Calculate memory usage of (compact) strings

Question

With java's compact strings feature, is there a public api to get the actual encoding or memory usage of a string? I could call either package private method coder or private method isLatin1 and adjust the calculation, but both will result in an Illegal reflective access warning.

Method isLatin1 = String.class.getDeclaredMethod("isLatin1");
isLatin1.setAccessible(true);
System.out.println((boolean)isLatin1.invoke("Jörn"));
System.out.println((boolean)isLatin1.invoke("foobar"));
System.out.println((boolean)isLatin1.invoke("\u03b1"));

There is a good reason for this warning. You’re querying an implementation detail which is not guaranteed to be there or to behave as you expect. What if a future version chooses between three different optimized encodings? — Holger
– Holger, Commented Sep 12, 2018 at 10:47

Eugene · Accepted Answer · 2018-09-10 14:06:09Z

2

That is pretty easy with JOL (but I am not entirely sure this is what you want):

String left = "Jörn"; 
System.out.println(GraphLayout.parseInstance(left).totalSize()); // 48 bytes

String right = "foobar";
System.out.println(GraphLayout.parseInstance(right).totalSize()); // 48 bytes

String oneMore = "\u03b1";
System.out.println(GraphLayout.parseInstance(oneMore).totalSize()); // 48 bytes

For encoding there isn't a public API, but you can deduce it...

private static String encoding(String s) {
    char[] arr = s.toCharArray();
    for (char c : arr) {
        if (c >>> 8 != 0) {
            return "UTF16";
        }
    }
    return "Latin1";
}

edited Sep 10, 2018 at 14:06

answered Sep 10, 2018 at 14:00

Eugene

122k17 gold badges219 silver badges335 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Jörn Horstmann Over a year ago

Thanks, I did not know about JOL, interesting to see the overhead of small strings. I'll probably not use it directly but estimate the size based on those values.

Eugene Over a year ago

@JörnHorstmann right. there are samples in that project that are far more interesting, like inheritance gaps, or a huge gap in .class, etc

Collectives™ on Stack Overflow

Calculate memory usage of (compact) strings

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related