What is in Java object header?

by

For HotSpot:

The object header consists of a mark word and a klass pointer.

The mark word has word size (4 byte on 32 bit architectures, 8 byte on 64 bit architectures) and

the klass pointer has word size on 32 bit architectures. On 64 bit architectures the klass pointer either has word size, but can also have 4 byte if the heap addresses can be encoded in these 4 bytes.

This optimization is called “compressed oops” and you can also control it with the option UseCompressedOops.

You can also find a wiki entry about this 1.

The mark word is actually used for many things.

  1. One is Biased Locking 2 through which HotSpot can implement efficient locking.
  2. It is also used during GC to set forward pointers, and to store the age of the objects. The identity hash code of an object can be stored inside the mark (the System.identityHashCode/Object.hashCode one).

There is a comment in the source code of markOop.hpp that describes the layout depending on the architecture:

//  32 bits:
//  --------
//             hash:25 ------------>| age:4    biased_lock:1 lock:2 (normal object)
//             JavaThread*:23 epoch:2 age:4    biased_lock:1 lock:2 (biased object)
//             size:32 ------------------------------------------>| (CMS free block)
//             PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
//
//  64 bits:
//  --------
//  unused:25 hash:31 -->| unused:1   age:4    biased_lock:1 lock:2 (normal object)
//  JavaThread*:54 epoch:2 unused:1   age:4    biased_lock:1 lock:2 (biased object)
//  PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
//  size:64 ----------------------------------------------------->| (CMS free block)
//
//  unused:25 hash:31 -->| cms_free:1 age:4    biased_lock:1 lock:2 (COOPs && normal object)
//  JavaThread*:54 epoch:2 cms_free:1 age:4    biased_lock:1 lock:2 (COOPs && biased object)
//  narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
//  unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)

You can also find the oop header file here.

More Related Contents:

  1. Memory effects of synchronization in Java
  2. How can I tell if I’m running in 64-bit JVM or 32-bit JVM (from within a program)?
  3. What is the exact meaning of static fields in Java?
  4. Running jmap getting Unable to open socket file
  5. How to fix java.lang.UnsupportedClassVersionError: Unsupported major.minor version
  6. Jvm takes a long time to resolve ip-address for localhost
  7. Can the JVM recover from an OutOfMemoryError without a restart
  8. Increasing the JVM maximum heap size for memory intensive applications
  9. How to find the JVM version from a program?
  10. How to tune Tomcat 5.5 JVM Memory settings without using the configuration program
  11. How do I find out what keystore my JVM is using?
  12. Error launching Android Studio: Failed to create JVM: error code-6
  13. JVM option -Xss – What does it do exactly?
  14. When does the main thread stop in Java?
  15. JFrame.dispose() vs System.exit()
  16. What is the JVM thread scheduling algorithm?
  17. try with resources introduce unreachable bytecode
  18. When will JVM use intrinsics
  19. Setting JVM/JRE to use Windows Proxy Automatically
  20. Would Java inline method(s) during optimization?
  21. What does “volatile” mean in Java?
  22. Java VM: reproducible SIGSEGV on both 1.6.0_17 and 1.6.0_18, how to report?
  23. Java concurrency: is final field (initialized in constructor) thread-safe?
  24. Why defining class as final improves JVM performance?
  25. What is the default maximum heap size for Sun’s JVM from Java SE 6?
  26. Why is there no GIL in the Java Virtual Machine? Why does Python need one so bad?
  27. Java’s “os.name” for Windows 10?
  28. How does the JVM terminate daemon threads? or How to write daemon threads that terminate gracefully
  29. Is it possible to use sun.misc.Unsafe to call C functions without JNI?
  30. JVM_FindSignal function continuously allocates native memory

Leave a Comment