly to this step. Each time an HTTP request arrives, the Service method of the target Servlet is called. The Service method starts a new thread each time it is called, depending on the type of HTTP request, whether to continue calling doGet() or doPost(). The Service method can be called multiple times during the lifetime of the Servlet.
3. Finally, the Servlet class is killed with a call to the destroy() method, where garbage collection cleans up resources.

Servlet life cycle

Note that there is only one instance of each particular Servlet class on each JVM, so there is no such thing as every instance of a Servlet.

Inheritance structure and methods of servlets

ServletConfig and ServletContext

PrintWriter out=response.getWriter();
out.println("59833576*@qq.com");Copy the code

ServletConfig

Write this in the deployment description file:

<servlet>
	<servlet-name>ActionServletName</servlet-name>
	<servlet-class>com.gyf.web.ActionServlet</servlet-class>
  	<init-param>
  		<param-name>adminEmail</param-name>
      	<param-value>59833576*@qq.com</param-value>
    </init-param>
</servlet>
<servlet-mapping>
	<servlet-name>ActionServletName</servlet-name>
	<url-pattern>/action.do</url-pattern>
</servlet-mapping>Copy the code

You can see that init-param is within the servlet tag, which means that it is only used within the servlet class, not globally.

In servlets we use this:

out.println(getServletConfig().getInitParameter("adminEmail"));Copy the code

Note that this method cannot be called in the constructor, and the Servlet gets the ServletConfig object only after init().

####ServletContext

The ServletContext is globally valid, as can be seen from where it is deployed:

<web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" Xsi: schemaLocation = "http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd" version = "2.4" > < servlet > <servlet-name>Test</servlet-name> <servlet-class>com.gyf.web.GetJarServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>Test</servlet-name> <url-pattern>/servlet-api.jar</url-pattern> </servlet-mapping> <context-param> <param-name>adminEmail</param-name> <param-value>59833576*@qq.com</param-value> </context-param> </web-app>Copy the code

Context-param is under the web-app tag, so it is valid for all servlets, in Servlet code:

out.println(getServletContext().getInitParamter("adminEmail"));Copy the code

Be aware of the difference between ServletContext and ServletConfig.

## Listener

What if we want to do something right away when the application is deployed? That’s when we need a listener. There are many types of listeners, each of which is used for different purposes. For example, ServletContextListener is required when an application is deployed.

ServletContextListener

package com.gyf; import javax.servlet.*; public class MyServletContextListener implements ServletContextListener { public void contextInitialized(ServletContextEvent event) { ServletContext sc=event.getServletContext(); String dogBreed=sc.getInitParameter("breed"); Dog d=new Dog(dogBreed); sc.setAttribute("dog",d); Public void contextDestroyed(ServletContextEvent Event) {// Close database}}Copy the code

<listener>
        <listener-class>
            com.gyf.MyServletContextListener
        </listener-class>
    </listener>Copy the code

There are also many listener classes available

Context initialization parameter thread safety

synchronized (getServletContext())
{
    getServletContext().setAttribute("foo",22);
    getServletContext().setAttribute("bar",42);
}Copy the code