How to calculate the memory usage of Java objects

As well as knowing the memory usage of common Java objects such as strings, dates, HashMaps etc, it is sometimes useful to be able to calculate the memory usage of arbitrary objects, given the fields and references they contain. The information given on this page will generally apply to JVMs based on OpenJDK using default parameters and are likely to be typical. It is important to note that strictly speaking, however, they could vary from one JVM to another.

General formula for calculating memory usage of Java objects

In general, the heap memory used by a Java object in Hotspot consists of:

Sizes of primitive types

In case you're not familiar with the byte size of the different Java primitive data types, here is the complete list:

Java primitive typeBytes required for Java primitive

You may have expected a boolean to take up a single bit, or an eighth of a byte, especially if an object had 8 boolean fields. In practice, for efficiency, the JVM will allocate a whole byte to each boolean. (This also means that the space taken up by a boolean array will be eight times greater than the theoretical requirement!)

Object overhead for "housekeeping" information

Instances of an object on the Java heap don't just take up memory for their actual fields. Inevitably, they also require some "housekeeping" information, such as recording an object's class, ID and status flags such as whether the object is currently reachable, currently synchronization-locked etc.

In Hotspot, the following generally holds:
  • a normal object requires 12 bytes of "housekeeping" space (note that this was 8 bytes in earlier JVMs);
  • arrays require 16 bytes (the same as a normal object, plus 4 bytes for the array length).

Object size granularity (alignment)

In Hotspot, every object occupies a number of bytes that is a multiple of 8. With default settings, if the number of bytes required by an object for its header and fields is not a multiple 8, then you round up to the next multiple of 8. In recent JVMs, the so-called object alignment is a tunable parameter (because the number of bits of alignment do not need to be specified in address offsets, the alignment also determines the overall size of heap that can be addressed).

The 8-byte object alignment rule in Java means, for example, that:

The table of memory usage of typical Java objects presented in our introduction was derived from empirical measurement, and generally reflects the observations above.

Memory used by commomn Java objects, strings and arrays

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Editorial page content written by Neil Coffey. Copyright © Javamex UK 2021. All rights reserved.