java - Singleton across JVM or Application instance or Tomcat instance

Question

If I deploy and run 2 instances of same application on a single instance of Tomcat(Or any other server). Then a single object(of a Singleton class) would be created:

1. Across single instance of Tomcat (but common for 2 instances of same application) OR
2. Across application instance (different for 2 instances of application)

So essentially I want to understand that is it always a case that a single object of Singleton class gets created per JVM? How does this works in case of application hosted on a Web server(or container).

Answer 1

If you have a singleton class and you run two webapps that use this class in Tomcat both webapps will get 2 different instances of this singleton in JVM running the Tomcat.

But if your webapp will use a singleton from JRE or Tomcat shared libs, eg Runtime.getRuntime webapps will get the same instance of Runtime.

This is because Tomcat uses individual class loaders for webapps. When a webapp class loader loads a class it first tries to find it on webapp class path, if the class is not found it asks parent class loader to load the class.