Customizing Web Applications with Annotations

8.1 Introduction to servlet annotations

Starting with version 7, Tomcat began to support the Servlet API 3.0 web servlet specification . In particular, a package appeared in it called javax.servlet.annotation. It contains various types of annotations that can be used to annotate a servlet class. If you use an annotation, then the deployment descriptor (web.xml) is not required.

List of the most useful annotations:

Example:

@WebServlet( urlPatterns = {"/api/*"} )
public class Example extends HttpServlet {
    protected void doGet( HttpServletRequest request, HttpServletResponse response) throws Exception {
         response.setContentType("text/html");
         PrintWriter out = response.getWriter();
         out.println("Hello World!");
    }
}

One annotation is enough to declare a servlet and map it to serve all requests that will come to the URL given by the /api/*.

8.2 Setting up servlet mapping

The annotation has @WebServletmany parameters that allow very flexible configuration of the servlet it describes. Let's take a look at the main ones:

There are several interesting points here.

First, note that valueand urlPatternsare interchangeable attributes. They both allow you to specify a list of URLs that the servlet should map to.

Secondly, the parameter asyncSupportedindicates whether the servlet will correctly process asynchronous requests over the HTTP/2 protocol.

And the third important attribute is initParams, it allows you to set a list of parameters that will be placed in the ServletContext.

Example:

@WebServlet( urlPatterns = {"/sendFile", "/uploadFile"}, loadOnStartup = 1, asyncSupported = true, initParams = { @WebInitParam(name = "saveDir", value = "c:/uploaded"), @WebInitParam(name = "allowedTypes", value = "jpg,gif,png") } )
public class ImageUploadServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws Exception {
        String saveDir = getInitParameter("saveDir");
        String fileTypes = getInitParameter("allowedTypes");

        PrintWriter writer = response.getWriter();

        writer.println("saveDir = " + saveDir);
        writer.println("fileTypes = " + fileTypes);
    }
}

8.3 Setting up filter mapping

As mentioned above, filters are a utility type of servlets, and they are configured like servlets. Here are the attributes an annotation has @WebFilter:

An example of a filter that intercepts all requests to specific servlets :

@WebFilter(servletNames = {"MyOwnServlet", "UploadServlet"})
public class MyFilter implements Filter {
    // implements Filter's methods here...
}

An example of a filter given multiple parameters :

@WebFilter( urlPatterns = "/uploadFilter", initParams = @WebInitParam(name = "fileTypes", value = "doc;xls;zip;txt;jpg;png;gif") )
public class UploadFilter implements Filter {
    // implements Filter's methods here...
}

An example of a filter given types for RequestDispatcher :

@WebFilter( urlPatterns = "/admin", dispatcherTypes = {DispatcherType.REQUEST, DispatcherType.FORWARD} )
public class MyFilter implements Filter {
    // implements Filter's methods here...
}

You will learn more about the purpose of filters and servlets in the following lectures.