Why Synchronizing on Boolean Variables is Not Recommended in Java

What are the risks associated with synchronizing on Boolean variables in Java?

// Example of improper synchronization on a Boolean
public class Example {
    private Boolean flag = true;

    public void toggleFlag() {
        synchronized(flag) {
            flag = !flag;
        }
    }
}

Synchronizing on Boolean variables in Java can lead to serious issues regarding thread safety and program predictability. Understanding the implications of this practice is crucial for developing robust multi-threaded applications.

// Proper synchronization using a dedicated lock object
public class ProperExample {
    private final Object lock = new Object();
    private boolean flag = true;

    public void toggleFlag() {
        synchronized(lock) {
            flag = !flag;
        }
    }
}

Causes

• Using wrapper classes like Boolean instead of primitives can introduce unnecessary overhead and unexpected behavior.
• Immutability of Boolean objects means that any modification leads to a new object being created, rendering the lock ineffective for synchronization.

Solutions

• Utilize synchronized blocks or methods with intrinsic locks (like the object's own monitor or a specific lock object) instead of Boolean variables.
• Prefer to use volatile flags or atomic variables (like AtomicBoolean) to manage state that requires concurrent access.

Mistake: Synchronizing on a mutable Boolean variable leads to unpredictable behavior.

Solution: Use an immutable object or a dedicated lock object to ensure consistent synchronization.

Mistake: Assuming the synchronized block on Boolean provides thread safety.

Solution: Understand that synchronization must be performed on the actual lockable object, not on a reference to a Boolean.