feat(examples): add Java WebFlux MCP server example

examples/basic-server-java-webflux/.gitignore

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+target/
+*.iml
+.idea/
+.vscode/
+*.class
+*.jar
+.DS_Store

examples/basic-server-java-webflux/.mvn/wrapper/maven-wrapper.properties

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+distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.9.9/apache-maven-3.9.9-bin.zip
+wrapperUrl=https://repo.maven.apache.org/maven2/org/apache/maven/wrapper/maven-wrapper/3.3.2/maven-wrapper-3.3.2.jar

examples/basic-server-java-webflux/README.md

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+# Example: Basic Server (Java WebFlux)
+
+A minimal Java MCP server that performs math operations with an interactive calculator UI. Demonstrates the MCP Java SDK with Spring Boot WebFlux and Streamable HTTP transport.
+
+## MCP Client Configuration
+
+This server uses HTTP transport only (no stdio). Add to your MCP client configuration:
+
+```json
+{
+  "mcpServers": {
+    "calculator": {
+      "url": "http://localhost:3001/mcp"
+    }
+  }
+}
+```
+
+### Local Development
+
+To test local modifications, first clone the repository and start the server:
+
+```bash
+git clone https://github.com/modelcontextprotocol/ext-apps.git
+cd ext-apps/examples/basic-server-java-webflux
+./mvnw spring-boot:run
+```
+
+Then configure your MCP client to connect to `http://localhost:3001/mcp`.
+
+## Features
+
+- Four basic operations: add, subtract, multiply, divide
+- Interactive calculator view using ext-apps SDK
+- Division by zero error handling
+- Structured content metadata for UI integration
+- Dark mode support
+- Three MCP prompts for guided interactions
+
+## Prerequisites
+
+- Java 17+ (`java -version`)
+- No global Maven needed (Maven wrapper included)
+
+## Running
+
+1. Start the server:
+
+   ```bash
+   ./mvnw spring-boot:run
+   # → Calculator listening on http://localhost:3001/mcp
+   ```
+
+2. View using the [`basic-host`](https://github.com/modelcontextprotocol/ext-apps/tree/main/examples/basic-host) example or another MCP Apps-compatible host.
+
+### Custom Port
+
+```bash
+PORT=3120 ./mvnw spring-boot:run
+```
+
+### Docker (accessing host server from container)
+
+```
+http://host.docker.internal:3001/mcp
+```
+
+## Tool: `calculate`
+
+Perform a math operation on two numbers. Supports add, subtract, multiply, and divide.
+
+### Parameters
+
+| Parameter   | Type   | Default    | Description                                             |
+| ----------- | ------ | ---------- | ------------------------------------------------------- |
+| `operation` | string | (required) | Math operation: `add`, `subtract`, `multiply`, `divide` |
+| `a`         | number | (required) | First operand                                           |
+| `b`         | number | (required) | Second operand                                          |
+
+### Example Inputs
+
+**Basic addition:**
+
+```json
+{ "operation": "add", "a": 5, "b": 3 }
+```
+
+**Decimal multiplication:**
+
+```json
+{ "operation": "multiply", "a": 3.14, "b": 2 }
+```
+
+**Division by zero (returns error):**
+
+```json
+{ "operation": "divide", "a": 10, "b": 0 }
+```
+
+## Prompts
+
+| Prompt            | Description                                   | Arguments    |
+| ----------------- | --------------------------------------------- | ------------ |
+| `quick-calc`      | Evaluate a math expression step by step       | `expression` |
+| `compare-numbers` | Compare two numbers using all four operations | `a`, `b`     |
+| `percentage`      | Calculate percentages                         | `question`   |
+
+## Architecture
+
+### Server
+
+Four Spring components wired via `McpConfig`:
+
+- **`McpConfig`** - Configures `WebFluxStreamableServerTransportProvider`, `RouterFunction`, and `McpSyncServer` with tools, resources, and prompts
+- **`CalculatorTools`** - Tool definition with UI metadata and calculation handler
+- **`CalculatorResources`** - Serves the interactive HTML view as an MCP resource with CSP metadata
+- **`CalculatorPrompts`** - Three guided prompts for common calculation patterns
+
+### App (`view.html`)
+
+Vanilla JavaScript UI that:
+
+- Loads the MCP Apps SDK from CDN (`@modelcontextprotocol/ext-apps`)
+- Provides number inputs and operation buttons
+- Calls `app.callServerTool()` on button click
+- Displays results from `ontoolresult` callback and `structuredContent` metadata
+- Adapts to dark mode via `prefers-color-scheme`
+
+### Key Files
+
+- [`src/main/java/.../McpConfig.java`](src/main/java/com/example/calculator/McpConfig.java) - Spring configuration with WebFlux transport
+- [`src/main/java/.../CalculatorTools.java`](src/main/java/com/example/calculator/CalculatorTools.java) - Tool definition and handler
+- [`src/main/java/.../CalculatorResources.java`](src/main/java/com/example/calculator/CalculatorResources.java) - HTML resource with CSP
+- [`src/main/java/.../CalculatorPrompts.java`](src/main/java/com/example/calculator/CalculatorPrompts.java) - MCP prompts
+- [`src/main/resources/view.html`](src/main/resources/view.html) - Interactive calculator UI
+
+## Dependencies
+
+- **Spring Boot 3.5** - WebFlux reactive web framework
+- **MCP Java SDK 0.17.2** (`io.modelcontextprotocol.sdk:mcp-spring-webflux`) - Streamable HTTP transport
+- **ext-apps SDK** (CDN) - Client-side MCP Apps protocol in `view.html`
+
+## Notes
+
+- This example uses HTTP transport only (Streamable HTTP via WebFlux). Stdio transport is not supported.
+- The server uses Netty (via WebFlux) instead of Tomcat for non-blocking request handling.
+- CSP metadata is declared on content items in `resources/read` to allow loading the ext-apps SDK from `unpkg.com`.
+
+## License
+
+MIT