I often see both terms — JMM (Java Memory Model) and JVM (Java Virtual Machine) — when learning about multithreading and memory management in Java. However, I'm confused about how they relate to each other and what their responsibilities are.
From what I understand:
The JVM is the engine that runs Java bytecode.
The JMM defines how variables are read and written in a concurrent environment.
But I'm still unclear on:
Does the JMM exist within the JVM or is it just a specification?
How does the JMM affect how the JVM works on different hardware?
Can the JVM implementation affect how the JMM behaves?
Is it possible for two JVMs to behave differently with regards to JMM?
Could someone clarify this with a concise explanation (or a practical example if possible)?
The JMM is a specification that limits how the JVM -- a piece of software, not a specification -- can behave.
The JMM leaves many behaviors undefined, which is to be expected and allows room for optimizations and different hardware constraints. In that way, two JVMs may behave differently with regards to their memory behavior, though both must follow the specification where it defines the correct behavior.