Calculating byte-size of Java object [duplicate]

Answer 1

Use getObjectSize() method of the Instrumentation package.

Look here for implementation details:

Answer 2

You may be interested in an article I wrote a while ago on how to calculate the memory usage of a Java object. It is admittedly aimed primarily at 32-bit Hotspot, although much of it applies in essence to other environments.

You can also download a simple agent for measuring Java object size from the same site which will take some of the hard work out of it for you and should work in 64-bit environments.

Note as others have I think mentioned that the serialised form of an object isn't the same as its form in memory, so using serialisation isn't suitable if you want to measure the memory footprint accurately.

Answer 3

The serialized size is definitely not the way to go, for two reasons:

• In the standard java serialization there can be quite a lot of overhead which would add to the size.
• It would not be any quicker than using the getObjectSize() method which we can presume will iterate over all the references, and use some kind of lookup to determine the size of the primitive values/references of an object.

If you need better performance then that really will depend on the distribution of your objects. One possiblility would be to do some random sampling of the values in your map, determine an average and calculate an estimate from this value.

For advice on how to look up a random value in a hashmap, see this question.