The following sections describe how to create and configure servlets.
With Java EE metadata annotations, the standard web.xml
deployment descriptor is now optional. The
Servlet 2.5 specification, states annotations can be defined on certain web components, such as servlets, filters, listeners, and tag handlers. The annotations are used to declare dependencies on external resources. The container will detect annotations on such components and inject necessary dependencies before the component’s life-cycle methods are invoked. See WebLogic Annotation for Web Components.
However, you can also define servlets as a part of a Web application in several entries in the 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 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 2.5 specification.
<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>
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-name>ServletServlet</servlet-name>
<url-pattern>/myservlet/*</url-pattern>
</servlet-mapping>
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:
<servlet-mapping>
<servlet-name>MyOwnDefaultServlet</servlet-name>
<url-pattern>/myservlet/*(</url-pattern>
</servlet-mapping>
FileServlet
to serve files with other extensions:<servlet-name>
, for example myFileServlet
.<servlet-class>
as weblogic.servlet.FileServlet
.<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 the docHome parameter (deprecated in this release) is not specified. As a result, it is possible to explicitly serve only files from specific directories by mapping the FileServlet to /dir/* , etc. |
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:
<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>
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 Java EE and Weblogic Server services such as JDBC, RMI, and JMS, in your servlet are discussed later in this document.
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
javax.servlet.http.HttpServlet
. For example:public class HelloWorldServlet extends HttpServlet{
service()
method.
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
{
res.setContentType("text/html");
java.io.PrintWriter
object to use for output, as follows:PrintWriter out = res.getWriter();
println()
method on the PrintWriter
object, as shown in the following example:out.println("<html><head><title>Hello World!</title></head>");
out.println("<body><h1>Hello World!</h1></body></html>");
}
}
WEB-INF/classes
directory of the Web Application that contains your servlet. For example:javac -d /myWebApplication/WEB-INF/classes myServlet.java
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:
host
is the name of the machine running WebLogic Server.port
is the port at which the above machine is listening for HTTP requests.webApplicationName
is the name of the Web Application containing the servlet.
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.
The preceding steps create a basic servlet. You will probably also use more advanced features of servlets:
init()
method.
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.
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.