How do Java Synchronized Static Methods Acquire Locks?

Java Synchronized Static Methods: Unlocking the Mystery of Class Locks

The Java synchronized keyword ensures that multiple threads will not interleave while executing synchronized methods on the same object. But what happens when a static method is declared as synchronized? Since static methods are not associated with an instance of the class, do they still acquire locks?

Exploring the Realm of Static Synchronized Methods

The answer lies in understanding the semantics of synchronized static methods. According to the Java Language Specification (JLS):

• For a static method, the monitor associated with the Class object for the method's class is used.
• For an instance method, the monitor associated with this (the object for which the method was invoked) is used.

Implications for Class-Level Locking

This means that when a static method is synchronized, it acquires the monitor associated with the Class object of its declaring class. Consequently, all synchronized static methods in the same class share the same monitor, preventing concurrent execution of any of these methods.

Example Use Case

Consider the following example:

public class Counter {
    private static int count = 0;

    public static synchronized void increment() {
        count++;
    }
}

In this example, the increment() method is declared as static and synchronized. This ensures that threads will not concurrently manipulate the count variable, maintaining its integrity across all instances of the Counter class.

Conclusion

While static methods may not be directly associated with an object, they still acquire locks on the class monitor. This ensures that synchronized static methods in the same class can execute exclusively, preventing data inconsistencies and maintaining thread safety within the class.

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