A retention policy determines at what point an annotation is discarded.
Java defines three such policies:SOURCE, CLASS, and RUNTIME.
SOURCE is retained only in the source file and is discarded during compilation.
CLASS is stored in the .class file during compilation. It is not available through the JVM during run time.
RUNTIME is stored in the .class file and is available through the JVM during run time.
A retention policy is specified for an annotation by using one of Java's built-in annotations