Thread Synchronization

Definition

Thread Synchronization is a mechanism in Java that ensures only one thread can access a shared resource or execute a specific block of code at any given time. It prevents data inconsistency and race conditions by enforcing a serialized access pattern to critical sections of a program.

Main Content

1. The Synchronized Keyword

• The synchronized keyword in Java is the primary tool for achieving mutual exclusion. It can be applied to methods or blocks of code.
• When a thread enters a synchronized block, it acquires an "intrinsic lock" (or monitor lock), preventing other threads from entering any synchronized code on the same object until the lock is released.

2. Monitor Locks

• Every object in Java is associated with an implicit lock, known as a monitor.
• When a thread invokes a synchronized method, it automatically acquires the monitor for that object. If another thread tries to access it, it is placed in a "blocked" state until the first thread completes its task.

3. Race Conditions

• A race condition occurs when multiple threads attempt to read and write shared data concurrently, leading to unpredictable results.
• Synchronization solves this by ensuring that the "Read-Modify-Write" sequence is atomic, meaning it happens as a single, uninterrupted operation.

Working / Process

1. Requesting Access

• A thread attempts to execute a synchronized method or block.
• It checks if the object's monitor lock is available or already held by another thread.

2. Lock Acquisition and Execution

• If the lock is free, the thread acquires the monitor and proceeds to execute the critical section.
• If the lock is held by another thread, the requesting thread moves into the "Blocked" state and waits in the entry set.

Thread A ----> [Has Lock] ----> [Critical Section] ----> [Releases Lock]
                                        |
Thread B -----------------------------> [Waiting in Entry Set]

3. Releasing the Lock

• Once the synchronized block is completed or an exception is thrown, the Java Virtual Machine (JVM) automatically releases the lock.
• Once released, the threads waiting in the entry set compete for the lock to enter the critical section.

Advantages / Applications

• Data Integrity: Ensures that shared variables are updated correctly, preventing corruption during multi-threaded operations.
• Thread Safety: Facilitates the creation of thread-safe singleton classes and collections (e.g., Vector or Hashtable).
• Resource Management: Prevents multiple threads from simultaneously writing to files, databases, or network sockets, which could lead to resource exhaustion or deadlocks.

Summary

Thread Synchronization is the process of controlling the access of multiple threads to shared resources to maintain data consistency. It utilizes monitor locks and the synchronized keyword to ensure that only one thread executes a critical section at a time, effectively preventing race conditions. Important terms to remember include: Mutual Exclusion (Mutex), Monitor Lock, Critical Section, and Race Condition.