Servlet 3.0 Annotations

Servlet 3.0 Specification has a number of new features and one of them is annotation. When an application loads, all classes into the application are examined for presence of the annotations. When these annotations are found on a particular class, the container registers instances of the classes as Listeners, Servlets, or Filters based on the annotation.
Servlet 3.0 Specification Annotations are as follows.

* @WebServlet
* @WebFilter
* @WebInitParam
* @WebListner
* @MultipartConfig

@WebServlet
This annotation is used to define a Servlet component in a web application and defines the metadata about the servlet being declared.
Classes annotated with @WebServlet annotation must extend javax.servlet.http.HttpServlet class.
@WebServlet annotation annotation has the following attributes.
* name
* description
* value
* urlPatterns
* initParams
* loadOnStartup
* asyncSupported
* smallIcon
* largeIcon
Note: the spec mandates either value or urlPatterns attribute be specififed.

Example:
@WebServlet(name = "myservlet", urlPatterns = { "/myservlet" })
public class MyServletExample extends HttpServlet {
}

@WebFilter
This annotation is used to define a filter in web application. The classes annotated with @WebFilter annotation must implement javax.servlet.Filter interface.
The @WebFilter annotation has the following attributes
* filterName
* description
* displayName
* initParams
* servletNames
* value
* urlPatterns
* dispatcherTypes
* asyncSupported

Example:
@WebFilter(filterName="samplefilter", urlPatterns={"/foo/*", "/bar"})
public class MyServletSampleFilter implements Filter {
}

@WebInitParam
This annotation is used to specify any init parameters that must be passed to the Servlet or the Filter.
The annotation defines the following attributes:
* name
* value
* description

Example:
@WebServlet(value="/hello",
initParams = {
@WebInitParam(name="name1", value="Hello "),
@WebInitParam(name="name2", value=" World!")
})
public class MyHelloServletExample extends HttpServlet {
}

@WebListner
This annotation is used to define a listener. The @WebListener annotation is used to register the following types of listeners
1. Context Listener (javax.servlet.ServletContextListener)
2. Context Attribute Listener (javax.servlet.ServletContextAttributeListener)
3. Servlet Request Listener (javax.servlet.ServletRequestListener)
4. Servlet Request Attribute Listener (javax.servlet.ServletRequestAttributeListener)
5. Http Session Listener (javax.servlet.http.HttpSessionListener)
6. Http Session Attribute Listener (javax.servlet.http.HttpSessionAttributeListener)

@WebListener is the easiest to use. Registering ServletContextListener.

Example:
@WebListener()
public class MySampleContextListner extends ServletContextListner {
public void contextInitialized(ServletContextEvent event) {
//do some thing on application init
}

public void contextDestroyed(ServletContextEvent event) {
//do some thing on application destroy
}
}

@MultipartConfig
Servlet 3.0 specification comes with the built in file upload support. This annotation, when specified on a Servlet, indicates that the request it expects is of type mime/multipart. The HttpServletRequest object of the corresponding servlet makes available the mime attachments via the getParts and getPart methods to iterate over the various mime attachments.

The @MultipartConfig annotation can be used to specify the location where the files can be stored, maximum size of the file being uploaded, maximum request size and the size threshold after which the file will be written to the disk. There is any equivalent element in web.xml that can be used for the same purpose.

@MultipartConfig annotation declares the following attributes:
* location
* maxFileSize
* maxRequestSize
* fileSizeThreshold

Example:
@WebServlet(name="UploadServlet", urlPatterns={"/upload"})
@MultipartConfig(location="d://tmp")
public class MyServletExample extends HttpServlet {
public void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//target directory and location are the same.
req.getPart("content").write("file.txt");
}
}

Hope this will help.