Java SizeOf /Instrumentation class

[AlastrionaAdair]

Hello. I'm looking for a way in java to create a buffer with a fixed memory size. For example I want to create a buffer which can hold 10MB of objects in memory, then when the buffer is full, it will deny any new objects being added to it (and write them to the disk instead). So far I haven't found anything in the Java API that suits my needs, so I decided to write a class myself.

However I quickly realised there seems to be no standard way of measuring the memory usage of an object in java (like c++ sizeof). I found the java.lang.instrument.Instrumentation class and it seems to be able to do what I want. However it doesn't seem very accurate, as it will round everything up to 8 bytes, so if I have an int and a long in a class, it will just show 8 bytes.

My current OS is fedora 14 and I'm using the sun JDK implementation (not the OpenJDK one that comes with fedora). I don't know if this matters, but I know that implementation of JVM differs for different systems.

Is there any way to do what I want, with the accuracy I want?

[dlorde]

Java isn't really suited to such direct memory management - in fact it's deliberately designed to avoid it. I suspect you'd be better off using a native language. If you have to have a Java front-end, use the JNI to interface with native code.

If the only tool you have is a hammer, you tend to see every problem as a nail...
A. Maslow

[AlastrionaAdair]

Java isn't really suited to such direct memory management - in fact it's deliberately designed to avoid it. I suspect you'd be better off using a native language. If you have to have a Java front-end, use the JNI to interface with native code.

If the only tool you have is a hammer, you tend to see every problem as a nail...
A. Maslow

If only we had such liberties in a business environment, then I would be using C++. I'm afraid use of Java is a requirement. I was kinda afraid of this response. I might have to just estimate the mem usage or throw the whole idea out the window and just make sure that the system that runs my tool will have enough memory assigned to the JVM :P.

[dlorde]

I guess you could make an empirical assessment of the amount of memory occupied by a certain number of objects, by reading in a large number of objects multiple times and tracking memory usage via a profiler. However, whether you could trust the results to be reliable on other hardware, OS's, or JVMs, I can't say. It may be a case of 'suck it and see'.

It is easier to measure something than to understand what you have measured...
Anon.