What is the significance of url-pattern in web.xml and how to configure servlet?

url-pattern is used in web.xml to map your servlet to specific URL. Please see below xml code, similar code you may find in your web.xml configuration file.

<servlet>
    <servlet-name>AddPhotoServlet</servlet-name>  //servlet name
    <servlet-class>upload.AddPhotoServlet</servlet-class>  //servlet class
</servlet>
 <servlet-mapping>
    <servlet-name>AddPhotoServlet</servlet-name>   //servlet name
    <url-pattern>/AddPhotoServlet</url-pattern>  //how it should appear
</servlet-mapping>

If you change url-pattern of AddPhotoServlet from /AddPhotoServlet to /MyUrl. Then, AddPhotoServlet servlet can be accessible by using /MyUrl. Good for the security reason, where you want to hide your actual page URL.

Java Servlet url-pattern Specification:

1. A string beginning with a '/' character and ending with a '/*' suffix is used for path mapping.
2. A string beginning with a '*.' prefix is used as an extension mapping.
3. A string containing only the '/' character indicates the "default" servlet of the application. In this case the servlet path is the request URI minus the context path and the path info is null.
4. All other strings are used for exact matches only.

Reference : Java Servlet Specification

You may also read this Basics of Java Servlet

Servlet-mapping has two child tags, url-pattern and servlet-name. url-pattern specifies the type of urls for which, the servlet given in servlet-name should be called. Be aware that, the container will use case-sensitive for string comparisons for servlet matching.

First specification of url-pattern a web.xml file for the server context on the servlet container at server .com matches the pattern in <url-pattern>/status/*</url-pattern> as follows:

http://server.com/server/status/synopsis               = Matches
http://server.com/server/status/complete?date=today    = Matches
http://server.com/server/status                        = Matches
http://server.com/server/server1/status                = Does not match

Second specification of url-pattern A context located at the path /examples on the Agent at example.com matches the pattern in <url-pattern>*.map</url-pattern> as follows:

 http://server.com/server/US/Oregon/Portland.map    = Matches
 http://server.com/server/US/server/Seattle.map     = Matches
 http://server.com/server/Paris.France.map          = Matches
 http://server.com/server/US/Oregon/Portland.MAP    = Does not match, the extension is uppercase
 http://example.com/examples/interface/description/mail.mapi  =Does not match, the extension is mapi rather than map`

Third specification of url-mapping,A mapping that contains the pattern <url-pattern>/</url-pattern> matches a request if no other pattern matches. This is the default mapping. The servlet mapped to this pattern is called the default servlet.

The default mapping is often directed to the first page of an application. Explicitly providing a default mapping also ensures that malformed URL requests into the application return are handled by the application rather than returning an error.

The servlet-mapping element below maps the server servlet instance to the default mapping.

<servlet-mapping>
  <servlet-name>server</servlet-name>
  <url-pattern>/</url-pattern>
</servlet-mapping>

For the context that contains this element, any request that is not handled by another mapping is forwarded to the server servlet.

And Most importantly we should Know about Rule for URL path mapping

1. The container will try to find an exact match of the path of the request to the path of the servlet. A successful match selects the servlet.
2. The container will recursively try to match the longest path-prefix. This is done by stepping down the path tree a directory at a time, using the ’/’ character as a path separator. The longest match determines the servlet selected.
3. If the last segment in the URL path contains an extension (e.g. .jsp), the servlet container will try to match a servlet that handles requests for the extension. An extension is defined as the part of the last segment after the last ’.’ character.
4. If neither of the previous three rules result in a servlet match, the container will attempt to serve content appropriate for the resource requested. If a “default” servlet is defined for the application, it will be used.

Reference URL Pattern