The Hitchhiker’s guide to Java class reloading
summary
Compares three concrete approaches and demos working code: the JVM HotSwap path using Instrumentation.redefineClasses preserves instance state but only allows method-internal edits; class loader–based restarts via Spring Boot DevTools drop context and lose session state but restart in seconds; and an agent that watches file timestamps, attaches with the Instrumentation API, and replays transformations (splitting state and behavior) to add fields/methods and refresh Spring/Hibernate mappings. Shows an agent loader that attaches to a running process, live-reloads controllers and entities, and preserves counters during many edits.
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chapters (12)
- 0:00Intro
- 3:14The solutions
- 4:07Hotswap
- 9:31Theory
- 13:45Dynamic Code Evolution VM
- 16:43Annotations
- 18:32Class loaders
- 21:03Application servers
- 26:59Java agents
- 28:54Demo
- 36:39Example
- 44:10Conclusion
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