Memory consistency in concurrent multi-threaded execution is primarily managed through the Java Memory Model (JMM). The JMM defines the rules and behaviors that govern how threads interact with memory when accessing shared variables. Here are the key concepts and mechanisms that help maintain memory consistency:
Shared Variables and Visibility
- When multiple threads are accessing shared variables, changes made by one thread should be visible to other threads. This is achieved through mechanisms like happens-before relationships and volatile variables.
- Volatile Variables: Reads and writes to volatile variables establish happens-before relationships. This ensures that changes to a volatile variable are visible to all threads immediately.
- Synchronization (Locks): Proper use of synchronized blocks or methods ensures that only one thread can execute a synchronized block at a time. It also ensures that changes to variables made within a synchronized block are visible to other threads when they subsequently acquire the same lock.
Happens-Before Relationship
- This is a crucial concept in JMM which defines the ordering of memory operations between threads. Specifically:
- Program order: Within a single thread, each action happens in the order specified by the program.
- Synchronization order: An unlock on a monitor happens-before every subsequent lock on that same monitor.
- Volatile variable access: A write to a volatile variable happens-before every subsequent read of that variable.
- Thread start and join: A call to Thread.start() on a thread happens-before any actions in the started thread.
Atomic Operations
- Certain operations, like reads and writes of reference variables and primitive variables (except for long and double types), are atomic. This means that no other thread can observe an intermediate state during these operations.
- Operations provided by classes in the java.util.concurrent.atomic package (e.g., AtomicInteger, AtomicReference) provide atomicity and can be used to safely perform compound actions on shared variables without explicit locking.
Thread Confinement and Thread Local Variables
- Keeping variables confined to a single thread reduces the need for synchronization because they are not shared among threads.
- ThreadLocal variables allow each thread to have its own instance of a variable, preventing thread interference and improving performance by reducing synchronization overhead.
Concurrent Collections
- Java provides thread-safe implementations of common collections (ConcurrentHashMap, ConcurrentLinkedQueue, etc.) that manage internal synchronization to ensure thread safety during concurrent access.
Executor Framework and Fork/Join Framework
- These frameworks provide higher-level abstractions for executing tasks concurrently while managing synchronization and coordination implicitly.
By adhering to these principles and using the provided mechanisms and classes, Java ensures that memory consistency is maintained in concurrent multi-threaded executions. Developers need to understand these concepts to write thread-safe code effectively and avoid subtle bugs related to concurrent access and visibility of shared variables.