3.2 A Servlet That Generates Plain Text

Listing 3.2 shows a simple servlet that outputs plain text, with the output shown in Figure 3-2. Before we move on, it is worth spending some time reviewing the process of installing, compiling, and running this simple servlet. See Chapter 2 (Server Setup and Configuration) for a much more detailed description of the process.

Figure 3-2. Result of http://localhost/servlet/HelloWorld.

First, be sure that you've already verified the basics:

• That your server is set up properly as described in Section 2.3 (Configure the Server).

• That your development CLASSPATH refers to the necessary three entries (the servlet JAR file, your top-level development directory, and ".") as described in Section 2.7 (Set Up Your Development Environment).

• That all of the test cases of Section 2.8 (Test Your Setup) execute successfully.

Second, type " javac HelloWorld.java " or tell your development environment to compile the servlet (e.g., by clicking Build in your IDE or selecting Compile from the emacs JDE menu). This step will compile your servlet to create HelloWorld.class .

Third, move HelloWorld.class to the directory that your server uses to store servlets that are in the default Web application. The exact location varies from server to server, but is typically of the form install_dir /.../WEB-INF/classes ( see Section 2.10 for details). For Tomcat you use install_dir /webapps/ROOT/WEB-INF/classes , for JRun you use install_dir /servers/default/default-ear/default-war/WEB-INF/classes , and for Resin you use install_dir /doc/WEB-INF/classes . Alternatively, you can use one of the techniques of Section 2.9 (Establish a Simplified Deployment Method) to automatically place the class files in the appropriate location.

Finally, invoke your servlet. This last step involves using either the default URL of http: //host /servlet /ServletName or a custom URL defined in the web.xml file as described in Section 2.11 (Web Applications: A Preview). During initial development, you will almost certainly find it convenient to use the default URL so that you don't have to edit the web.xml file each time you test a new servlet. When you deploy real applications, however, you almost always disable the default URL and assign explicit URLs in the web.xml file (see Section 2.11, "Web Applications: A Preview"). In fact, servers are not absolutely required to support the default URL, and a few, most notably BEA WebLogic, do not.

Figure 3-2 shows the servlet being accessed by means of the default URL, with the server running on the local machine.

Listing 3.2 HelloWorld.java

 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWorld extends HttpServlet {   public void doGet(HttpServletRequest request,                     HttpServletResponse response)       throws ServletException, IOException {     PrintWriter out = response.getWriter();     out.println("Hello World");   } }