Java BigDecimal memory usage?

问题:

Is there a guideline for estimating the amount of memory consumed by a BigDecimal?

Looking for something similar to these guidelines for estimating String memory usage.

回答1:

If you look at the fields in the source for BigDecimal there is:

BigDecimal:   long intCompact +8 bytes   int precision +4 bytes   int scale +4 bytes   String stringCache +?   BigInteger intVal +?  BigInteger:   int bitCount +4 bytes   int bitLength +4 bytes   int firstNonzeroIntNum +4 bytes   int lowestSetBit +4 bytes   int signum +4 bytes   int[] mag +? 

The comment for stringCache is "Used to store the canonical string representation, if computed.", so assuming that you don't call toString we will leave that as zero bytes, so in total there are (8+4+4)=16 bytes + BigInteger in BigDecimal and 4+4+4+4+4=20 bytes + mag for BigInteger. So a total of 36 bytes plus the magnitude. As far as I can tell magnitude is always the minimum number of bits necessary to represent the full integer, so for a number n it will need log2(n) bits, which can be converted to ints. So in general you should be using about:

36 + Ceiling(log2(n)/8.0) bytes 

(note this doesn't include any of the other object descriptor overhead as your example link for strings does, but it should give you a good general idea.)

回答2:

If you dig into the internals of BigDecimal you'll see that it uses a compact representation if the significand is Long.MAX_VALUE. Hence, the memory usage can vary depending on the actual values you're representing.

文章来源: Java BigDecimal memory usage?