Do Synchronized Static Methods Lock on the Class Object in Java?

Java Synchronized Static Methods: Synchronization Mechanism Analysis

In Java, the synchronized keyword is used to protect critical sections of code from being accessed concurrently. It ensures that only one thread at a time can execute a synchronized method or block. However, the behavior of synchronized static methods differs slightly from that of instance methods, as static methods do not have an associated object.

Question:

Given that static methods do not have an associated object, how does the synchronized keyword affect them? Specifically, does it lock on the class rather than an object?

Answer:

Yes. When a synchronized static method is invoked, it acquires a monitor associated with the Class object for the method's class. This means that multiple threads attempting to execute the same static synchronized method will have to acquire the lock on the class before proceeding.

Details:

As mentioned in Java Language Specification §8.4.3.6, "Synchronized Methods":

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

This ensures that static synchronized methods can be used to protect resources associated with the class itself, rather than specific instances. For example, if a static method accesses a shared count variable within the class, it can be synchronized to prevent multiple threads from accessing it concurrently and potentially creating data inconsistencies.

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