Understanding Synchronized Methods in Java: Can Two Threads Execute Simultaneously?

This guide explores the mechanics of `synchronized` instance methods in Java, particularly regarding how multiple threads can interact with different instances. Learn if two threads can execute the same synchronized method simultaneously with different monitor objects.
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Understanding Synchronized Methods in Java: Can Two Threads Execute Simultaneously?

Java's multithreading capabilities are powerful, allowing developers to efficiently manage concurrent operations. However, concepts like synchronization can be puzzling for newcomers. A common question arises regarding synchronized instance methods: Can two threads execute the same synchronized instance method if the monitor object is different?

In this post, we will clarify this question with an easy-to-understand explanation and example.

The Basics of Synchronization

When a method in Java is declared as synchronized, it means that only one thread can execute that method at any point in time for a given instance (or monitor object). This is to prevent cohesion issues where multiple threads might interfere with each other while accessing shared resources.

Key Points:

Synchronized Keyword: Ensures only one thread can access a method simultaneously for the same instance.

Monitor Object: In our synchronization context, the monitor object is the instance of the class where the synchronized method exists.

The Scenario Explained

Consider the following example code:

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

Breaking It Down

Two Distinct Instances: In the code above, exchanger and exchanger1 refer to two separate instances of the SynchronizedExchanger class.

Execution at the Same Time: Since both threads are calling the synchronized method setObject on their own respective instances (exchanger for thread1 and exchanger1 for thread2), they can execute the method simultaneously.

Important Concepts to Remember:

synchronized(this): When you mark a method as synchronized, it works effectively as if you had written:

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

In this case, this only represents the current instance of the class.

Accessing the Same Instance: If only one instance of SynchronizedExchanger were created, both threads would contend for it, resulting in only one thread being able to execute setObject at any given time. They wouldn't be able to run the synchronized method simultaneously.

Conclusion

To summarize, two threads can indeed execute the same synchronized instance method at the same time, provided they are operating on different instances of the object. This is a fundamental aspect of Java multithreading that aids in maintaining object integrity while allowing for concurrency when necessary.

Understanding this concept allows developers to better design their applications to make full use of concurrency while avoiding potential pitfalls associated with synchronization.

By grasping how synchronization and monitor objects work, you can write more efficient and robust multithreaded applications in Java.