How to Synchronize a Static Variable Across Multiple Thread Instances in Java?

How can I synchronize a static variable across different instances of a class in Java, specifically when multiple threads are involved?

public class Test  {
   private static int count = 0;  
   private static final Object lock = new Object();    
   public static void foo() {
      synchronized(lock) {
         count++;  
      }
   }
}

In Java, to synchronize access to a static variable across different instances of a class when multiple threads are involved, it is essential to use synchronized blocks that lock on a static object. This strategy ensures that thread execution is properly managed and that the static variable is modified safely without race conditions. In your example, the use of a static lock object achieves this goal effectively.

public class Test {
   private static int count = 0;  
   private static final Object lock = new Object(); 
   
   public static void foo() {
      synchronized(lock) {
         count++;  
      }
   }
}

Causes

• Poorly managed synchronization can lead to race conditions, where two or more threads access shared resources simultaneously, causing unexpected behavior.
• Using instance-level locks instead of static locks prevents proper synchronization across different instances of a class.

Solutions

• Use a static lock object for synchronization to ensure that all threads, regardless of the instance they work with, access the static variable under the same lock.
• Implement synchronized static methods that allow safe access to static variables when they are modified.

Mistake: Not using a static lock object for synchronizing static variables.

Solution: Always declare a static final lock object to synchronize access to static variables.

Mistake: Using instance methods for synchronization on static variables.

Solution: Use static methods or synchronized blocks that lock on static objects to ensure correct synchronization.