Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## Creating and Initializing a Servlet

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 the `urlPatterns` 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 the `Servlet` 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.

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