7.16 Object Layout in Memory

This is a historical learning note and may contain outdated or incomplete understanding.

1. Object Composition

There are two kinds of objects: ordinary objects and array objects.

1.1. Object Header

Mark Word

Stores three kinds of information: the object’s hashCode, GC information, and lock information.

Lock information: the meaning of bits differs by state. The final three bits are related to locks: 001 represents the unlocked state, 00 represents a lightweight lock, 10 represents a heavyweight lock, and 101 represents a biased lock.

Class pointer: points to the class. For example, if I call new Test(), this points to the address of Class<Test> in the method area.
Array length: for an array, this value represents the length of the array.

1.2. Instance Data

For example, if Test has an Integer val field, its value is stored here.

1.3. Padding

Ensures 8-byte alignment; insufficient space is padded.

2. Experiment

pom.xml

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

ObjectTest

public class ObjectTest
{
    public static void main(String[] args)
    {
        Object object = new Object();
        String s = ClassLayout.parseInstance(object).toPrintable();
        System.out.println(s);
    }
}

Output

2.1. How Many Bytes Does an Object Occupy?

16 bytes.