AWS Tomcat server memory is too high

My application is running on REDHAT 6.0 on Tomcat 8. Sometimes memory usage is 100% when my application is used by 30 users at a time

Server Configuration Details

Machine ec-2 M3 Medium on AWS 
RAM : 4GB
DISK : 30GB
Processors : Intel Xeon E5-2670 v2 (Ivy Bridge) Processors

Is that normal? I am new to Java. What should i do to resolve it ?