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:

Answer 1

• The Tomcat creates new class loader for each web application.
• So if your Singleton class is stored in war file, the same war file will have two instances in Tomcat container i.e. it creates two separate Singleton class for each war file.
• If the Singleton class is in Tomcat's share library path, Tomcat creates only one Singleton instance for the both application.

JVM analogy:

JVM is like big mansion. It contains combined family with serveral applicance and libraries.

ClassLoaders are family members, each family member represents one ClassLoader (works as delegate hierarchy not inheritance hierarchy). Note: ClassLoader is class, it can create multiple instances.

Applications are like appliance. for example: Washing Machine, Fridge, Air Cooler, Television, Dining table, Sofa and so on...

Libraries are each one having his own individual library. Every one search in parent's library if not found then search in his own library.

limitations: If father buy an appliance their children can use it, but it can not used by his parent and siblings. Each application might use different versions of the same libraries. i.e. If library contain two or more versions of same books, it picks whichever the book is available first.

Each family number can use only one unique appliance.

In home, we can use multiple appliance of same version. So, JVM allows us to run multiple applications of same versions.

Garbage Collector is a servant in Mansion, who roams as a daemon, who can clear any kind of Objects.

The scope of a static variable is limited to one per ClassLoader.