Quarkus Debug

quarkus-debug

Keyword: quarkus-debug | Platforms: gemini,claude,codex

Expert AI Agent Skill for Quarkus Debugging - Advanced techniques for diagnosing and fixing issues across the entire Quarkus lifecycle, from development mode to native executables.

Core Mandates

• Dev Mode First: Always leverage quarkus:dev for immediate feedback and live reload.
• Context-Aware Debugging: Distinguish between Imperative (Blocking) and Reactive (Event Loop) contexts.
• Binary Parity: Ensure behavior consistency between JVM mode and Native Image mode.
• Augmentation Insight: Distinguish between build-time (deployment) and run-time errors.
• No-Block Rule: Never block the Event Loop during debugging unless using specific thread-aware tools.

🛠 Debugging Domains

1. Development Phase (Dev Mode)

• JPDA/Remote Debug: Default port 5005. Use quarkus.debug.host and quarkus.debug.port to customize.
• Dev UI (/q/dev):
  • Inspect CDI Beans, Configuration, and Extension status.
  • Use the Arc extension UI to debug dependency injection issues.
  • Continuous Testing: Debug tests as they run in the background.
• Hot Reload Issues: If changes don't reflect, check quarkus.live-reload.password or ClassLoader isolation settings.

2. Reactive & Asynchronous (Mutiny)

• Stack Trace Unwrapping: Reactive stack traces are often unhelpful. Use .onFailure().invoke(Throwable::printStackTrace) or Mutiny's infrastructure tools.
• Context Propagation:
  • Debug ContextNotActiveException by ensuring DuplicatedContext is propagated correctly.
  • Use quarkus.arc.context-propagation.enabled=true.
• Event Loop Blocking: Enable quarkus.vertx.warning-exception-time to detect long-running tasks blocking the Event Loop.
• Mutiny Infrastructure: Use Infrastructure.setCanClearThreadLocals(false) carefully to debug ThreadLocal issues.

3. Native Executables (GraalVM AOT)

• AOT Issues: Most native errors are due to Reflection, Resources, or Dynamic Proxies missing from reflect-config.json.
• GraalVM Agent: Run in JVM mode with the agent to auto-generate configs:

  java -agentlib:native-image-agent=config-output-dir=./config -jar target/*-runner.jar
  

• Native Debugging: Build with -H:GenerateDebugInfo=1 and use GDB or LLDB.
• Static vs Runtime Init: Debug InitializerError by checking quarkus.native.additional-build-args=--trace-class-initialization=....

4. Build-Time (Augmentation)

• BuildStep Failure: If the build fails during "Augmenting phase", it's a deployment issue.
• Log Verbosity: Use -Dquarkus.log.level=DEBUG during build to see extension internal logs.
• Bytecode Inspection: Inspect generated classes in target/quarkus-app/lib/main/ or using tools like javap.
• Bazel (rules_quarkus):
  • Debug augmentation by running with --sandbox_debug --verbose_failures.
  • Investigate QuarkusBootstrap by checking the generated quarkus-bootstrap.json.

🔍 Troubleshooting Workflows

ClassLoader & Dependency Conflicts

• Issue: ClassCastException or NoClassDefFoundError in Dev Mode.
• Solution: Quarkus uses a multi-layered ClassLoader. Check if a library is being loaded by the "Runtime ClassLoader" but expected by the "Base ClassLoader".
• Action: Use quarkus.class-loading.parent-first-artifacts to force specific libraries to the parent ClassLoader.

Database & Dev Services

• Issue: Testcontainers/Dev Services fail to start.
• Action: Check Docker connectivity. Inspect logs using docker logs <container_id>. Use quarkus.datasource.devservices.port to pin ports for external inspection.

Memory Leaks in Dev Mode

• Issue: OutOfMemoryError after several hot reloads.
• Action: Often caused by static fields or threads not being shut down by an extension. Use JFR (Java Flight Recorder) to profile:

  mvn quarkus:dev -Dquarkus.profile=dev -Djava.arg.1=-XX:StartFlightRecording=filename=recording.jfr
  

🌐 Troubleshooting Sources

Directive: When dealing with cryptic reactive stack traces or native crashes, use web_fetch on these specialized troubleshooting guides.

• Reactive Diagnostics: Mutiny Infrastructure Guide - Clear ThreadLocals and debug handlers.
• Native Crash Analysis: GraalVM Native Image Diagnostics - Debugging native executables.
• Context Propagation: SmallRye Context Propagation Guide - Dealing with ThreadLocal loss in async code.
• OTel Tracing: Quarkus OpenTelemetry Guide - Tracing requests across microservices.

📚 References & Tools

• Quarkus - Debugging Guide
• Mutiny - Troubleshooting Guide
• GraalVM - Native Image Debugging
• rules_quarkus - Integration Troubleshooting

Skill Interoperability

The quarkus-debug 🔍 skill is an advanced troubleshooting layer built on:

• java-expert ☕: JVM internals, JFR, and basic JPDA.
• quarkus-expert ⚡: CDI, Augmentation, and Dev Mode internals.
• vertx-expert 🌀: Event Loop and non-blocking I/O debugging.
• graalvm-expert 🚀: AOT compilation and native runtime issues.
• rules-quarkus 🔧: Bazel-specific augmentation and orchestration.