memory usage of byte[] ?

[Ulf Zibis]

Peter, thanks for your extensive explanation. This is what I wanted to know.

-Ulf


Am 05.07.2008 22:26, Peter B. Kessler schrieb:
> Ulf Zibis wrote:
>> Hi all,
>>
>> can you tell me something about memory consuming in the current Sun JVM?
>> How much bytes do byte arrays consume per byte?
>> ... or in other words: is it less memory-consuming using byte arrays
>> than int arrays, if these are large?
>> Same question about short and char arrays?
>>
>> Thanks for a short answer.
>>
>> -Ulf
>>
>> -- please CC to my email.
>
> Every object in the HotSpot JVM has a 2-word header, where the
> word size is 32-bits in the 32-bit JVM and 64-bits in the 64-bit
> JVM (duh).  An array then has a word that holds the length of
> the array.  Following that comes the data, in whatever size is
> appropriate: boolean and byte elements take 1 byte each, chars
> and shorts take 2 bytes, ints and floats take 4 bytes, and longs
> and doubles take 8 bytes.  References to other objects take either
> 4 or 8 bytes depending on whether you are in a 32-bit JVM or a
> 64-bit one (with a twist with compressed oops).  If the data needs
> to be aligned (e.g., doubles), then padding is inserted between
> the array length and the data if needed.  After the data there's
> enough bytes of padding to get us to an 8-byte boundary (so that
> all objects start on an 8-byte boundary).
>
> So in answer to your direct questions: byte arrays are less
> memory-consuming than int arrays, especially if the arrays are
> large.  Short and char arrays are the same size, though there
> are published schemes to squeeze char arrays if you aren't using
> the high-order bytes.  We don't use any of those, yet.  (But you
> are welcome to contribute to that implementation if you think
> it's important.  The trick is not slowing down the people that
> aren't using it.)
>
> You can use -XX:+PrintClassHistgram on the java command line and
> "kill -QUIT" (from another shell) to see how big things are.
> A more user-friendly way of browsing the heap of a Java program
> is "jmap -dump" followed by "jhat".  See the manual pages for
> those tools to see how to use them.
>
>             ... peter
>
>
>