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
Spring-specific transport implementations are available as an optional dependencies io.modelcontextprotocol.sdk:mcp-spring-webflux, io.modelcontextprotocol.sdk:mcp-spring-webmvc for Spring Framework users.
The server supports both synchronous and asynchronous APIs, allowing for flexible integration in different application contexts.
// Register tools, resources, and prompts syncServer.addTool(syncToolSpecification); syncServer.addResource(syncResourceSpecification); syncServer.addPrompt(syncPromptSpecification);
// Send logging notifications syncServer.loggingNotification(LoggingMessageNotification.builder() .level(LoggingLevel.INFO) .logger("custom-logger") .data("Server initialized") .build());
// Close the server when done syncServer.close();
</Tab>
<Tab title="Async API">
```java
// Create an async server with custom configuration
McpAsyncServer asyncServer = McpServer.async(transportProvider)
.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(asyncToolSpecification)
.doOnSuccess(v -> logger.info("Tool registered"))
.subscribe();
asyncServer.addResource(asyncResourceSpecification)
.doOnSuccess(v -> logger.info("Resource registered"))
.subscribe();
asyncServer.addPrompt(asyncPromptSpecification)
.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 Providers
The transport layer in the MCP SDK is responsible for handling the communication between clients and servers. It provides different implementations to support various communication protocols and patterns. The SDK includes several built-in transport provider implementations:
```java
StdioServerTransportProvider transportProvider = new StdioServerTransportProvider(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:
<ul>
<li>Bidirectional communication through stdin/stdout</li>
<li>Process-based integration support</li>
<li>Simple setup and configuration</li>
<li>Lightweight implementation</li>
</ul>
</>
```java
@Configuration
class McpConfig {
@Bean
WebFluxSseServerTransportProvider webFluxSseServerTransportProvider(ObjectMapper mapper) {
return new WebFluxSseServerTransportProvider(mapper, "/mcp/message");
}
@Bean
RouterFunction<?> mcpRouterFunction(WebFluxSseServerTransportProvider transportProvider) {
return transportProvider.getRouterFunction();
}
}
```
<p>Implements the MCP HTTP with SSE transport specification, providing:</p>
<ul>
<li>Reactive HTTP streaming with WebFlux</li>
<li>Concurrent client connections through SSE endpoints</li>
<li>Message routing and session management</li>
<li>Graceful shutdown capabilities</li>
</ul>
</>
```java
@Configuration
@EnableWebMvc
class McpConfig {
@Bean
WebMvcSseServerTransportProvider webMvcSseServerTransportProvider(ObjectMapper mapper) {
return new WebMvcSseServerTransportProvider(mapper, "/mcp/message");
}
@Bean
RouterFunction<ServerResponse> mcpRouterFunction(WebMvcSseServerTransportProvider transportProvider) {
return transportProvider.getRouterFunction();
}
}
```
<p>Implements the MCP HTTP with SSE transport specification, providing:</p>
<ul>
<li>Server-side event streaming</li>
<li>Integration with Spring WebMVC</li>
<li>Support for traditional web applications</li>
<li>Synchronous operation handling</li>
</ul>
</>
```java
@Configuration
@EnableWebMvc
public class McpServerConfig implements WebMvcConfigurer {
@Bean
public HttpServletSseServerTransportProvider servletSseServerTransportProvider() {
return new HttpServletSseServerTransportProvider(new ObjectMapper(), "/mcp/message");
}
@Bean
public ServletRegistrationBean customServletBean(HttpServletSseServerTransportProvider transportProvider) {
return new ServletRegistrationBean(transportProvider);
}
}
```
<p>
Implements the MCP HTTP with SSE transport specification using the traditional Servlet API, providing:
</p>
<ul>
<li>Asynchronous message handling using Servlet 6.0 async support</li>
<li>Session management for multiple client connections</li>
<li>
Two types of endpoints:
<ul>
<li>SSE endpoint (<code>/sse</code>) for server-to-client events</li>
<li>Message endpoint (configurable) for client-to-server requests</li>
</ul>
</li>
<li>Error handling and response formatting</li>
<li>Graceful shutdown support</li>
</ul>
</>
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 logging 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 Specification
The Model Context Protocol allows servers to expose tools that can be invoked by language models. The Java SDK allows implementing a Tool Specifications with their handler functions. Tools enable AI models to perform calculations, access external APIs, query databases, and manipulate files:
The Tool specification includes a Tool definition with name, description, and parameter schema followed by a call handler that implements the tool's logic.
The function's first argument is McpAsyncServerExchange for client interaction, and the second is a map of tool arguments.
Resource Specification
Specification of a resource with its handler function. Resources provide context to AI models by exposing data such as: File contents, Database records, API responses, System information, Application state. Example resource specification:
The resource specification comprised of resource definitions and resource read handler.
The resource definition including name, description, and MIME type.
The first argument of the function that handles resource read requests is an McpAsyncServerExchange upon which the server can
interact with the connected client.
The second arguments is a McpSchema.ReadResourceRequest.
Prompt Specification
As part of the Prompting capabilities, MCP provides a standardized way for servers to expose prompt templates to clients. The Prompt Specification is a structured template for AI model interactions that enables consistent message formatting, parameter substitution, context injection, response formatting, and instruction templating.
The prompt definition includes name (identifier for the prompt), description (purpose of the prompt), and list of arguments (parameters for templating).
The handler function processes requests and returns formatted templates.
The first argument is McpAsyncServerExchange for client interaction, and the second argument is a GetPromptRequest instance.
Using Sampling from a Server
To use Sampling capabilities, connect to a client that supports sampling. No special server configuration is needed, but verify client sampling support before making requests. Learn about client sampling support.
Once connected to a compatible client, the server can request language model generations:
// Define a tool that uses sampling var calculatorTool = new McpServerFeatures.SyncToolSpecification( new Tool("ai-calculator", "Performs calculations using AI", schema), (exchange, arguments) -> { // Check if client supports sampling if (exchange.getClientCapabilities().sampling() == null) { return new CallToolResult("Client does not support AI capabilities", false); }
// Create a sampling request
McpSchema.CreateMessageRequest request = McpSchema.CreateMessageRequest.builder()
.messages(List.of(new McpSchema.SamplingMessage(McpSchema.Role.USER,
new McpSchema.TextContent("Calculate: " + arguments.get("expression")))
.modelPreferences(McpSchema.ModelPreferences.builder()
.hints(List.of(
McpSchema.ModelHint.of("claude-3-sonnet"),
McpSchema.ModelHint.of("claude")
))
.intelligencePriority(0.8) // Prioritize intelligence
.speedPriority(0.5) // Moderate speed importance
.build())
.systemPrompt("You are a helpful calculator assistant. Provide only the numerical answer.")
.maxTokens(100)
.build();
// Request sampling from the client
McpSchema.CreateMessageResult result = exchange.createMessage(request);
// Process the result
String answer = result.content().text();
return new CallToolResult(answer, false);
}
);
// Add the tool to the server server.addTool(calculatorTool);
</Tab>
<Tab title="Async API">
```java
// Create a server
McpAsyncServer server = McpServer.async(transportProvider)
.serverInfo("my-server", "1.0.0")
.build();
// Define a tool that uses sampling
var calculatorTool = new McpServerFeatures.AsyncToolSpecification(
new Tool("ai-calculator", "Performs calculations using AI", schema),
(exchange, arguments) -> {
// Check if client supports sampling
if (exchange.getClientCapabilities().sampling() == null) {
return Mono.just(new CallToolResult("Client does not support AI capabilities", false));
}
// Create a sampling request
McpSchema.CreateMessageRequest request = McpSchema.CreateMessageRequest.builder()
.content(new McpSchema.TextContent("Calculate: " + arguments.get("expression")))
.modelPreferences(McpSchema.ModelPreferences.builder()
.hints(List.of(
McpSchema.ModelHint.of("claude-3-sonnet"),
McpSchema.ModelHint.of("claude")
))
.intelligencePriority(0.8) // Prioritize intelligence
.speedPriority(0.5) // Moderate speed importance
.build())
.systemPrompt("You are a helpful calculator assistant. Provide only the numerical answer.")
.maxTokens(100)
.build();
// Request sampling from the client
return exchange.createMessage(request)
.map(result -> {
// Process the result
String answer = result.content().text();
return new CallToolResult(answer, false);
});
}
);
// Add the tool to the server
server.addTool(calculatorTool)
.subscribe();
The CreateMessageRequest object allows you to specify: Content - the input text or image for the model,
Model Preferences - hints and priorities for model selection, System Prompt - instructions for the model's behavior and
Max Tokens - maximum length of the generated response.
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.