mirror of
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Burak Kaygusuz
957edc6f32
- Replace SDK 0.14.1 patterns with SDK 2.0.1 builders and Map schemas. - Add the BOM, Gradle, and packaging rules. - Document schema validation, isError results, and structuredContent. - Add transport security, CORS, and Spring AI 2.0 guidance. - Replace sampling, protocol logging, and roots with alternatives. - Rewrite the Common Patterns with code that compiles on SDK 2.0.1. - Update the description in docs/README.instructions.md.
313 lines
16 KiB
Markdown
313 lines
16 KiB
Markdown
---
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description: 'Instructions for building Model Context Protocol (MCP) servers in Java with the MCP Java SDK 2.x'
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applyTo: '**/*.java, **/pom.xml, **/build.gradle, **/build.gradle.kts'
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---
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# Java MCP Server Development
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These instructions apply to the [MCP Java SDK](https://github.com/modelcontextprotocol/java-sdk) 2.x. SDK 2.0.x supports the MCP specification revision [2025-11-25](https://modelcontextprotocol.io/specification/2025-11-25) and the earlier revisions. SDK 3.x will add the revision [2026-07-28](https://modelcontextprotocol.io/specification/2026-07-28).
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## Instructions
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- Import the BOM `io.modelcontextprotocol.sdk:mcp-bom:2.0.1` and add the `io.modelcontextprotocol.sdk:mcp` artifact.
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- In Gradle, import the BOM with `implementation(platform("io.modelcontextprotocol.sdk:mcp-bom:2.0.1"))`. Also add `testRuntimeOnly("org.junit.platform:junit-platform-launcher")`. Without it, the `test` task fails.
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- The `mcp` artifact contains the SDK core and the Jackson 3 JSON mapper. For Jackson 2, use `mcp-core` and `mcp-json-jackson2`.
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- Do not use code from SDK 0.x or 1.x. Read the [migration guide](https://github.com/modelcontextprotocol/java-sdk/blob/main/MIGRATION-2.0.md) when you update a server.
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- The packages are `io.modelcontextprotocol.server`, `io.modelcontextprotocol.spec`, and `io.modelcontextprotocol.json`. The protocol types are nested in `McpSchema`.
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- Make a server with `McpServer.sync(transport)` or `McpServer.async(transport)`. Set `serverInfo(name, version)` and `capabilities(...)`, and then call `build()`.
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- Use the sync server for most servers. Use the async server with Project Reactor (`Mono`) only when the tool code does not block a thread.
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- For stdio transport, use `new StdioServerTransportProvider(McpJsonDefaults.getMapper())`.
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- Enable only the capabilities that the server uses: `ServerCapabilities.builder().tools(false).resources(false, true).prompts(true).completions().build()`. `tools(listChanged)` and `prompts(listChanged)` enable list change notifications. `resources(subscribe, listChanged)` also enables resource subscriptions.
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- Register tools with `SyncToolSpecification.builder().tool(tool).callHandler((exchange, request) -> ...)`. The handler gets the arguments as a `Map` from `request.arguments()`.
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- Use the builders that take the required fields first: `Tool.builder(name, inputSchema)`, `Resource.builder(uri, name)`, `ResourceTemplate.builder(uriTemplate, name)`, `Prompt.builder(name)`, `TextContent.builder(text)`, and `ReadResourceResult.builder(contents)`. The builders without arguments are deprecated.
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- A required field must not be `null`. A `null` value causes an `IllegalArgumentException`.
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- Give `inputSchema` and `outputSchema` as `Map<String, Object>` values. The `JsonSchema` record is deprecated.
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- The server checks each schema against JSON Schema 2020-12 when you call `build()` or `addTool()`. An incorrect schema causes an `IllegalArgumentException`.
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- The SDK validates the arguments against `inputSchema` before it calls the handler. It also validates `structuredContent` against `outputSchema`. If the validation fails, the client gets a tool result with `isError` set to `true`.
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- If a tool has an `outputSchema`, return `structuredContent` in each successful result. Without it, the SDK returns a tool result with `isError` set to `true`.
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- For machine-readable output, set `outputSchema` on the tool and return `CallToolResult.builder().structuredContent(map)`. Also add the same data as text content for older clients.
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- Set tool hints with `ToolAnnotations.builder().readOnlyHint(true).build()`. Use `destructiveHint(true)` for a destructive tool.
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- Return `CallToolResult.builder().addTextContent(message).isError(true).build()` for an error that the model must read.
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- Do not throw an exception or an `McpError` for a tool failure. The client gets a JSON-RPC error, not a tool result. Many hosts do not show this error to the model.
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- Use `McpError.builder(McpSchema.ErrorCodes.INVALID_PARAMS).message(...).build()` only for a protocol error, for example an unknown resource URI.
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- Register resources with `SyncResourceSpecification`, resource templates with `SyncResourceTemplateSpecification`, prompts with `SyncPromptSpecification`, and completions with `SyncCompletionSpecification`.
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- Define resource templates with RFC 6570 URI templates, for example `users://{id}`.
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- Report progress with `exchange.progressNotification(...)` only when `request.progressToken()` is not `null`.
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- Get user input with `exchange.createElicitation(ElicitFormRequest.builder(message, requestedSchema).build())`. First, make sure that `exchange.getClientCapabilities().elicitation()` is not `null`.
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- Do not use sampling (`exchange.createMessage()`). The revision 2026-07-28 deprecates it. Call the LLM provider API directly.
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- Do not use protocol logging (`exchange.loggingNotification()`). The revision 2026-07-28 deprecates it. Log with SLF4J.
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- Do not use roots (`exchange.listRoots()`). The revision 2026-07-28 deprecates it. Get paths from tool arguments, resource URIs, or the server configuration.
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- Add or remove tools at runtime with `server.addTool(...)` and `server.removeTool(name)`. Then call `server.notifyToolsListChanged()`.
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- In a stdio server, stdout is the protocol channel. Do not call `System.out.println()`. Use `slf4j-simple` (it writes to stderr), or set the target of the Logback console appender to `System.err`.
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- For a remote server, use Streamable HTTP. The SSE transports are deprecated.
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- For a servlet container, use `HttpServletStreamableServerTransportProvider`. For a server without sessions, use `HttpServletStatelessServerTransport`.
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- The HTTP transports do not check the `Host` and `Origin` headers by default. Always set `securityValidator(DefaultServerTransportSecurityValidator.builder()...build())`. This also applies to the Spring AI starters.
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- For Spring Boot, use the MCP starters of [Spring AI](https://docs.spring.io/spring-ai/reference/api/mcp/mcp-overview.html) 2.0 or later, for example `spring-ai-starter-mcp-server-webmvc`. Do not use the `io.modelcontextprotocol.sdk:mcp-spring-*` artifacts.
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- In Spring AI, write tools as bean methods with `@McpTool` and `@McpToolParam` (from `org.springframework.ai.mcp.annotation`).
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- For browser clients, configure CORS. Allow the `Mcp-*` request headers. Expose the `Mcp-Session-Id` response header.
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- Package a stdio server as one executable JAR with the `maven-shade-plugin`. In Gradle, use the `application` plugin and the start script from `installDist`. Do not use `./gradlew run` as the client command.
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- Test a server with `McpClient.sync(new StdioClientTransport(params, McpJsonDefaults.getMapper()))`. The SDK has no in-memory transport.
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- Test servers manually with the MCP Inspector.
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## Best Practices
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- Give each tool one responsibility.
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- Write clear tool descriptions. The description tells the model when to use the tool.
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- Add a `description` to each property in `inputSchema`.
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- Put each tool specification in its own method. This keeps the tools easy to test.
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- In the async server, do not block the event loop. Run code that blocks with `Mono.fromCallable(...).subscribeOn(Schedulers.boundedElastic())`.
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- Use environment variables for configuration.
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- Be careful when a tool gives access to the file system or the network.
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## Common Patterns
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The snippets do not show all imports. The protocol types are nested in `io.modelcontextprotocol.spec.McpSchema`. `Mono` is in `reactor.core.publisher`, and `Schedulers` is in `reactor.core.scheduler`.
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### Basic Server Setup (stdio)
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```java
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import java.util.List;
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import java.util.Map;
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import io.modelcontextprotocol.json.McpJsonDefaults;
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import io.modelcontextprotocol.server.McpServer;
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import io.modelcontextprotocol.server.McpServerFeatures.SyncToolSpecification;
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import io.modelcontextprotocol.server.transport.StdioServerTransportProvider;
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import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
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import io.modelcontextprotocol.spec.McpSchema.ServerCapabilities;
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import io.modelcontextprotocol.spec.McpSchema.Tool;
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public final class McpServerApp {
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static SyncToolSpecification addTool() {
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Tool tool = Tool.builder("add", Map.of(
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"type", "object",
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"properties", Map.of(
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"a", Map.of("type", "integer", "description", "First number"),
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"b", Map.of("type", "integer", "description", "Second number")),
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"required", List.of("a", "b")))
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.description("Add two integers.")
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.build();
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return SyncToolSpecification.builder()
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.tool(tool)
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.callHandler((exchange, request) -> {
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long a = ((Number) request.arguments().get("a")).longValue();
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long b = ((Number) request.arguments().get("b")).longValue();
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return CallToolResult.builder().addTextContent(String.valueOf(a + b)).build();
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})
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.build();
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}
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public static void main(String[] args) {
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McpServer.sync(new StdioServerTransportProvider(McpJsonDefaults.getMapper()))
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.serverInfo("my-server", "0.1.0")
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.capabilities(ServerCapabilities.builder().tools(false).build())
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.tools(addTool())
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.build();
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}
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}
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```
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### Tool with Structured Output
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```java
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static SyncToolSpecification wordCountTool() {
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Tool tool = Tool.builder("word_count", Map.of(
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"type", "object",
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"properties", Map.of("text", Map.of("type", "string")),
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"required", List.of("text")))
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.description("Count the words in a text.")
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.outputSchema(Map.of(
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"type", "object",
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"properties", Map.of("words", Map.of("type", "integer")),
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"required", List.of("words")))
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.annotations(ToolAnnotations.builder().readOnlyHint(true).build())
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.build();
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return SyncToolSpecification.builder()
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.tool(tool)
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.callHandler((exchange, request) -> {
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String text = (String) request.arguments().get("text");
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int words = text.isBlank() ? 0 : text.trim().split("\\s+").length;
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return CallToolResult.builder()
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.addTextContent("{\"words\":" + words + "}")
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.structuredContent(Map.of("words", words))
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.build();
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})
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.build();
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}
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```
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### Error Handling
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```java
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static SyncToolSpecification divideTool() {
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Tool tool = Tool.builder("divide", Map.of(
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"type", "object",
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"properties", Map.of("a", Map.of("type", "number"), "b", Map.of("type", "number")),
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"required", List.of("a", "b")))
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.description("Divide a by b.")
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.build();
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return SyncToolSpecification.builder()
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.tool(tool)
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.callHandler((exchange, request) -> {
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double a = ((Number) request.arguments().get("a")).doubleValue();
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double b = ((Number) request.arguments().get("b")).doubleValue();
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if (b == 0) {
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return CallToolResult.builder().addTextContent("b must not be zero.").isError(true).build();
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}
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return CallToolResult.builder().addTextContent(String.valueOf(a / b)).build();
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})
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.build();
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}
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```
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### Resources and Resource Templates
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```java
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static SyncResourceSpecification configResource() {
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return new SyncResourceSpecification(
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Resource.builder("config://app", "app-config").mimeType("application/json").build(),
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(exchange, request) -> ReadResourceResult.builder(List.of(
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TextResourceContents.builder(request.uri(), "{\"debug\":false}")
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.mimeType("application/json")
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.build()))
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.build());
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}
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static SyncResourceTemplateSpecification userResource() {
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return new SyncResourceTemplateSpecification(
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ResourceTemplate.builder("users://{id}", "user").build(),
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(exchange, request) -> {
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String id = request.uri().substring("users://".length());
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return ReadResourceResult.builder(List.of(
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TextResourceContents.builder(request.uri(), "User " + id).build()))
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.build();
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});
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}
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```
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Register them with `.resources(configResource())` and `.resourceTemplates(userResource())`.
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### Prompt with Completion
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```java
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static SyncPromptSpecification reviewPrompt() {
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Prompt prompt = Prompt.builder("code-review")
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.description("Review code in a language.")
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.arguments(List.of(PromptArgument.builder("language").required(true).build()))
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.build();
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return new SyncPromptSpecification(prompt, (exchange, request) -> GetPromptResult.builder(List.of(
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PromptMessage.builder(Role.USER,
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TextContent.builder("Review this " + request.arguments().get("language") + " code.").build())
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.build()))
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.build());
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}
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static SyncCompletionSpecification languageCompletion() {
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List<String> languages = List.of("java", "kotlin", "scala");
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return new SyncCompletionSpecification(new PromptReference("code-review"),
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(exchange, request) -> new CompleteResult(new CompleteResult.CompleteCompletion(
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languages.stream().filter(l -> l.startsWith(request.argument().value())).toList(), null, false)));
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}
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```
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Register them with `.prompts(reviewPrompt())` and `.completions(languageCompletion())`.
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### Async Tool with Progress
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```java
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static AsyncToolSpecification lookupTool() {
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Tool tool = Tool.builder("lookup", Map.of(
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"type", "object",
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"properties", Map.of("query", Map.of("type", "string")),
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"required", List.of("query")))
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.description("Look up a query in a slow backend.")
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.build();
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return AsyncToolSpecification.builder()
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.tool(tool)
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.callHandler((exchange, request) -> {
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Object token = request.progressToken();
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Mono<Void> progress = token == null ? Mono.empty()
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: exchange.progressNotification(
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ProgressNotification.builder(token, 0.5).total(1.0).message("Halfway").build());
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return progress.then(Mono.fromCallable(() -> slowLookup((String) request.arguments().get("query")))
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.subscribeOn(Schedulers.boundedElastic())
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.map(text -> CallToolResult.builder().addTextContent(text).build()));
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})
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.build();
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}
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```
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Register it on `McpServer.async(transport)`.
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### Streamable HTTP Server (Servlet)
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```java
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HttpServletStreamableServerTransportProvider transport = HttpServletStreamableServerTransportProvider.builder()
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.jsonMapper(McpJsonDefaults.getMapper())
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.mcpEndpoint("/mcp")
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.securityValidator(DefaultServerTransportSecurityValidator.builder()
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.allowedHost("localhost:8080")
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.allowedOrigin("http://localhost:*")
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.build())
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.build();
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McpServer.sync(transport)
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.serverInfo("my-server", "0.1.0")
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.capabilities(ServerCapabilities.builder().tools(false).build())
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.tools(addTool())
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.build();
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```
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The transport is a Jakarta Servlet. Register it in a servlet container at `/mcp` with async support.
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### Spring AI Tool
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```java
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@Component
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class CalculatorTools {
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@McpTool(name = "add", description = "Add two integers")
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public int add(@McpToolParam(description = "First number") int a,
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@McpToolParam(description = "Second number") int b) {
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return a + b;
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}
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}
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```
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### Integration Test
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```java
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class McpServerAppTest {
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private static McpSyncClient client;
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@BeforeAll
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static void startServer() {
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String java = Path.of(System.getProperty("java.home"), "bin", "java").toString();
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ServerParameters server = ServerParameters.builder(java)
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.args("-cp", System.getProperty("java.class.path"), McpServerApp.class.getName())
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.build();
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client = McpClient.sync(new StdioClientTransport(server, McpJsonDefaults.getMapper())).build();
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client.initialize();
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}
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@AfterAll
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static void stopServer() {
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client.close();
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}
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@Test
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void add() {
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CallToolResult result = client.callTool(
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CallToolRequest.builder("add").arguments(Map.of("a", 2, "b", 3)).build());
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assertEquals("5", ((TextContent) result.content().get(0)).text());
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}
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}
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```
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