Why is Synchronizing on String Objects in Java Problematic?

Synchronizing on String Objects in Java

Understanding why synchronizing on String objects in Java can be problematic underlies the issue you've encountered. By default, String objects are passed by reference, meaning that the reference to the same String object is shared among different parts of the program. This can lead to unexpected behavior when multiple threads try to access the same String object concurrently.

The key to fixing this issue lies in understanding that interning String objects ensures they are unique for each value. Intern String objects are stored in a pool within the Java Virtual Machine (JVM), and any subsequent attempt to create a string with the same value will return a reference to the interned string instead of creating a new object. This ensures that there is only one instance of each unique String value in the JVM.

By using interned Strings, you can ensure that the key you use for synchronization is unique and that all threads will obtain a reference to the same object. This will result in the expected behavior, where only one thread at a time enters the synchronized block to perform the get/set operation.

To intern a String object, you can use the intern() method. Here's an example:

final String key = "Data-" + email;
final String internedKey = key.intern();

By interning the key string, you create a unique object for it, and any subsequent attempt to create a string with the same value will return the same interned object. This ensures that the reference to the key is consistent across all threads, leading to proper synchronization.

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