quarkusio/quarkusdev-skills/java-decompile

Java Class Decompiler

Decompile and view the source code for Java classes from project dependencies.

Overview

This skill decompiles Java classes from Maven dependencies using Vineflower (a modern Java decompiler) via jbang, allowing you to inspect library internals, understand API implementations, and find method signatures without leaving your workflow.

When to Use

• Need to understand how a library method is implemented
• Looking for exact method signatures or parameter types
• Exploring the internals of a dependency
• Debugging issues related to third-party libraries
• Finding which overloads are available for a class

Quick Reference

Input formats:

• Fully qualified: com.fasterxml.jackson.databind.ObjectMapper
• Simple name: ObjectMapper (will search for matches)

Tools used:

• Vineflower decompiler (via jbang)
• Maven classpath
• Fallback: javap for bytecode disassembly

Requirements:

• jbang installed (https://www.jbang.dev/)

Implementation

Step 1: Determine the Class Name

Parse the user's input (available as $ARGUMENTS in command context):

• If it contains dots (.), treat as fully qualified name
• If it's a simple name, you'll need to search for it first

Step 2: Find the JAR Containing the Class

Convert class name to file path: replace . with /, append .class

Option A: Use Maven classpath file (fast for first search)

# Build classpath file if missing
if [ ! -f /tmp/quarkus-classpath.txt ]; then
  mvn dependency:build-classpath -Dmdep.outputFile=/tmp/quarkus-classpath.txt -q 2>/dev/null
fi
 
# Search for the class in classpath JARs
CLASSPATH=$(cat /tmp/quarkus-classpath.txt)

Option B: Search Maven local repository directly

find ~/.m2/repository -name "*.jar" | while read jar; do
  if jar tf "$jar" 2>/dev/null | grep -q "com/example/ClassName.class"; then
    echo "$jar"
    break
  fi
done

Option C: Check local build output (for project classes)

find . -path "*/target/classes/com/example/ClassName.class" 2>/dev/null

Step 3: Decompile the Class

Primary method: Vineflower via jbang

Vineflower works on JAR files, so extract the specific class or decompile the entire JAR:

# Option A: Decompile specific class from JAR
# Create temp directory for output
TEMP_DIR=$(mktemp -d)
 
# Decompile the JAR containing the class
jbang org.vineflower:vineflower:RELEASE "/path/to/the.jar" "$TEMP_DIR"
 
# Find and display the decompiled class
find "$TEMP_DIR" -name "ClassName.java" -exec cat {} \;
 
# Cleanup
rm -rf "$TEMP_DIR"

# Option B: For single class extraction (faster)
# Extract the class file first
CLASS_FILE="com/example/ClassName.class"
jar xf "/path/to/the.jar" "$CLASS_FILE"
 
# Decompile just that class
TEMP_DIR=$(mktemp -d)
jbang org.vineflower:vineflower:RELEASE "$CLASS_FILE" "$TEMP_DIR"
cat "$TEMP_DIR/com/example/ClassName.java"
 
# Cleanup
rm -rf "$CLASS_FILE" "$TEMP_DIR"

Fallback method: javap (if jbang/Vineflower unavailable)

javap -p -c -cp "/path/to/the.jar" "fully.qualified.ClassName"

Note: jbang automatically downloads and caches Vineflower on first use.

Step 4: Present Decompiled Source

Show the decompiled code with syntax highlighting. Highlight key elements:

• Public API methods and their signatures
• Constructor parameters
• Important fields
• Annotations
• Package and imports

Step 5: Handle Multiple Matches

If the user provided a simple class name and multiple matches exist:

1. List all matching classes with their packages
2. Show which JAR each comes from
3. Ask which one to decompile

Common Patterns

Searching within a specific JAR:

jar tf /path/to/some.jar | grep -i ClassName

Finding all classes with a name pattern:

# Use the class-index if available
grep -i "ClassName" "$HOME/.cache/quarkusdev-skills/class-index.txt"

Tips

• Quarkus internal classes may be in target/classes/ rather than .m2
• Vineflower produces more readable output than javap
• jbang automatically manages Vineflower versions and dependencies
• Use -p flag with javap to show private members
• For inner classes, include the $ in the class name
• Vineflower decompiles entire JARs - for single classes, extract first for faster results

Platform Notes

This skill works with:

• Claude Code: Direct execution
• Gemini CLI: Use equivalent file/bash tools
• Codex: Use RunBash/ReadFile equivalents

Tool scripts are platform-independent (bash/python).