Object Layout

Java object layout¶

Provides a good guide to the size of the object header in Java and what it is made up of, for 64bit (compressed oops on and off) and 32 bit JVM.

https://gist.github.com/arturmkrtchyan/43d6135e8a15798cc46c

Excellent guide to memory layout: http://psy-lob-saw.blogspot.com/2013/05/know-thy-java-object-memory-layout.html

The tool jol is a library that can be used to identify the expected size of an object.

Here we see the output for testing with jol on a 64bit compressed oops JVM.

public final class MyClassSingleInt {
    public int value;
}

com.bvb.MyClassSingleInt object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0    12        (object header)                           N/A
     12     4    int MyClassSingleInt.value                    N/A
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

public final class MyClassTwoInts {
    public int int1;
    public int int2;
}

com.bvb.MyClassTwoInts object internals:
 OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
      0    12        (object header)                           N/A
     12     4    int MyClassTwoInts.int1                       N/A
     16     4    int MyClassTwoInts.int2                       N/A
     20     4        (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

You can see that the second object with two ints, each of size 4 bytes, will lose a total of 4 bytes due to the alignment of the object on 8 byte boundaries.

Object header¶

See the link above for 32bit, not used that often at this point.

64bit compressed OOP¶

|--------------------------------------------------------------------------------------------------------------|--------------------|
|                                            Object Header (96 bits)                                           |        State       |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
|                                  Mark Word (64 bits)                           |    Klass Word (32 bits)     |                    |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
| unused:25 | identity_hashcode:31 | cms_free:1 | age:4 | biased_lock:1 | lock:2 |    OOP to metadata object   |       Normal       |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
| thread:54 |       epoch:2        | cms_free:1 | age:4 | biased_lock:1 | lock:2 |    OOP to metadata object   |       Biased       |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
|                         ptr_to_lock_record                            | lock:2 |    OOP to metadata object   | Lightweight Locked |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
|                     ptr_to_heavyweight_monitor                        | lock:2 |    OOP to metadata object   | Heavyweight Locked |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|
|                                                                       | lock:2 |    OOP to metadata object   |    Marked for GC   |
|--------------------------------------------------------------------------------|-----------------------------|--------------------|

64bit¶

|------------------------------------------------------------------------------------------------------------|--------------------|
|                                            Object Header (128 bits)                                        |        State       |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
|                                  Mark Word (64 bits)                         |    Klass Word (64 bits)     |                    |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
| unused:25 | identity_hashcode:31 | unused:1 | age:4 | biased_lock:1 | lock:2 |    OOP to metadata object   |       Normal       |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
| thread:54 |       epoch:2        | unused:1 | age:4 | biased_lock:1 | lock:2 |    OOP to metadata object   |       Biased       |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
|                       ptr_to_lock_record:62                         | lock:2 |    OOP to metadata object   | Lightweight Locked |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
|                     ptr_to_heavyweight_monitor:62                   | lock:2 |    OOP to metadata object   | Heavyweight Locked |
|------------------------------------------------------------------------------|-----------------------------|--------------------|
|                                                                     | lock:2 |    OOP to metadata object   |    Marked for GC   |
|------------------------------------------------------------------------------|-----------------------------|--------------------|

Klass word¶

The class word contains a pointer to the native memory representation of the class.
Link to Klass from j11: https://hg.openjdk.java.net/jdk/jdk11/file/1ddf9a99e4ad/src/hotspot/share/oops/klass.hpp
Because of the compressed ordinary object pointers, we have a shorthand reference to the native memory location. See the following for more info: https://stuefe.de/posts/metaspace/what-is-compressed-class-space/

Last update: 2025-08-16