Singleton across JVM or Application instance or Tomcat instance

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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.

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