Use the @WebServlet annotation to define a servlet component in a web application. This annotation is specified on a class and contains metadata about the servlet being declared. The annotated servlet must specify at least one URL pattern. This is done by using the urlPatterns or value attribute on the annotation. All other attributes are optional, with default settings. Use the value attribute when the only attribute on the annotation is the URL pattern; otherwise use theurlPatterns attribute when other attributes are also used.

Classes annotated with @WebServlet must extend the javax.servlet.http.HttpServlet class. For example, the following code snippet defines a servlet with the URL pattern /report:

import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;

@WebServlet("/report")
public class MoodServlet extends HttpServlet {
    ...

The web container initializes a servlet after loading and instantiating the servlet class and before delivering requests from clients. To customize this process to allow the servlet to read persistent configuration data, initialize resources, and perform any other one-time activities, you can either override the init method of theServlet interface or specify the initParams attribute of the @WebServlet annotation. The initParams attribute contains a @WebInitParam annotation. If it cannot complete its initialization process, a servlet throws an UnavailableException.

Use an initialization parameter to provide data needed by a particular servlet. By contrast, a context parameter provides data that is available to all components of a web application.

JavaEE学习之路-Creating and Initializing a Servlet