Eviction Advisors

You can affect which elements are selected for eviction from the cache by providing a class that implements the org.ehcache.config.EvictionAdvisor interface.

EvictionAdvisor implementations are invoked when Ehcache is attempting to evict entries from the cache (in order to make room for new entries) in order to determine whether the given entry should not be considered a good candidate for eviction. If the eviction is advised against, Ehcache will try to honor the preference of preserving that entry in the cache, though there is no full guarantee of such.

CacheConfiguration<Long, String> cacheConfiguration = 
    CacheConfigurationBuilder.newCacheConfigurationBuilder(Long.class, 
    String.class, 
    ResourcePoolsBuilder.heap(2L))   // 1
    .withEvictionAdvisor(new OddKeysEvictionAdvisor<>()) // 2
    .build();

CacheManager cacheManager = CacheManagerBuilder.newCacheManagerBuilder()
    .withCache("cache", cacheConfiguration)
    .build(true);

Cache<Long, String> cache = cacheManager.getCache("cache", Long.class, 
    String.class);

// Work with the cache
cache.put(42L, "The Answer!");
cache.put(41L, "The wrong Answer!");
cache.put(39L, "The other wrong Answer!");

cacheManager.close();

In this particular example, the OddKeysEvictionAdvisor class will advise against eviction of any key that is an odd number. The cache is constrained to only be allowed to contain two entries, however the code has put three entries into the cache - which will trigger capacity eviction. By the time the cache manager gets closed, only mappings with odd keys should be left in the cache as their prime candidacy for eviction would have been advised against.