Why should url-pattern in servlet mapping start with forward slash(/)

Question

I was reading Head First JSP and Servlets book. I was going through the mapping of servlet. And my doubt here is

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Answer 1

The / in the url-pattern means the root of the web application that uses this web.xml. So when you give /ServletBeer.do. That means http://<yourhost>:<port>/<your context Root>/ServletBeer.do. When ever you hit this url, the servlet you which have mapped with this i.e com.avinash.TestServlet will be invoked.

Its not mandatory that you have to use / in all your url mapping - that purely depends on how you want to configure it. / is just a regular expression. You can also use *.jsp in your url-pattern - which will actually invoke this servlet everytime you request any jsp in your web- application. Consider the section 12.2 Specification of Mappings ** from **JSR-000315 Java Servlet 3.0 Final Release

12.2 Specification of Mappings In the Web application deployment descriptor, the following syntax is used to define mappings:

■ A string beginning with a ‘/’ character and ending with a ‘/*’ suffix is used for path mapping.

■ A string beginning with a ‘*.’ prefix is used as an extension mapping.

■ The empty string ("") is a special URL pattern that exactly maps to the application's context root, i.e., requests of the form http://host:port/<contextroot>/. In this case the path info is ’/’ and the servlet path and context path is empty string (““).

■ 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.

■ All other strings are used for exact matches only

Answer 2

On the above questions i could find answer for one of the questions.

What happens if the url-pattern do not start with forward slash(/). I have checked it but i don't remember

When i started tomcat by removing the / in the url-pattern i got the following error

Caused by: java.lang.IllegalArgumentException: Invalid <url-pattern> ServletBeer.do in servlet mapping
    at org.apache.catalina.core.StandardContext.addServletMapping(StandardContext.java:3267)
    at org.apache.catalina.core.StandardContext.addServletMapping(StandardContext.java:3242)
    at org.apache.catalina.deploy.WebXml.configureContext(WebXml.java:1324)
    at org.apache.catalina.startup.ContextConfig.webConfig(ContextConfig.java:1348)
    at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:880)
    at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:378)
    at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
    at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:90)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5343)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)

This indicates that if we are not using extension mapping compulsory the url-pattern should start with /