SizeOf an object? Why so hard?

[Roger Riggs]

Hi David,

What is the use case? What are you trying to optimize?

Thanks, Roger

On 6/19/19 10:10 PM, David Ryan wrote:
> I'm in the process of moving a project from Java 8 to Java 11 and I've hit
> a stumbling block on what I thought should be a rather simple problem.
> During runtime, how do I get the size of an object in memory?  Here are the
> methods I've found so far:
>
> 1. The  Unsafe way.
>
> Use sun.misc.Unsafe to get the declared fields and find the field with the
> largest objectFieldOffset. Use the sun.arch.data.model to calculate the
> machine word (WORD = NR_BITS/8) size.  Return ((maxOffset / WORD ) +1 ) *
> WORD.  This is reasonably accurate but obviously not friendly in Java 11.
> Found on Stackoverflow [1].
>
> 2. Use java.lang.instrument.Instrumentation
>
> Been around for a long time [2].  However, this requires configuring
> special manifests for the project.  Not very friendly for application
> deployment, etc.
>
> 3. Use before/after memory consumption
>
> Also mentioned in various stack overflow responses is the idea of calling
> the garbage collector and memory usage before and after allocating and
> using the difference for the size.  I suppose this had the advantage of
> providing a deeper estimate of the object.  However, with other threads
> doing work in the background this is mostly luck that we get the right
> answer.
>
> 4. Add up the base types.
>
> Also observed, investigate the types of each field and estimate the size of
> the object based on the size of an arbitrary header length plus sizes for
> each boolean, int, long, reference, etc.  This misses padding added by the
> JVM, so not as accurate as using UNSAFE in 1.
>
>
> A few questions:
> Is there another method I've missed?
> Have I missed an obvious method that has been added in Java 11+?
> Is there any plans to add anything in a future release?
>
>
> [1]
> https://stackoverflow.com/questions/9368764/calculate-size-of-object-in-java
> [2]
> https://amitstechblog.wordpress.com/2011/05/19/estimating-the-memory-usage-of-a-java-object/