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Developing Web Applications, Servlets, and JSPs for WebLogic Server

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Creating and Configuring Servlets

The following sections describe how to create and configure servlets.

 


Configuring Servlets

You define servlets as a part of a Web application in several entries in the J2EE standard Web Application deployment descriptor, web.xml. The web.xml file is located in the WEB-INF directory of your Web application.

The first entry, under the root servlet element in web.xml, defines a name for the servlet and specifies the compiled class that executes the servlet. (Or, instead of specifying a servlet class, you can specify a JSP.) The servlet element also contains definitions for initialization attributes and security roles for the servlet.

The second entry in web.xml, under the servlet-mapping element, defines the URL pattern that calls this servlet.

Servlet Mapping

Servlet mapping controls how you access a servlet. The following examples demonstrate how you can use servlet mapping in your Web application. In the examples, a set of servlet configurations and mappings (from the web.xml deployment descriptor) is followed by a table (see url-patterns and Servlet Invocation) showing the URLs used to invoke these servlets.

For more information on servlet mappings, such as general servlet mapping rules and conventions, refer to Section 11 of the Servlet 2.4 specification.

Listing 4-1 Servlet Mapping Example

<servlet>
  <servlet-name>watermelon</servlet-name>
  <servlet-class>myservlets.watermelon</servlet-class>
</servlet>
<servlet>
  <servlet-name>garden</servlet-name>
  <servlet-class>myservlets.garden</servlet-class>
</servlet>
<servlet>
  <servlet-name>list</servlet-name>
  <servlet-class>myservlets.list</servlet-class>
</servlet>
<servlet>
  <servlet-name>kiwi</servlet-name>
  <servlet-class>myservlets.kiwi</servlet-class>
</servlet>
<servlet-mapping>
  <servlet-name>watermelon</servlet-name>
  <url-pattern>/fruit/summer/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
  <servlet-name>garden</servlet-name>
  <url-pattern>/seeds/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
  <servlet-name>list</servlet-name>
  <url-pattern>/seedlist</url-pattern>
</servlet-mapping>
<servlet-mapping>
  <servlet-name>kiwi</servlet-name>
  <url-pattern>*.abc</url-pattern>
</servlet-mapping>

Table 4-1 url-patterns and Servlet Invocation

URL

Servlet
Invoked

http://host:port/mywebapp/fruit/summer/index.html

watermelon

http://host:port/mywebapp/fruit/summer/index.abc

watermelon

http://host:port/mywebapp/seedlist

list

http://host:port/mywebapp/seedlist/index.html

The default servlet, if configured, or an HTTP 404 File Not Found error message.

If the mapping for the list servlet had been /seedlist*, the list servlet would be invoked.

http://host:port/mywebapp/seedlist/pear.abc

kiwi

If the mapping for the list servlet had been /seedlist*, the list servlet would be invoked.

http://host:port/mywebapp/seeds

garden

http://host:port/mywebapp/seeds/index.html

garden

http://host:port/mywebapp/index.abc

kiwi

ServletServlet can be used to create a default mappings for servlets. For example, to create a default mapping to map all servlets to /myservlet/*, so the servlets can be called using http://host:port/web-app-name/myservlet/com/foo/FooServlet, add the following to your web.xml file. (The web.xml file is located in the WEB-INF directory of your Web application.)

<servlet>
  <servlet-name>ServletServlet</servlet-name>
  <servlet-class>weblogic.servlet.ServletServlet</servlet-class>
</servlet>

<servlet-mapping>

<servlet-name>ServletServlet</servlet-name>
  <url-pattern>/myservlet/*</url-pattern>
</servlet-mapping>

 


Setting Up a Default Servlet

Each Web application has a default servlet. This default servlet can be a servlet that you specify, or, if you do not specify a default servlet, WebLogic Server uses an internal servlet called the FileServlet as the default servlet.

You can register any servlet as the default servlet. Writing your own default servlet allows you to use your own logic to decide how to handle a request that falls back to the default servlet.

Setting up a default servlet replaces the FileServlet and should be done carefully because the FileServlet is used to serve most files, such as text files, HTML file, image files, and more. If you expect your default servlet to serve such files, you will need to write that functionality into your default servlet.

To set up a user-defined default servlet:

  1. Define your servlet as described in Configuring How a Client Accesses a Web ApplicationAdd a servlet-mapping with url-pattern = "/" as follows:
  2. <servlet-mapping> 
    <servlet-name>MyOwnDefaultServlet</servlet-name>
    <url-pattern>/myservlet/*(</url-pattern>
    </servlet-mapping>
  3. If you still want the FileServlet to serve files with other extensions:
    1. Define a servlet and give it a <servlet-name>, for example myFileServlet.
    2. Define the <servlet-class> as weblogic.servlet.FileServlet.
    3. Using the <servlet-mapping> element, map file extensions to the myFileServlet (in addition to the mappings for your default servlet). For example, if you want the myFileServlet to serve.gif files, map *.gif to the myFileServlet.

Note: The FileServlet includes the SERVLET_PATH when determining the source filename if docHome is not specified. As a result, it is possible to explicitly serve only files from specific directories by mapping the FileServlet to /dir/*, etc.

 


Servlet Initialization Attributes

You define initialization attributes for servlets in the Web application deployment descriptor, web.xml, in the init-param element of the servlet element, using param-name and param-value tags. The web.xml file is located in the WEB-INF directory of your Web application. For example:

Listing 4-2 Example of Configuring Servlet Initialization Attributes in web.xml

<servlet>
  <servlet-name>HelloWorld2</servlet-name>
  <servlet-class>examples.servlets.HelloWorld2</servlet-class>
  <init-param>
    <param-name>greeting</param-name>
    <param-value>Welcome</param-value>
  </init-param>
  <init-param>
    <param-name>person</param-name>
    <param-value>WebLogic Developer</param-value>
  </init-param>
</servlet>

 


Writing a Simple HTTP Servlet

The section provides a procedure for writing a simple HTTP servlet, which prints out the message Hello World. A complete code example (the HelloWorldServlet) illustrating these steps is included at the end of this section. Additional information about using various J2EE and Weblogic Server services such as JDBC, RMI, and JMS, in your servlet are discussed later in this document.

  1. Import the appropriate package and classes, including the following:
  2. import javax.servlet.*;
    import javax.servlet.http.*;
    import java.io.*;
  3. Extend javax.servlet.http.HttpServlet. For example:
  4. public class HelloWorldServlet extends HttpServlet{
  5. Implement a service() method.
  6. The main function of a servlet is to accept an HTTP request from a Web browser, and return an HTTP response. This work is done by the service() method of your servlet. Service methods include response objects used to create output and request objects used to receive data from the client.

    You may have seen other servlet examples implement the doPost() and/or doGet() methods. These methods reply only to POST or GET requests; if you want to handle all request types from a single method, your servlet can simply implement the service() method. (However, if you choose to implement the service() method, you cannot implement the doPost() or doGet() methods, unless you call super.service() at the beginning of the service() method.) The HTTP servlet specification describes other methods used to handle other request types, but all of these methods are collectively referred to as service methods.

    All the service methods take the same parameter arguments. An HttpServletRequest provides information about the request, and your servlet uses an HttpServletResponse to reply to the HTTP client. The service method looks like the following:

    public void service(HttpServletRequest req,
    HttpServletResponse res) throws IOException
    {
  7. Set the content type, as follows:
  8. res.setContentType("text/html");
  9. Get a reference to a java.io.PrintWriter object to use for output, as follows:
  10. PrintWriter out = res.getWriter();
  11. Create some HTML using the println() method on the PrintWriter object, as shown in the following example:
  12. out.println("<html><head><title>Hello World!</title></head>");
    out.println("<body><h1>Hello World!</h1></body></html>");
    }
    }
  13. Compile the servlet, as follows:
    1. Set up a development environment shell with the correct classpath and path settings.
    2. From the directory containing the Java source code for your servlet, compile your servlet into the WEB-INF/classes directory of the Web Application that contains your servlet. For example:
    3. javac -d /myWebApplication/WEB-INF/classes myServlet.java
  14. Deploy the servlet as part of a Web Application hosted on WebLogic Server.
  15. Call the servlet from a browser.
  16. The URL you use to call a servlet is determined by: (a) the name of the Web Application containing the servlet and (b) the name of the servlet as mapped in the deployment descriptor of the Web Application. Request parameters can also be included in the URL used to call a servlet.

    Generally the URL for a servlet conforms to the following:

    http://host:port/webApplicationName/mappedServletName?parameter

    The components of the URL are defined as follows:

    For example, to use a Web browser to call the HelloWorldServlet (the example featured in this document), which is deployed in the examplesWebApp and served from a WebLogic Server running on your machine, enter the following URL:

    http://localhost:7001/examplesWebApp/HelloWorldServlet

    The host:port portion of the URL can be replaced by a DNS name that is mapped to WebLogic Server.

 


Advanced Features

The preceding steps create a basic servlet. You will probably also use more advanced features of servlets:

 


Complete HelloWorldServlet Example

This section provides the complete Java source code for the example used in the preceding procedure. The example is a simple servlet that provides a response to an HTTP request. Later in this document, this example is expanded to illustrate how to use HTTP parameters, cookies, and session tracking.

Listing 4-3 HelloWorldServlet.java

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
public class HelloWorldServlet extends HttpServlet {
public void service(HttpServletRequest req,
HttpServletResponse res)
throws IOException
{
// Must set the content type first
res.setContentType("text/html");
// Now obtain a PrintWriter to insert HTML into
PrintWriter out = res.getWriter();
    out.println("<html><head><title>" +
"Hello World!</title></head>");
out.println("<body><h1>Hello World!</h1></body></html>");
}
}

You can find the source code and instructions for compiling and running examples in the samples/examples/servlets directory of your WebLogic Server distribution.

 

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