Why and How Should You Synchronize on String Objects in Java?

Synchronizing on String Objects in Java

Synchronizing operations on multi-threaded applications using String objects can present challenges due to the existence of multiple instances of the same string value. To ensure proper synchronization on the desired cache key, it's essential to use the intern() method on the String object.

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

synchronized(internedKey) { ... }

The intern() method returns a canonical representation of the string, ensuring that all references to that string value will reference the same object. This allows for consistent synchronization across multiple threads.

However, synchronizing on interned strings can introduce new performance considerations. Internally, the VM may acquire a lock during the intern process, potentially causing performance degradation.

To mitigate potential issues, consider using a lock object associated with each key rather than the key itself. This approach avoids the potential overhead associated with interning strings.

Map<String, Object> lockMap = new HashMap<>();
Object lock = lockMap.get(key);

if (lock == null) {
    lock = new Object();
    lockMap.put(key, lock);
}

synchronized(lock) { ... }

With this approach, each key is associated with a unique lock object, ensuring that only one thread can access the corresponding cache operation at a time.

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