MCP协议

原理讲解

基础概念

Introduction - Model Context Protocol

MCP Host：想要通过 MCP 访问数据的程序，例如 Claude Desktop、IDE 或 AI 工具
MCP Clients：与服务器保持 1:1 连接的协议客户端
MCP Servers：轻量级程序，每个程序通过标准化模型上下文协议公开特定功能
Local Data Sources: MCP 服务器可以安全访问的计算机文件、数据库和服务
Remote Services: MCP 服务器可以通过互联网（例如，通过 API）连接的外部系统

核心组件

协议层

协议层处理消息框架，请求/响应链接和高级通信模式。

class Protocol<Request, Notification, Result> {// Handle incoming requests, 处理请求setRequestHandler<T>(schema: T, handler: (request: T, extra: RequestHandlerExtra) => Promise<Result>): void// Handle incoming notifications, 处理通知, 不需要对方响应的请求setNotificationHandler<T>(schema: T, handler: (notification: T) => Promise<void>): void// Send requests and await responses, 请求request<T>(request: Request, schema: T, options?: RequestOptions): Promise<T>// Send one-way notifications, 通知notification(notification: Notification): Promise<void>
}

传输层

运输层处理客户与服务器之间的实际通信。 MCP支持多种运输机制：

Stdio transport

- 使用标准输入/输出进行通信
- 适用于本地流程的处理

HTTP with SSE transport

- 使用服务器范围的事件进行服务器到客户消息
- 使用 http POST方式发送消息，用于客户到服务器消息

所有运输都使用JSON-RPC 2.0交换消息。有关模型上下文协议消息格式的详细信息，请参见规格。

消息类型

MCP具有这些主要消息类型：

请求期望对方有回应：

interface Request {method: string;params?: { ... };
}

结果是对请求的成功响应：

interface Result {[key: string]: unknown;
}

错误表明请求失败：

interface Error {code: number;message: string;data?: unknown;
}

通知是单向消息，不会期望回复：

interface Notification {method: string;params?: { ... };
}

生命周期

初始化

客户以协议版本和功能发送initialize请求
服务器响应其协议版本和功能
客户将initialized通知发送为确认
正常消息交换开始

init client, connect server
init model, get tools, query, tool call
tool result -> model
model summary

尝试使用

通过mcp工具读取本地文件内容

clone 官方提供的mcp服务项目:

git clone git@github.com:modelcontextprotocol/servers.git

构建本地服务:

docker build -t mcp/filesystem -f src/filesystem/Dockerfile .

镜像构建完成后, 采用LangChain4j进行测试用例构建:

package dev.langchain4j.example.mcp;import dev.langchain4j.mcp.McpToolProvider;
import dev.langchain4j.mcp.client.DefaultMcpClient;
import dev.langchain4j.mcp.client.McpClient;
import dev.langchain4j.mcp.client.transport.McpTransport;
import dev.langchain4j.mcp.client.transport.stdio.StdioMcpTransport;
import dev.langchain4j.model.chat.ChatLanguageModel;
import dev.langchain4j.model.openai.OpenAiChatModel;
import dev.langchain4j.service.AiServices;
import dev.langchain4j.service.tool.ToolProvider;import java.io.File;
import java.util.List;public class McpToolsExampleOverStdio {// We will let the AI read the contents of this file/*** This example uses the `server-filesystem` MCP server to showcase how* to allow an LLM to interact with the local filesystem.* <p>* Running this example requires npm to be installed on your machine,* because it spawns the `server-filesystem` as a subprocess via npm:* `npm exec @modelcontextprotocol/server-filesystem@0.6.2`.* <p>* Of course, feel free to swap out the server with any other MCP server.* <p>* The communication with the server is done directly via stdin/stdout.* <p>* IMPORTANT: when executing this, make sure that the working directory is* equal to the root directory of the project* (`langchain4j-examples/mcp-example`), otherwise the program won't be able to find* the proper file to read. If you're working from another directory,* adjust the path inside the StdioMcpTransport.Builder() usage in the main method.*/public static void main(String[] args) throws Exception {ChatLanguageModel model = OpenAiChatModel.builder().apiKey(System.getenv("OPENAI_API_KEY")).baseUrl(System.getenv("OPENAI_API_URL")).modelName("gpt-4o-mini").logRequests(true).logResponses(true).build();McpTransport transport = new StdioMcpTransport.Builder().command(List.of("docker", "run", "-i", "--rm","--mount", "type=bind,src=/Users/louye/fun-project/langchain4j-examples/mcp-example/src/main/resources,dst=/projects","mcp/filesystem","/projects")).logEvents(true).build();McpClient mcpClient = new DefaultMcpClient.Builder().transport(transport).build();ToolProvider toolProvider = McpToolProvider.builder().mcpClients(List.of(mcpClient)).build();Bot bot = AiServices.builder(Bot.class).chatLanguageModel(model).toolProvider(toolProvider).build();try {String response = bot.chat("修改文件 /projects/file.txt 的内容, 将内容修改为: Hello, world!");System.out.println("RESPONSE: " + response);} finally {mcpClient.close();}}
}