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

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  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 /