MCP Server

Overview

The MCP Server is a foundational component in the Model Context Protocol (MCP) architecture that provides tools, resources, and capabilities to clients. It implements the server-side of the protocol, responsible for:

Exposing tools that clients can discover and execute
Managing resources with URI-based access patterns
Providing prompt templates and handling prompt requests
Supporting capability negotiation with clients
Implementing server-side protocol operations
Managing concurrent client connections
Providing structured logging and notifications

The server supports both synchronous and asynchronous APIs, allowing for flexible integration in different application contexts.

// Create a server with custom configuration
McpSyncServer syncServer = McpServer.sync(transport)
    .serverInfo("my-server", "1.0.0")
    .capabilities(ServerCapabilities.builder()
        .resources(true)     // Enable resource support
        .tools(true)         // Enable tool support
        .prompts(true)       // Enable prompt support
        .logging()           // Enable logging support
        .build())
    .build();

// Register tools, resources, and prompts
syncServer.addTool(syncToolRegistration);
syncServer.addResource(syncResourceRegistration);
syncServer.addPrompt(syncPromptRegistration);

// Send logging notifications
syncServer.loggingNotification(LoggingMessageNotification.builder()
    .level(LoggingLevel.INFO)
    .logger("custom-logger")
    .data("Server initialized")
    .build());

// Close the server when done
syncServer.close();

// Create an async server with custom configuration
McpAsyncServer asyncServer = McpServer.async(transport)
    .serverInfo("my-server", "1.0.0")
    .capabilities(ServerCapabilities.builder()
        .resources(true)     // Enable resource support
        .tools(true)         // Enable tool support
        .prompts(true)       // Enable prompt support
        .logging()           // Enable logging support
        .build())
    .build();

// Register tools, resources, and prompts
asyncServer.addTool(asyncToolRegistration)
    .doOnSuccess(v -> logger.info("Tool registered"))
    .subscribe();

asyncServer.addResource(asyncResourceRegistration)
    .doOnSuccess(v -> logger.info("Resource registered"))
    .subscribe();

asyncServer.addPrompt(asyncPromptRegistration)
    .doOnSuccess(v -> logger.info("Prompt registered"))
    .subscribe();

// Send logging notifications
asyncServer.loggingNotification(LoggingMessageNotification.builder()
    .level(LoggingLevel.INFO)
    .logger("custom-logger")
    .data("Server initialized")
    .build());

// Close the server when done
asyncServer.close()
    .doOnSuccess(v -> logger.info("Server closed"))
    .subscribe();

Server Transport

Create in-process based transport:

StdioServerTransport transport = new StdioServerTransport(new ObjectMapper());

Provides bidirectional JSON-RPC message handling over standard input/output streams with non-blocking message processing, serialization/deserialization, and graceful shutdown support.

Key features:

Bidirectional communication through stdin/stdout
Process-based integration support
Simple setup and configuration
Lightweight implementation

Creates WebFlux-based SSE server transport. Requires the mcp-spring-webflux dependency.

@Configuration
class McpConfig {
    @Bean
    WebFluxSseServerTransport webFluxSseServerTransport(ObjectMapper mapper) {
        return new WebFluxSseServerTransport(mapper, "/mcp/message");
    }

    @Bean
    RouterFunction<?> mcpRouterFunction(WebFluxSseServerTransport transport) {
        return transport.getRouterFunction();
    }
}

Implements the MCP HTTP with SSE transport specification, providing:

Reactive HTTP streaming with WebFlux
Concurrent client connections through SSE endpoints
Message routing and session management
Graceful shutdown capabilities

Creates WebMvc-based SSE server transport. Requires the mcp-spring-webmvc dependency.

@Configuration
@EnableWebMvc
class McpConfig {
    @Bean
    WebMvcSseServerTransport webMvcSseServerTransport(ObjectMapper mapper) {
        return new WebMvcSseServerTransport(mapper, "/mcp/message");
    }

    @Bean
    RouterFunction<ServerResponse> mcpRouterFunction(WebMvcSseServerTransport transport) {
        return transport.getRouterFunction();
    }
}

Implements the MCP HTTP with SSE transport specification, providing:

Server-side event streaming
Integration with Spring WebMVC
Support for traditional web applications
Synchronous operation handling

Creates a Servlet-based SSE server transport. It is included in the core mcp module. The HttpServletSseServerTransport can be used with any Servlet container. To use it with a Spring Web application, you can register it as a Servlet bean:

@Configuration
@EnableWebMvc
public class McpServerConfig implements WebMvcConfigurer {

    @Bean
    public HttpServletSseServerTransport servletSseServerTransport() {
        return new HttpServletSseServerTransport(new ObjectMapper(), "/mcp/message");
    }

    @Bean
    public ServletRegistrationBean customServletBean(HttpServletSseServerTransport servlet) {
        return new ServletRegistrationBean(servlet);
    }
}

Implements the MCP HTTP with SSE transport specification using the traditional Servlet API, providing:

Asynchronous message handling using Servlet 6.0 async support
Session management for multiple client connections
Two types of endpoints:
- SSE endpoint (/sse) for server-to-client events
- Message endpoint (configurable) for client-to-server requests
Error handling and response formatting
Graceful shutdown support

Server Capabilities

The server can be configured with various capabilities:

var capabilities = ServerCapabilities.builder()
    .resources(false, true)  // Resource support with list changes notifications
    .tools(true)            // Tool support with list changes notifications
    .prompts(true)          // Prompt support with list changes notifications
    .logging()              // Enable logging support (enabled by default with loging level INFO)
    .build();

Logging Support

The server provides structured logging capabilities that allow sending log messages to clients with different severity levels:

// Send a log message to clients
server.loggingNotification(LoggingMessageNotification.builder()
    .level(LoggingLevel.INFO)
    .logger("custom-logger")
    .data("Custom log message")
    .build());

Clients can control the minimum logging level they receive through the mcpClient.setLoggingLevel(level) request. Messages below the set level will be filtered out. Supported logging levels (in order of increasing severity): DEBUG (0), INFO (1), NOTICE (2), WARNING (3), ERROR (4), CRITICAL (5), ALERT (6), EMERGENCY (7)

Tool Registration

// Sync tool registration
var syncToolRegistration = new McpServerFeatures.SyncToolRegistration(
    new Tool("calculator", "Basic calculator", Map.of(
        "operation", "string",
        "a", "number",
        "b", "number"
    )),
    arguments -> {
        // Tool implementation
        return new CallToolResult(result, false);
    }
);

// Async tool registration
var asyncToolRegistration = new McpServerFeatures.AsyncToolRegistration(
    new Tool("calculator", "Basic calculator", Map.of(
        "operation", "string",
        "a", "number",
        "b", "number"
    )),
    arguments -> {
        // Tool implementation
        return Mono.just(new CallToolResult(result, false));
    }
);

Resource Registration

// Sync resource registration
var syncResourceRegistration = new McpServerFeatures.SyncResourceRegistration(
    new Resource("custom://resource", "name", "description", "mime-type", null),
    request -> {
        // Resource read implementation
        return new ReadResourceResult(contents);
    }
);

// Async resource registration
var asyncResourceRegistration = new McpServerFeatures.AsyncResourceRegistration(
    new Resource("custom://resource", "name", "description", "mime-type", null),
    request -> {
        // Resource read implementation
        return Mono.just(new ReadResourceResult(contents));
    }
);

Prompt Registration

// Sync prompt registration
var syncPromptRegistration = new McpServerFeatures.SyncPromptRegistration(
    new Prompt("greeting", "description", List.of(
        new PromptArgument("name", "description", true)
    )),
    request -> {
        // Prompt implementation
        return new GetPromptResult(description, messages);
    }
);

// Async prompt registration
var asyncPromptRegistration = new McpServerFeatures.AsyncPromptRegistration(
    new Prompt("greeting", "description", List.of(
        new PromptArgument("name", "description", true)
    )),
    request -> {
        // Prompt implementation
        return Mono.just(new GetPromptResult(description, messages));
    }
);

Error Handling

The SDK provides comprehensive error handling through the McpError class, covering protocol compatibility, transport communication, JSON-RPC messaging, tool execution, resource management, prompt handling, timeouts, and connection issues. This unified error handling approach ensures consistent and reliable error management across both synchronous and asynchronous operations.

Overview MCP Client