Ehcache & MultiThreading
This may help answer your question, from the FAQ:
Is it thread safe to modify Element values after retrieval from a Cache?
Remember that a value in a cache element is globally accessible from multiple threads. It is inherently not thread safe to modify the value. It is safer to retrieve a value, delete the cache element and then reinsert the value.
(emphasis added by me)
From the official documentation appears it is specifically built and tested to run well under highly concurrent access, as long as you do not modify Element from the multiple threads.
But, of course, this does not mean that using Ehcache makes other parts of your code thread safe. If you fetch the same value from multiple threads (using the same key), the instance may be shared and you need to know what you are doing before modifying it. Best seems to use immutable objects like strings as cached values.