HotSwap

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.

Latest patch update: February 2, 2010
Patch name: hotswap.patch
Please send any comments or questions to: wuerthinger@ssw.jku.at

Implementation Notes

Initial implementation description (April 1, 2009): Thomas Wuerthinger, Dynamic Code Evolution for the Java HotSpotVirtual Machine

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

Status

Mutator Methods

Classes can implement a method "void $mutator()".
This method is called on every object whose class was redefined and can therefore be used to initialize the new instances of a class.
Example code:

class A {
  int x;
}

class A' {
  int x;
  int doubleX;

  void $mutator() {
    doubleX = 2 * x;
  }
}

Future Work

• Identify possible problems introduced by deleting a method or field (bytecode analysis on existing fields).
• Determine safety of type narrowing changes.
• Automatic generation of mutator methods
• Investigate ways to match method execution points.