A DESCRIPTION OF THE REQUEST :
There is no description whether or not java.util.ResourceBundle is thread-safe.
ResourceBundle#getBundle
ResourceBundle#getObject
ResourceBundle#getString

either these functions' specs of the whole class description should contain this information.

According to the current specification all the operations on ResourceBundle are to be performed within a critical section, since there are no guarantees about thread-safety.

JUSTIFICATION :
It is unsafe to rely that the use of ResourceBundle is thread-safe, even though resource bundles are often used in multi-threading environments.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
There should be a clear one-sentence indication whether or not synchronisation on ResourceBundle is needed.