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The project website including binaries for download and a bugtracking system can be found at

The class data structures of the Java HotSpot virtual machine are immutable during the execution of a program. While new classes can be loaded, it is not possible to change existing classes (e.g. add/remove methods or fields). This projects tries to relax this condition to allow arbitrary changes to existing Java classes (including changes to the class hierarchy) while running a Java program.

Work in progress.

Latest patch update: December 17, 2010
Patch base: jdk7-b118
Full patch (advanced version): hotswap.patch
Patch with VM modifications only and limited feature set: hotswaplight.patch
Please send any comments or questions to:

Implementation Notes

Initial implementation description (April 1, 2009): Thomas Wuerthinger, Dynamic Code Evolution for the Java HotSpotVirtual Machine
For more recent information, see the list of publications.

General Design Decisions

  • Arbitrary changes possible (including changes to subtype relationships)
  • No performance penalty before or after the change
  • No introduced indirections
  • Continued execution of old active methods
  • Only simple, comprehensible strategies for field matching or method transitions
  • Change possible at any point when the VM is suspended
  • Graceful handling of accesses to deleted fields or calls of deleted methods


Type of Change Supported? Possible problems after resume
Swap Method Body yes  
Add Method yes  
Remove Method yes NoSuchMethodError / none in advanced version
Add Field yes  
Remove Field yes NoSuchFieldError / none for static fields in advanced version
Add Supertype yes  
Remove Supertype no / yes in advanced version none, but not always guaranteed to be possible

Transformer Methods

Classes can implement a method "void $transformer()".
This method is called on every object whose class was redefined and can therefore be used to initialize the new instances of a class.
Additionally, it is possible to implement a static method "void $staticTransformer()" that will be called on the class only.
Example code:

Initial version
New version

Future Work

  • NetBeans support for advanced features such as transformer methods and method forwarding.
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