How Much Memory Does a Java Object Really Consume?

Memory Consumption of an Object in Java

Determining the memory consumption of an object in Java is a multifaceted topic, as it depends on various factors.

Object Header and References

In modern 64-bit JDKs, objects have a 12-byte header that is padded to a multiple of 8 bytes, resulting in a minimum object size of 16 bytes. Objects on 32-bit JVMs have an 8-byte header padded to a multiple of 4 bytes.

References to objects typically occupy 4 bytes on 32-bit platforms or up to -Xmx32G on 64-bit platforms, and 8 bytes above 32Gb (-Xmx32G).

Measurement Methods

To estimate the storage consumed by an object, you can use Instrumentation.getObjectSize(). The JOL (Java Object Layout) tool can be used to visualize the actual object layout, footprint, and references.

Object Contents and Alignment

The contents of an object, such as member variables and references, contribute to the memory consumption. To optimize memory usage, the JVM employs alignment, where memory is allocated in multiples of 8 bytes. This means that even if an object contains only a few small variables, the JVM might allocate more memory to ensure alignment.

Boxed Types, Arrays, and Strings

Boxed wrapper classes, such as Integer or Long, have overhead compared to primitive types due to their additional size and the object header. Multidimensional arrays and strings can also be memory intensive due to the overhead of nested arrays and the String's internal char array and additional overhead.

Therefore, while answering the specific question given becomes difficult due to the JVM's flexibility in memory management, these factors provide insight into the memory consumption of objects in Java. To optimize memory usage, carefully consider object design, data structures, and alignment.

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