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Understanding the first level JPA cache

I can bet that every Java developer at least heard about L1 (aka EntityManager or Session) cache. But is your level of understanding it good enough? If you're not sure, consider going through this post.

At first, we need to know what the persistence context is. Following EntityManager JavaDoc we know, that:
"A persistence context is a set of entity instances in which for any persistent entity identity there is a unique entity instance. Within the persistence context, the entity instances and their lifecycle are managed." 
In fact, the first level cache is the same as the persistence context. That means operations such as persist(), merge(), remove() are changing only internal collections in the context and are not synchronized to the underlying database. What is the mosts important here is what happens when you invoke the clear() method. It clears the L1 cache. But we know L1 == persistence context. Does it mean clearing L1 removes all entities? In fact yes - all entities…