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Synchronized vs ReentrantLock

This page compares the two primary mechanisms for managing thread synchronization and locking in Java: the synchronized keyword and the ReentrantLock class.

Overview

Both mechanisms serve the fundamental purpose of ensuring mutual exclusion in concurrent programming, preventing multiple threads from accessing shared resources simultaneously.^[600-developer-juc.md]

Key Differences

The core distinctions between the two approaches are typically categorized into the following areas:

  • Usage and Flexibility: synchronized is a keyword implicit to the Java language, typically used for method or block-level locking. ReentrantLock is a class from the java.util.concurrent.locks package that offers explicit locking and unlocking.
  • Functionality: ReentrantLock provides advanced features not available with synchronized, such as fairness (the ability to specify the order in which threads acquire the lock), interruptibility (allowing threads to respond to interrupts while waiting for a lock), and the ability to have multiple Condition objects for fine-grained thread signaling.
  • Performance: Historically, ReentrantLock offered significantly better performance than synchronized. However, optimizations in modern Java versions (JDK 1.6+) have narrowed this gap considerably, making synchronized highly competitive in many standard scenarios.

Synchronized

The synchronized keyword is the built-in mechanism for synchronization in Java.

  • Simplicity: It is easier to use than ReentrantLock because the lock is automatically acquired and released by the JVM when entering and exiting the synchronized block or method. This eliminates the risk of forgetting to release the lock (deadlock due to coding error), provided the code structure is correct.
  • Lock Handling: The locking and unlocking process is implicit and managed by the Java Virtual Machine.

ReentrantLock

ReentrantLock is an explicit lock implementation provided by the JUC package.

  • Explicit Control: The programmer is responsible for manually acquiring the lock with lock() and releasing it in a finally block via unlock(). While this provides greater control, it introduces a higher risk of deadlocks if the lock is not released properly due to exceptions.
  • Interruption: It supports methods like lockInterruptibly(), allowing a thread to interrupt itself while waiting to acquire a lock.
  • Fairness: It allows the creation of a "fair" lock, which guarantees that the longest-waiting thread gets the lock, reducing the risk of thread starvation (though often at the cost of throughput).

Sources

  • 600-developer-juc.md
  • 600-developer__JUC.md