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MCP Java Decompiler Server

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Decompile Java class files from file paths, package names, or JAR files using a JavaScript port of the CFR decompiler.

🔥🔥🔥✓ VerifiedFreeAdvanced setup
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MCP Java Decompiler Server (v1.2.4)

A Model Context Protocol (MCP) server for decompiling Java class files. This server allows AI assistants and tools that implement the MCP protocol to decompile Java bytecode into readable source code.

Features

  • Decompile Java .class files from file path
  • Decompile Java classes from package name (e.g., java.util.ArrayList)
  • Decompile Java classes from JAR files
  • Specify which class to extract from JAR files
  • Full MCP-compatible API
  • Stdio transport for seamless integration
  • Clean error handling
  • Temporary file management

MCP Tools

The server provides three main tools:

1. decompile-from-path

Decompiles a Java .class file from a file path.

Parameters:

  • classFilePath: Absolute path to the Java .class file

Example request:

{
  "jsonrpc": "2.0",
  "id": "1",
  "method": "mcp.tool.execute",
  "params": {
    "tool": "decompile-from-path",
    "args": {
      "classFilePath": "/path/to/Example.class"
    }
  }
}

2. decompile-from-package

Decompiles a Java class from a package name.

Parameters:

  • packageName: Fully qualified Java package and class name (e.g., java.util.ArrayList)
  • classpath: (Optional) Array of classpath directories to search

Example request:

{
  "jsonrpc": "2.0",
  "id": "2",
  "method": "mcp.tool.execute",
  "params": {
    "tool": "decompile-from-package",
    "args": {
      "packageName": "java.util.ArrayList",
      "classpath": ["/path/to/rt.jar", "/path/to/classes"]
    }
  }
}

3. decompile-from-jar

Decompiles a Java class from a JAR file.

Parameters:

  • jarFilePath: Absolute path to the JAR file (required)
  • className: Fully qualified class name to extract from the JAR (required) (e.g., "com.example.MyClass")

Example request:

{
  "jsonrpc": "2.0",
  "id": "3",
  "method": "mcp.tool.execute",
  "params": {
    "tool": "decompile-from-jar",
    "args": {
      "jarFilePath": "/path/to/example.jar",
      "className": "com.example.MyClass"
    }
  }
}

Development

# Run in development mode
npm run dev

# Create test fixtures (creates sample Java class for testing)
npm run test:setup

# Run tests 
npm test

# Run linting
npm run lint

# Fix linting issues
npm run lint:fix

# Format code
npm run format

# Run with MCP Inspector for interactive testing
npx @modelcontextprotocol/inspector node ./index.js

Testing with MCP Inspector

You can use the official MCP Inspector tool to test the server functionality interactively:

# Install and run the MCP Inspector with the decompiler server
npx @modelcontextprotocol/inspector node ./index.js

The Inspector provides a user-friendly web interface that allows you to:

  • List all available tools
  • Execute the decompilation tools with custom parameters
  • View and explore the decompiled output
  • Test different inputs and error scenarios

This is especially useful for debugging and understanding the MCP server's capabilities before integrating it with other applications.

How It Works

  1. The server uses the CFR decompiler (@run-slicer/cfr - a JavaScript port of the popular CFR Java decompiler)
  2. When a decompile request is received, the server:
    • Reads the class file data directly or extracts it from a JAR file
    • Processes the class file with CFR decompiler
    • Returns the formatted source code
  3. For JAR files, the server:
    • Creates a temporary directory for extraction
    • Extracts the JAR contents
    • Decompiles the specified class (or first class if none specified)
    • Cleans up the temporary directory