Concurrent Modification handling in Java's Guava Cache During Cache Cleanup
I'm writing unit tests and I'm working on a project and hit a roadblock. I'm collaborating on a project where I'm working with a `ConcurrentModificationException` when using Guava's `CacheBuilder` to implement an in-memory cache with a maximum size. The cache is supposed to expire entries after 10 minutes, but during the cleanup process, I encounter the exception when multiple threads try to access the cache simultaneously. I have a setup like this: ```java Cache<String, String> cache = CacheBuilder.newBuilder() .maximumSize(100) .expireAfterWrite(10, TimeUnit.MINUTES) .build(); ``` In my application, I have multiple threads adding and retrieving data from this cache. Here is how I'm currently adding entries: ```java public void addToCache(String key, String value) { cache.put(key, value); } ``` And this is how I'm retrieving entries: ```java public String getFromCache(String key) { return cache.getIfPresent(key); } ``` However, when I try to run concurrent retrievals and additions, I sometimes get this behavior: ``` Exception in thread "Thread-2" java.util.ConcurrentModificationException at java.util.HashMap$HashIterator.nextNode(HashMap.java:1447) at java.util.HashMap$KeyIterator.next(HashMap.java:1471) at com.google.common.cache.LocalCache$LocalManualCache.cleanUp(LocalCache.java:2414) ``` I’ve tried synchronizing access to the cache with `synchronized` blocks around both the `put` and `get` methods, but this hasn't resolved the scenario. Here’s how I implemented that: ```java public synchronized void addToCache(String key, String value) { cache.put(key, value); } public synchronized String getFromCache(String key) { return cache.getIfPresent(key); } ``` Despite this, the exception still occurs. I've also checked the version of Guava I’m using, which is 30.1-jre, and it seems to be the latest stable version. Is there something I'm missing in configuring the cache or managing concurrent access? Any guidance on how to prevent this exception from being thrown would be greatly appreciated. This is happening in both development and production on Windows 11. What are your experiences with this? Hoping someone can shed some light on this.