What Does the `synchronized` Keyword Lock on in a Static Method in Java?

Synchronized Static Methods in Java: Locking on Class or Object

Unlike instance methods, which are associated with an object, static methods have no explicit object reference. This begs the question: when a static method is synchronized, does the keyword lock on the class or the object?

According to the Java documentation, synchronized methods ensure that "it is not possible for two invocations of synchronized methods on the same object to interleave." However, for a static method, there is no associated object.

Lock Acquisition on Class Object

To clarify, the synchronized keyword in a static method locks on the class object, not the object itself. In the Java Language Specification (Section 8.4.3.6), it states that:

"For a class (static) method, the monitor associated with the Class object for the method's class is used."

Therefore, when two threads execute synchronized static methods on the same class, they will be serialized, ensuring that only one thread executes at a time.

In summary, it is important to understand that synchronization in static methods operates differently than in instance methods. Static methods acquire a lock on the class object, providing controlled access to shared data and preventing concurrent execution.

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