Jurassic JDK: Migrate or Extinct
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A practical guide to migrating legacy Java applications from JDK 7 to JDK 21 (and beyond), based on 18 months of real-world experience across 15+ production projects. The core advice is to hop LTS version by LTS version (7→8→11→17→21) rather than attempting a single big jump. Key steps include running a dependency audit first, using OpenRewrite for automated refactoring, manually updating build file Java version targets, and budgeting significant time for Hibernate 6 breaking changes (ResultSet reading by position, @Type rework, removal of IdGeneratorStrategyInterpreter). The guide also covers JDK 25 changes, CI/CD pipeline updates, and a phased migration plan: inventory, dependency cleanup, one hop at a time, then repeat.
