Ehcache’s ARC (Automatic Resource Control) is pretty great – it more or less automatically and optimally handles cache sizing/allocation. Using ARC, one can effectively say, “Here’s 2GB to allocate for caching – no more than that – manage the cache as best as possible.” Of course, more complex rules can also be done (on a per cache basis instead of globally, for instance) but that’s the general idea.
I’ve used ARC on my last couple of projects to great success. However, it’s not without a bit of complexity. ARC has to calculate the size of objects in the cache to determine how much space is used by the cache, and calculating sizes of Java objects isn’t easy or obvious. Besides difficult to do yet seemingly simple sound tasks as answering “How big is an object reference pointer?” there are object graph traversal problems, and those object graph issues are the ones that I’ve run into a few times in my ARC adventures.
When using Ehcache with ARC to store Thymeleaf templates (for example, when using Spring Cache abstraction backed by Ehcache as the Thymeleaf cache implementation as I previous discussed), you’ll run into messages like this one:
WARN [pool-1-thread-1] [ehcache.pool.impl.DefaultSizeOfEngine] sizeOf The configured limit of 100 object references was reached while attempting to calculate the size of the object graph. This can be avoided by adding stop points with @IgnoreSizeOf annotations. Since the CacheManger or Cache elements maxDepthExceededBehavior is set to “abort”, the sizing operation has stopped and the reported cache size is not accurate. If performance degradation is NOT an issue at the configured limit, raise the limit value using the CacheManager or Cache elements maxDepth attribute. For more information, see the Ehcache configuration documentation.
To fix this problem, you need to tell ARC what parts of Thymeleaf objects shouldn’t be counted when calculating object sizes so ARC doesn’t get into an infinite loop (when it hits circular references) or try to calculate the size of the entire Spring context.
A system property needs to be set pointing to a file that lists the classes and fields that should be ignored by ARC. In my Spring @Configuation class for configuring ehcache, I have this block:
static { System.setProperty(net.sf.ehcache.pool.impl.DefaultSizeOfEngine.USER_FILTER_RESOURCE, "net.sf.ehcache.sizeof.filter.properties"); }
But you can really do this anywhere (including simply adding “-Dnet.sf.ehcache.sizeof.filter=net.sf.ehcache.sizeof.filter.properties” to the Java command line).
Next, create a file named “net.sf.ehcache.sizeof.filter.properties” in the classpath (in a Maven project, the src/main/resources directory makes sense) with these contents:
org.thymeleaf.templateresolver.TemplateResolution.resourceResolver org.thymeleaf.dom.Node.processors
That’s it!
Note that I’ve requested that Thymeleaf make the org.thymeleaf.Template class serializable, which would remove the need to do anything (ARC would “Just Work” if this change is made): https://github.com/thymeleaf/thymeleaf/issues/378 So hopefully, with a future version of Thymeleaf, all of this information will be useless and nothing more than of historical interest.
