general_time | Allows getting general time between two instances | Object-Relational Mapping library

Your variable is defined on a stack, which would normally be garbage collected after the method quits. However, using the value in a lambda requires Java to share the value in a closure, which requires it to be final or effectively final (not modified by any code it is shared to). Any value to remain mutable across threads must be, therefore, wrapped into an instance which itself is final or effectively final.

The easiest, and also thread safe, way to achieve this is by using classes from java.util.concurrent.atomic, in your case AtomicLong. The variable itself will be effectively final or can be declared final, but the value can be set (and read) in any lambda and thread: