Understanding synchronized Methods: Why They Can Execute Simultaneously in Java

Explore the rules of synchronization in Java that might lead to unexpected concurrent execution of synchronized methods. Learn why your synchronized methods can run simultaneously and how to manage thread safety effectively.
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Understanding synchronized Methods: Why They Can Execute Simultaneously in Java

In the world of Java programming, particularly when dealing with multithreading, the synchronized keyword holds a critical role in ensuring thread safety. However, there are times when you might encounter unexpected behavior, such as synchronous methods executing concurrently. This post will explore this phenomenon and provide an in-depth explanation of why it occurs and how to manage synchronized methods correctly.

The Problem at Hand

Imagine you have a Java class with two synchronized methods. Intuitively, one would expect that if one thread is executing a synchronized method, no other thread should be able to execute another synchronized method of the same class instance until the first has completed. However, in the example code below, we see threads running concurrently with synchronized methods—a behavior that may seem contrary to the expectations of many developers.

Example Code

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Observations from the Output

As you run the code, you might see thread 1 and thread 2 executing simultaneously:

[[See Video to Reveal this Text or Code Snippet]]

This is unexpected because the output suggests that both abc() and pqr() methods are being executed concurrently, defying the principle of synchronization.

Understanding the Synchronization Mechanism

To clear up this confusion, let’s delve into how Java handles synchronization and thread safety.

Instance vs. Class Level Synchronization

Instance Synchronization:
The synchronized keyword applied to non-static methods allows access control on the instance level. This means that if a method is synchronized, the lock is applied to the particular instance (this) being used. In your example, you create two separate instances of the class ncuie:

Thread t = new ncuie(5);

Thread t1 = new ncuie(10);

Since these threads are operating on different instances of the ncuie class, they are not blocking each other. Hence, both threads can execute abc() and pqr() simultaneously without waiting for the other to finish.

Class Level Synchronization:

Key Points to Remember

Separate Instances: When using instance-level synchronization, multiple instances of the same class can run synchronized methods concurrently.

Class-level Synchronization: Use this when multiple threads need to access static methods or shared data across instances.

Independent Synchronization: It's important to remember that synchronization on an instance and synchronization on a class object are completely independent.

Conclusion

In summary, if you find that synchronized methods are executing simultaneously in Java, remember that the synchronization is tied to the instance level. Separate instances can run synchronized methods at the same time. To enforce thread safety across all threads for a particular method, consider using class-level synchronization or ensuring that only one instance is being used if synchronization is necessary.

Understanding these concepts will empower you to manage concurrency in your applications more effectively, leading to robust and error-free multi-threaded applications.