Slash required on urlPatterns for @WebServlet annotations

@pla1 commented on [Thu May 30 2019](https://github.com/gradle-guides/building-java-web-applications/issues/25)

Using Apache Tomcat/9.0.20 servlet API 4.0.1 I had to change
`@WebServlet(name = "HelloServlet", urlPatterns = {"hello"}, loadOnStartup = 1)`
to
`@WebServlet(name = "HelloServlet", urlPatterns = {"/hello"}, loadOnStartup = 1)`

---

@ETLJ commented on [Tue Nov 12 2019](https://github.com/gradle-guides/building-java-web-applications/issues/25#issuecomment-552702763)

That fixed the problem I was having too. Tomcat/7.0.86 servlet-api:3.1.0

`21:07:11 ERROR A child container failed during start
java.util.concurrent.ExecutionException: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Tomcat].StandardHost[localhost].StandardContext[/awebdemo]]
at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[na:na]
at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191) ~[na:na]
at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:1241) ~[tomcat-embed-core-7.0.86.jar:7.0.86]
at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:819) ~[tomcat-embed-core-7.0.86.jar:7.0.86]
at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145) ~[tomcat-embed-core-7.0.86.jar:7.0.86]
at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1702) ~[tomcat-embed-core-7.0.86.jar:7.0.86]
at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1692) ~[tomcat-embed-core-7.0.86.jar:7.0.86]
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[na:na]
at java.base/java.lang.Thread.run(Thread.java:834) ~[na:na]
Caused by: org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Tomcat].StandardHost[localhost].StandardContext[/awebdemo]]
at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:162) ~[tomcat-embed-core-7.0.86.jar:7.0.86]
... 6 common frames omitted
Caused by: java.lang.IllegalArgumentException: Invalid hello in servlet mapping`

Search the guide for the @WebServlet example containing urlPatterns = {"hello"}. Update the example to use the required leading slash, then verify that the documented application configuration matches the servlet URL pattern expected by Tomcat.

Written by the indexing model from the issue text.