How to Properly Synchronize Access to Cache in Java Using String Keys?

How can I synchronize access to a cache when using String keys in a Java web application to prevent concurrency issues?

private SomeData[] getSomeDataByEmail(WebServiceInterface service, String email) {
    final String key = "Data-" + email;
    SomeData[] data = (SomeData[]) StaticCache.get(key);
    
    if (data == null) {
        data = service.getSomeDataForEmail(email);
        StaticCache.set(key, data, CACHE_TIME);
    }
    else {
        logger.debug("getSomeDataForEmail: using cached object");
    }
    
    return data;
}

In a multithreaded Java application, synchronizing access to shared resources is critical to ensure data consistency. When using String objects to synchronize, it is essential to understand the behavior of String interning and the potential pitfalls of using mutable state in a concurrent context.

private SomeData[] getSomeDataByEmail(WebServiceInterface service, String email) {
    final String key = "Data-" + email;
    SomeData[] data;   
    synchronized(getLockForKey(key)) {  
        data = (SomeData[]) StaticCache.get(key);
        if (data == null) {
            data = service.getSomeDataForEmail(email);
            StaticCache.set(key, data, CACHE_TIME);
        }
        else {
            logger.debug("getSomeDataForEmail: using cached object");
        }
    }
    return data;
}

private Object getLockForKey(String key) {
    // Logic to return a lock object for the given key, possibly stored in a ConcurrentHashMap
}

Causes

• Synchronizing on String objects can lead to unintended behavior due to the String interning mechanism in Java, where different String instances with the same content may refer to different objects in memory.
• If the strings used for synchronization are created locally within the method, multiple threads may end up synchronizing on different String objects rather than the same one.

Solutions

• Instead of synchronizing on the String object directly, use a dedicated lock object or use a ConcurrentHashMap to manage access to the cache.
• Implement a locking mechanism that utilizes the `java.util.concurrent` package, specifically `ReentrantLock`, which provides more control and avoids the pitfalls of sintering strings.
• Consider caching strategy improvements, such as using an in-memory caching solution that inherently handles concurrency.

Mistake: Synchronizing on mutable objects or Strings can lead to deadlocks or unexpected logging issues when multiple threads access the same data simultaneously.

Solution: Always synchronize on immutable objects or use dedicated lock objects instead of Strings.

Mistake: Using the same string value for synchronization in different threads without ensuring they point to the same object instance can create multiple locks.

Solution: Opt for a static constant lock object or a dedicated locking mechanism.