Singleton across JVM or Application instance or Tomcat instance

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).

Solution 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.

Solution 2:

A singleton is normally tied to a ClassLoader only.

So if you have a singleton based on a .class file in your .war file, and you deploy this web application multiple times, each application gets its own singleton.

On the other hand, if the .class file of your singleton is in the classpath of tomcat, then you only have one instance. This .class does not belong to a specific web application (it belongs to the tomcat instance).

If you have the singleton in both locations, it depends on the class loader hierarchy, and you can possibly select between "parent first" or "web application first".