jbang-mcp-server

name	jbang-mcp-server
description	Scaffolds and installs zero boilerplate Java-based MCP STDIO servers using JBang and LangChain4j for Gemini CLI. Use this to quickly bootstrap an MCP server from scratch.
license	Apache-2.0
compatibility	Requires a modern version of Java, and the installation of the JBang command-line tool
metadata	{"author":"Guillaume Laforge","version":"0.1"}

JBang LangChain4j MCP Server Creator

This skill helps quickly scaffold a new Java-based MCP STDIO server using JBang and LangChain4j, and installs it into Gemini CLI's settings.json.

Process

1. Ask User for Details:

   • Desired file name (e.g., McpToolServer.java) and path to save it.
   • The name of the server to register in ~/.gemini/settings.json (e.g., java-calc).
   • (Optional) High-level description of the tools they want to add initially.

2. Scaffold the Server:

   • Read the template file located at assets/McpServerTemplate.java using the read_file tool. Note that the path to assets/McpServerTemplate.java needs to be resolved relative to the skill directory or read from the skill's bundled assets. As an alternative if the absolute path is unknown, directly write out the template contents described below.
   • Replace {SERVER_CLASS_NAME} with the base name of the requested Java file (e.g., McpToolServer if file is McpToolServer.java).
   • Replace {TOOL_CLASS_NAME} with a related name (e.g., MyTools).
   • (Optional) Modify the @Tool annotated methods to reflect the user's requirements.
   • Write the finalized content to the user's requested path using the write_file tool.
   • Make the file executable using chmod +x <path_to_file> via run_shell_command.

3. Verify the Server:

   • Run jbang build <path_to_file> using the run_shell_command tool to check for any compilation errors.
   • If there are compilation errors (e.g., unclosed string literals due to unescaped newlines), use the replace tool to fix them.
   • Repeat the compilation check until successful.

4. Install the Server in Gemini CLI:

   • Read ~/.gemini/settings.json.
   • Use the replace tool or jq via run_shell_command to inject a new entry under mcpServers.
   • The new entry should look like this:

     "{server_name}": {
       "command": "jbang",
       "args": [
         "run",
         "--quiet",
         "{absolute_path_to_java_file}"
       ]
     }
     

   • Inform the user that the server has been created and configured, and remind them that Gemini CLI automatically reloads configurations.

Template Backup

If assets/McpServerTemplate.java cannot be read, use this template:

///usr/bin/env jbang "$0" "$@" ; exit $?
//DEPS dev.langchain4j:langchain4j-core:1.11.0
//DEPS dev.langchain4j:langchain4j-community-mcp-server:1.11.0-beta19
//DEPS org.slf4j:slf4j-simple:2.0.17
//JAVA 21

import dev.langchain4j.agent.tool.Tool;
import dev.langchain4j.community.mcp.server.McpServer;
import dev.langchain4j.community.mcp.server.transport.StdioMcpServerTransport;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;
import java.util.concurrent.CountDownLatch;

public class {SERVER_CLASS_NAME} {

    static {
        // Configure SLF4J Simple Logger to write to System.err
        // This is crucial for MCP servers over STDIO to avoid polluting stdout
        System.setProperty("org.slf4j.simpleLogger.logFile", "System.err");
    }

    private static final Logger log = LoggerFactory.getLogger({SERVER_CLASS_NAME}.class);

    public static void main(String[] args) throws Exception {
        log.info("Starting LangChain4j MCP Server...");

        // Instantiate tools
        {TOOL_CLASS_NAME} tools = new {TOOL_CLASS_NAME}();

        // Create Server
        McpServer server = new McpServer(List.of(tools));

        // Start Transport
        StdioMcpServerTransport transport = new StdioMcpServerTransport(server);

        log.info("MCP Server started successfully on STDIO.");

        // Keep Alive
        new CountDownLatch(1).await();
    }

    // --- Tool Definition ---
    public static class {TOOL_CLASS_NAME} {

        @Tool("Description of your tool")
        public String sampleTool(String input) {
            log.info("Called sampleTool with {}", input);
            return "Processed: " + input;
        }
    }
}

Key Rules

• Logging: JBang STDIO servers MUST write all logs to System.err to avoid polluting the JSON-RPC standard output stream. This is already handled in the template via System.setProperty("org.slf4j.simpleLogger.logFile", "System.err"); but ensure this is maintained if modifying the file structure.
• Dependencies: The template relies on LangChain4j and slf4j-simple. Do not remove the //DEPS directives at the top of the template.