What is the difference between getDefaultInstance() and getInstance() in Session class?

Question

What is the difference between Session.getDefaultInstance(props, authenticator) and getInstance(props, authenticator)? In general, when will you choos

Answer 1

FAQ says: https://javaee.github.io/javamail/FAQ#getdefaultinstance

Q: When should I use Session.getDefaultInstance and when should I use Session.getInstance?

A: Almost all code should use Session.getInstance. The Session.getDefaultInstance method creates a new Session the first time it's called, using the Properties that are passed. Subsequent calls will return that original Session and ignore any Properties you pass in. If you want to create different Sessions with different properties, Session.getDefaultInstance won't do that. If some other code in the same JVM (e.g., in the same app server) has already created the default Session with their properties, you may end up using their Session and your properties will be ignored. This often explains why your property settings seem to be ignored. Always use Session.getInstance to avoid this problem.