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.

Prototype
Work in progress.

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

Type of Change	Supported?	Possible problems after resume
Swap Method Body	yes
Add Method	yes
Remove Method	yes	NoSuchMethodError
Add Field	yes
Remove Field	yes	NoSuchFieldError
Add Supertype	yes
Remove Supertype	yes	possible VM crash

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:

Initial version

class A {
  int x;
}

New version

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 active methods).
Determine safety of type narrowing changes.
Automatic generation of mutator methods
Investigate ways to match method execution points.