java-debugger

java-debugger

A Claude Code skill that teaches Claude to debug Java code using JDI (Java Debug Interface) instead of adding System.out.println statements or writing throwaway programs.

What it does

When Claude needs to understand why Java code misbehaves, this skill provides a DebugInspector tool that:

• Sets breakpoints and inspects local variables, object fields, and collection contents
• Sets watchpoints to track field modifications with old/new values and call stacks
• Evaluates expressions at breakpoints (field access, method calls, reference equality)
• Catches exceptions to inspect state at the throw site
• Returns structured JSON — not text to parse
• Runs in 3-5 seconds per invocation

All without modifying a single line of source code.

Requirements

• JDK 21+ (uses JDI from the jdk.jdi module and single-file source execution)
• Maven or Gradle for test execution

Installation

Via marketplace

/plugin marketplace add kroepke/claude-marketplace
/plugin install java-debugger@kroepke-marketplace

Direct install

/plugin install kroepke/java-debugger-skill

Example

When you tell Claude "the FooTest is failing, can you debug it?", instead of adding print statements, it runs:

java ${CLAUDE_SKILL_DIR}/scripts/DebugInspector.java \
  --maven-project . \
  --test "com.example.FooTest#testMethod" \
  --break "com.example.Foo:42" \
  --port 6001

And gets structured output like:

[{
  "location": "com.example.Foo:42",
  "method": "process",
  "locals": {
    "input": "hello",
    "result": {"type": "HashMap", "id": 1832, "fields": {"size": 0}},
    "expected": {"type": "HashMap", "id": 1833, "fields": {"size": 3}}
  }
}]

Development

The test-projects/debug-testbed/ directory contains a Maven project with intentional bugs and failing tests for development and evaluation. Run mvn test in that directory to see the failures.