determining java memory usage

Question

Hmmm. Is there a primer anywhere on memory usage in Java? I would have thought Sun or IBM would have had a good article on the subject but I can\'t find anything that looks

Answer 1

As of Java 5, on Hotspot and other VMs that support it, you can use the Instrumentation interface to ask the VM the memory usage of a given object. It's fiddly but you can do it. In case you want to try this method, I've added a page to my web site on querying the memory size of a Java object using the Instrumentation framework.

As a rough guide in Hotspot on 32 bit machines:

• objects use 8 bytes for "housekeeping"
• fields use what you'd expect them to use given their bit length (though booleans tend to be allocated an entire byte)
• object references use 4 bytes
• overall obejct size has a granularity of 8 bytes (i.e. if you have an object with 1 boolean field it will use 16 bytes; if you have an object with 8 booleans it will also use 16 bytes)

There's nothing special about collections in terms of how the VM treats them. Their memory usage is the total of their internal fields plus -- if you're counting this -- the usage of each object they contain. You need to factor in things like the default array size of an ArrayList, and the fact that that size increases by 1.5 whenever the list gets full. But either asking the VM or using the above metrics, looking at the source code to the collections and "working it through" will essentially get you to the answer.

If by "closure" you mean something like a Runnable or Callable, well again it's just a boring old object like any other. (N.B. They aren't really closures!!)