How Much Memory Does a Java Object Really Use? A JOL Walkthrough

To determine how many bytes a Java object occupies, you can use the OpenJDK JOL (Java Object Layout) tool.

Add the following dependency to your pom.xml:

<dependency>
  <groupId>org.openjdk.jol</groupId>
  <artifactId>jol-core</artifactId>
  <version>0.10</version>
</dependency>

Java objects are aligned to 8‑byte boundaries, so their total size is always a multiple of 8. An object consists of three parts: the object header, the instance data, and any padding needed to reach the alignment.

The object header includes a mark word (8 bytes) and a klass pointer (4 bytes). For array types, an additional field stores the array length.

For a simple object, after accounting for the sizes of its fields and the required padding, the total size in the example is 40 bytes .

For an array example:

char[] cs = new char[36];
System.out.println(ClassLayout.parseInstance(cs).toPrintable());

The JOL output shows:

[C object internals:
OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
0       4      (object header)                           01 00 00 00 (1)
4       4      (object header)                           00 00 00 00 (0)
8       4      (object header)                           41 00 00 20 (536870977)
12      4      (object header)                           24 00 00 00 (36)
16      72     char [C.<elements>                         N/A
Instance size: 88 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

Since each char occupies 2 bytes and the array length is 36, the calculated instance size is 88 bytes .

These calculations illustrate how JOL can be used to inspect object memory layout, understand padding, and verify the actual byte consumption of Java objects.