Why Should You Avoid Synchronizing on String Objects in Java?

Synchronizing on String Objects in Java

String objects are immutable in Java, which means that once a String object is created, its contents cannot be altered. This immutability has certain implications when it comes to synchronization.

The Issue with Synchronizing on String Objects

The problem arises when multiple threads attempt to access the same String object concurrently. In such cases, the JVM may create multiple instances of the same String object, even though they all have the same value. This can occur due to the following reasons:

• String Interning: To optimize String operations, the JVM has a String pool that stores canonical instances of Strings. When a new String object is created with the same value as an existing string in the pool, the JVM returns the canonical instance instead of creating a new one.
• String Creation: If a new String object is created using the new keyword, a new String instance is created even if another String with the same value exists in the pool.

Consequences of Concurrent Access to String Keys

In your example, you synchronize on the string key that is used to access the cache. However, if multiple threads access the cache concurrently, they may end up creating multiple instances of the same key, leading to incorrect synchronization behavior. Specifically, some threads may enter the synchronization block while others are blocked, even if they are accessing the same key.

Recommended Solution

To address this issue, you should use an object dedicated to synchronization instead of synchronizing directly on the string key. This can be achieved using any of the following methods:

• Using an Object as the Synchronization Lock: Create a new object that will serve as a "lock" for each unique key and synchronize on that object.
• Using ConcurrentHashMap: Consider using the ConcurrentHashMap class, which provides thread-safe data structures that automatically handle synchronization.

Conclusion

Understanding the behavior of String objects with respect to synchronization is crucial for preventing concurrency issues in your Java applications. By using dedicated objects or concurrent data structures for synchronization, you can ensure that your code operates correctly and efficiently, even under high load conditions.

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