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 really solid. I'm interested in knowing two things:

1. at runtime, figuring out how much memory the classes in my package are using at a given time
2. at design time, estimating general memory overhead requirements for various things like:
   • how much memory overhead is required for an empty object (in addition to the space required by its fields)
   • how much memory overhead is required when creating closures
   • how much memory overhead is required for collections like ArrayList

I may have hundreds of thousands of objects created and I want to be a "good neighbor" to not be overly wasteful of RAM. I mean I don't really care whether I'm using 10% more memory than the "optimal case" (whatever that is), but if I'm implementing something that uses 5x as much memory as I could if I made a simple change, I'd want to use less memory (or be able to create more objects for a fixed amount of memory available).

I found a few articles (Java Specialists' Newsletter and something from Javaworld) and one of the builtin classes java.lang.instrument.getObjectSize() which claims to measure an "approximation" (??) of memory use, but these all seem kind of vague...

(and yes I realize that a JVM running on two different OS's may be likely to use different amounts of memory for different objects)

Answer 1

I used JProfiler a number of years ago and it did a good job, and you could break down memory usage to a fairly granular level.

Answer 2

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!!)

Answer 3

You can use JMP, but it's only caught up to Java 1.5.