Java Memory Model (JMM) is a specification that defines the behavior of object references and the visibility of changes made to shared variables. Understanding JMM is crucial for writing correct and efficient multi-threaded Java programs.

What is JMM?

The Java Memory Model ensures that the changes made to shared variables by one thread are visible to other threads. It defines the rules for the ordering of instructions and memory consistency guarantees.

Key Concepts

  • Shared Variables: Variables that are accessible by multiple threads.
  • Visibility: When one thread updates a variable, the change must be visible to other threads.
  • Atomicity: Ensures that an operation is performed as a single, indivisible step.
  • Ordering: Defines the order in which instructions are executed.

JMM Components

  • Thread Instructions: Instructions executed by a thread.
  • Locks: Synchronization mechanisms such as synchronized blocks and ReentrantLock.
  • Variables: Shared variables that can be accessed by multiple threads.

Memory Consistency

Memory consistency ensures that the behavior of the program is predictable and consistent across different threads.

  • Sequential Consistency: The program behaves as if all operations were executed in a single thread.
  • Atomicity: Operations on shared variables are atomic.
  • Happens-Before Relationship: Defines the order in which events occur.

Tips for Writing Multi-threaded Code

  • Use volatile keyword for variables that need to be visible across threads.
  • Use synchronized blocks or methods to ensure atomicity and visibility.
  • Use final keyword to prevent reassignment of shared variables.

Further Reading

For more detailed information about Java Memory Model, you can refer to the following resources:

Concurrency in Java