MCP Java Decompiler Server

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.

- 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

- Node.js 16+
- npm
- No Java requirement (using JavaScript port of CFR decompiler)

You can run the server directly with npx without installing:

# Run the server npx -y @idachev/mcp-javadc

# Install globally npm install -g @idachev/mcp-javadc # Run the server mcpjavadc

# Clone the repository git clone https://github.com/idachev/mcp-javadc.git cd mcp-javadc # Install dependencies npm install # Run the server npm start

To use with an MCP client (like Claude or another MCP-compatible AI assistant):

# Configure the MCP client to use this server npx some-mcp-client --server "node /path/to/mcp-javadc/index.js"

claude mcp add javadc -s project -- npx -y @idachev/mcp-javadc

{ "mcpServers": { "javaDecompiler": { "command": "npx", "args": ["-y", "@idachev/mcp-javadc"], "env": { "CLASSPATH": "/path/to/java/classes" } } } }

Decompiles a Java .class file from a file path.

- classFilePath: Absolute path to the Java .class file

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

Decompiles a Java class from a package name.

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

{ "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"] } } }

Decompiles a Java class from a JAR file.

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

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

The CFR decompiler (@run-slicer/cfr) is a JavaScript port of the popular CFR Java decompiler. It works well with:
- Standard Java class files
- Classes that are part of a known package structure
- Modern Java features (all Java versions)
- JAR files containing Java classes

If you encounter issues with a specific class file, try:

- Using thedecompile-from-packagetool with explicit classpath
- Using thedecompile-from-jartool with explicit class name
- Ensuring the class file is a valid Java bytecode file
- Checking for corrupt class files or JAR archives

When working with JAR files from Maven repositories:

- Use thefind ~/.m2 -name "dependency-namejar"command to locate JAR files
- Filter out source and javadoc JARs usinggrep -v source | grep -v javadoc
- Usejar tf your-jar-file.jar | grep .classto list available classes in a JAR
- Check that class names match the package structure in the JAR

- CLASSPATH: Java classpath for finding class files (used when no classpath is specified)

# 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

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.
- The server uses the CFR decompiler (@run-slicer/cfr - a JavaScript port of the popular CFR Java decompiler)
- 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

- 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

This is a web browser that enables your coding agent, such as Claude Code, to visit websites on your behalf and assist you in identifying bugs or creating UI test cases.

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