Modernize Java with Cursor and GitLab (opens in new tab)
The post argues that modernizing Java 8 to Java 21 should be handled as a series of small, reviewable changes rather than one large AI-generated merge request. Cursor is effective for bounded coding tasks, while GitLab provides the planning, CI/CD, security, review, and lifecycle context needed to make those changes safe. The recommended approach is to begin with a focused test fix, establish quality gates, and then modernize one application boundary at a time. ## AI-Assisted Java Modernization - Java modernization affects the build, runtime, dependencies, APIs, concurrency, tests, containers, and production behavior. - A single broad prompt can produce an oversized merge request that is difficult to validate or review. - Cursor works best when given a focused issue, such as one failing test or one bounded implementation problem. - GitLab complements Cursor with: - Durable planning through epics and issue hierarchies - GitLab MCP context inside Cursor - CI/CD and security scanning - Code Review Flow and Developer Flow - Code-owner approvals and impact analysis - Cross-service testing and review evidence ## The Java HTTP Metrics Collector - The tutorial uses Tanuki IoT Platform’s Java HTTP metrics collector. - The collector: - Checks HTTP health and maintenance endpoints - Records response status and timing metrics - Sends readings to a Rust metrics-store backend through `POST /api/metrics` - This creates a realistic boundary for modernization because both the Java client and Rust backend contract must continue working. ## Project Setup and Guardrails - Required tools include Cursor, Java 8 and Java 21, Maven, Docker, Docker Compose, and GitLab MCP. - GitLab Duo Code Review Flow, Developer Flow, and an impact-analysis flow should be enabled for the project. - The repository includes `AGENTS.md`, which provides Cursor with project structure, instructions, and Maven test commands. - The workflow begins by importing the GitLab project, cloning it, and opening it in Cursor. ## Fixing the Failing End-to-End Test - The collector allows users to configure an expected HTTP status code. - The implementation incorrectly treats every 2xx response as successful and rejects configured responses such as `503`, even when they are expected. - An existing end-to-end test exposes the mismatch, but the CI job is initially allowed to fail, turning the failure into ignored background noise. - Cursor is prompted to: - Analyze the problem first - Trace the configuration through `HttpCollector` - Fix the implementation - Run the focused tests and the full Maven test suite - Once the fix passes, Cursor creates a branch and merge request. - The formerly non-blocking end-to-end job can then become a required check once it is deterministic and green. ## Review and Merge Controls - Each merge request triggers CI/CD, tests, and security scanning. - GitLab Duo Code Review evaluates the change against Java-specific project instructions. - Concrete review findings are addressed through Developer Flow before merging. - The merge request remains the central collaboration and decision point, even when Cursor performs most of the implementation work. - Fixing the test first establishes a behavioral baseline without combining it with the Java runtime migration. ## Planning the Java 21 Migration - The Java 8-to-21 migration is treated as a larger, planned effort rather than an isolated coding task. - The modernization epic contains: - Child work items - Team discussions - Research merge requests - Pipeline history - Dependencies - Security findings - This project context gives the agent information beyond the local source code and helps define the quality gates required before changing production behavior. The practical recommendation is to use Cursor for fast, narrowly scoped implementation while relying on GitLab to provide durable planning, automated evidence, and consistent review controls. This combination allows teams to modernize incrementally without sacrificing safety or reviewability.