How 660 Java Projects Upgraded from JDK 8 to JDK 21 with Zero Failures

Background and Motivation

For years the backend teams used JDK 8, but growing services hit performance, resource, compatibility and security limits, prompting a migration to JDK 21.

Upgrade Value

Performance

Throughput up ~50% with G1 GC.

Memory usage reduced ~60%.

New Language Features

Records, pattern matching, switch expressions.

Virtual threads for lightweight concurrency.

Improved diagnostics and monitoring tools.

Ecosystem

Support for modern frameworks such as Spring Boot 3.x.

Risks and Challenges

Compatibility issues from the module system, third‑party libraries, and deprecated JVM flags.

Operational risks: manual configuration errors and inconsistent environment settings.

Upgrade Process

Compatibility Scan

Used the open‑source EMT4J tool to scan >2,800 dependencies, finding 130 problematic packages. Scans covered services, test frameworks, CI/CD scripts and monitoring.

Problem Classification

Reflection access – solved with --add‑opens JVM arguments.

Dependency incompatibilities – ignored, configured, upgraded or patched.

Parameter changes – mapped old flags to new ones (e.g., -XX:+UseParNewGC removed, -XX:InitialRAMFraction replaced by -XX:InitialRAMPercentage).

Tooling

Developed a “JDK Upgrade Wizard” that automates parameter checks, code analysis, Dockerfile updates and provides one‑click rollback.

Batch Rollout

Projects were divided into three priority batches, each released after a stability window, using gray‑release, monitoring and automated regression tests.

Results

All 660 services upgraded in three months with zero P3‑level incidents.

Average memory usage dropped 51.33% (several TB saved).

CPU usage reduced for ~13% of services by 10‑30%.

Throughput and response time improved 10‑30% for compute‑intensive services.

The standardized, automated process proved fast, low‑cost and invisible to end users.