Miles to go …

Servlets 2.x allowed to create a mapping between an HTTP error code or an exception type to the path of a resource in the Web application. This is achieved by specifying an "error-page" element in the "web.xml". The element definition looks like:

So any HTTP error code or an exception thrown within the application can be mapped to a resource bundled with the application. Here is a sample:

<error-page>
    <error-code>404</error-code>
    <location>/error-404.jsp</location>
</error-page>

Adding the above fragment in "web.xml" of an application will display "error-404.jsp" page to the client if a non-existing resource is accessed. This mapping can be easily done for other HTTP status codes as well by adding other <error-page> elements.

Similarly, <exception-type> element can be used to map an exception to a resource in the web application. This allows fine-grained mapping of errors from your web application to custom pages.

Starting with Servlets 3.0, <error-code> and <exception-type> elements are optional. An <error-page> without any <exception-type> and <error-code> will be considered as the webapp’s default error page, and will act as a "catch-all" for any error codes or exception types. It will be an error if a web.xml contains more than one such default error page.

A default error page may be overridden for specific exception types and error codes. For example:

     <error-page>
       <location>/error-default.jsp</location>
     </error-page>
     <error-page>
       <error-code>404</error-code>
       <location>/error-404.jsp</location>
     </error-page>

Any response with a status code other than 404 will be error-dispatched to /default.jsp, while a 404 response will be error-dispatched to /error-404.jsp.

So if the Servlet code looks like:

@WebServlet(name="HelloServlet", urlPatterns={"/HelloServlet"})
public class HelloServlet extends HttpServlet {
protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws IOException {
String type = (String)request.getParameter("type");
if (type == null) {
response.getWriter().print("hello world");
return;
}
if (type.equals("helloex")) {
throw new HelloException();
} else if (type.equals("ncdfe")) {
throw new NoClassDefFoundError();
} else {
throw new NullPointerException();
}
}
}

And the "web.xml" looks like:

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
<error-page>
<exception-type>java.lang.NoClassDefFoundError</exception-type>
<location>/error-ncdfe.jsp</location>
</error-page>
<error-page>
<exception-type>server.HelloException</exception-type>
<location>/error-helloex.jsp</location>
</error-page>
<error-page>
<error-code>404</error-code>
<location>/error-404.jsp</location>
</error-page>
<error-page>
<location>/error-default.jsp</location>
</error-page>
</web-app>

Lets say the directory structure looks like:

WEB-INF/classes/
WEB-INF/classes/server/
WEB-INF/classes/server/HelloException.class
WEB-INF/classes/server/HelloServlet.class
WEB-INF/web.xml
error-404.jsp
error-default.jsp
error-helloex.jsp
error-ncdfe.jsp

and this application is deployed as "DefaultErrorPage.war". Then here is a table of the page that gets displayed when the URL mentioned in the first column is accessed:

URL	Response	Comment
http://localhost:8080/DefaultErrorPage/HelloServlet	"hello world"	Expected result
http://localhost:8080/DefaultErrorPage/HelloServlet2	error-404.jsp	HTTP 404
http://localhost:8080/DefaultErrorPage/HelloServlet?type=ncdfe	error-ncdfe.jsp	System exception
http://localhost:8080/DefaultErrorPage/HelloServlet?type=helloex	error-helloex.jsp	User exception
http://localhost:8080/DefaultErrorPage/HelloServlet?type	error-default.jsp	Catch-all exception

Try this and other Java EE 6 features in GlassFish Server Open Source Edition 3 or Oracle GlassFish Server today!

The complete source code used in this blog can be downloaded here.

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