How Do Synchronized Static Methods Work in Java: Do They Lock on the Class or Object?

Do synchronized static methods in Java lock on the object or the class?

public class Example {
    public static synchronized void staticMethod() {
        // method implementation
    }

    public synchronized void instanceMethod() {
        // method implementation
    }
}

In Java, synchronized static methods behave differently compared to instance methods. When a static method is declared as synchronized, the lock is associated with the class object rather than instances of the class. This means that synchronized static methods cannot interleave execution across multiple threads, ensuring thread safety at the class level.

public class MyClass {
    // Synchronized static method
    public static synchronized void syncStatic() {
        // critical section code
    }

    // Instance method
    public synchronized void syncInstance() {
        // critical section code
    }
}

Causes

• Static methods do not operate on instances, but on the class itself.
• Synchronization in Java relies on locks, which can be associated with objects or class objects.

Solutions

• To ensure that static methods are thread-safe, use the synchronized keyword, which will lock the class object.
• For instance methods, the synchronized keyword locks the specific instance of the class.
• Be cautious with synchronized static methods in multi-threaded environments to avoid deadlocks.

Mistake: Assuming synchronized static methods lock on instance-level objects.

Solution: Remember that synchronized static methods lock on the class-level object, not instances.

Mistake: Neglecting to consider potential deadlocks when using multiple synchronized methods.

Solution: Plan method calls carefully and consider using locks or other concurrency tools to avoid deadlocks.