项目：Spring Boot REST API

Spring Boot 是 Java 后端的事实标准。但它需要 Maven/Gradle 与多文件——Playground 跑不了。

本章的做法：

先按 真实项目结构 拆文件展示（pom.xml、Application、Controller、Service、Model）
再用 JDK 自带的 com.sun.net.httpserver.HttpServer 写一个单文件可运行的极简 HTTP 服务，体验「注解 + 路由 + 依赖注入」的核心思想

本章 Playground 不会发起真实网络请求（沙箱禁外网），全部用 HttpClient 访问本机端口完成端到端演示。

1. 真实 Spring Boot 项目结构

user-service/
├── pom.xml
└── src/
    ├── main/
    │   ├── java/com/example/userservice/
    │   │   ├── UserServiceApplication.java     // 启动类
    │   │   ├── controller/UserController.java   // REST 控制器
    │   │   ├── service/UserService.java         // 业务逻辑
    │   │   ├── model/User.java                  // 领域模型
    │   │   └── dto/CreateUserRequest.java       // 入参 DTO
    │   └── resources/
    │       └── application.yml
    └── test/
        └── java/com/example/userservice/
            └── UserControllerTest.java

`pom.xml`

<project xmlns="http://maven.apache.org/POM/4.0.0">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.3.0</version>
    </parent>
 
    <groupId>com.example</groupId>
    <artifactId>user-service</artifactId>
    <version>1.0.0</version>
 
    <properties>
        <java.version>21</java.version>
    </properties>
 
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-validation</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>
</project>

启动类

package com.example.userservice;
 
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
 
@SpringBootApplication
public class UserServiceApplication {
    public static void main(String[] args) {
        SpringApplication.run(UserServiceApplication.class, args);
    }
}

Controller

package com.example.userservice.controller;
 
import com.example.userservice.model.User;
import com.example.userservice.service.UserService;
import org.springframework.web.bind.annotation.*;
 
import java.util.List;
 
@RestController
@RequestMapping("/users")
public class UserController {
    private final UserService service;
 
    public UserController(UserService service) {   // 构造器注入
        this.service = service;
    }
 
    @GetMapping
    public List<User> list() {
        return service.findAll();
    }
 
    @GetMapping("/{id}")
    public User get(@PathVariable Long id) {
        return service.findById(id);
    }
 
    @PostMapping
    public User create(@RequestBody CreateUserRequest req) {
        return service.create(req.name(), req.email());
    }
}

Service

package com.example.userservice.service;
 
import com.example.userservice.model.User;
import org.springframework.stereotype.Service;
 
import java.util.*;
import java.util.concurrent.atomic.AtomicLong;
 
@Service
public class UserService {
    private final Map<Long, User> store = new HashMap<>();
    private final AtomicLong seq = new AtomicLong(0);
 
    public List<User> findAll() {
        return store.values().stream().toList();
    }
 
    public User findById(long id) {
        User u = store.get(id);
        if (u == null) throw new NoSuchElementException("user " + id);
        return u;
    }
 
    public User create(String name, String email) {
        long id = seq.incrementAndGet();
        User u = new User(id, name, email);
        store.put(id, u);
        return u;
    }
}

Model + DTO

package com.example.userservice.model;
 
public record User(long id, String name, String email) {}
 
package com.example.userservice.dto;
 
import jakarta.validation.constraints.Email;
import jakarta.validation.constraints.NotBlank;
 
public record CreateUserRequest(
    @NotBlank String name,
    @Email     String email
) {}

启动后访问

$ mvn spring-boot:run
 
$ curl http://localhost:8080/users
[]
$ curl -X POST -H "Content-Type: application/json" \
       -d '{"name":"Alice","email":"a@x.com"}' \
       http://localhost:8080/users
{"id":1,"name":"Alice","email":"a@x.com"}
$ curl http://localhost:8080/users/1
{"id":1,"name":"Alice","email":"a@x.com"}

ℹ️Spring Boot 关键注解

注解	作用
`@SpringBootApplication`	一键启用：自动配置 + 组件扫描 + 配置类
`@RestController`	`@Controller` + `@ResponseBody`：方法返回值直接写响应体（JSON）
`@RequestMapping("/users")`	类级别 URL 前缀
`@GetMapping / @PostMapping`	等价于 `@RequestMapping(method = GET/POST)`
`@PathVariable`	取 URL 里的占位符 `/users/{id}`
`@RequestBody`	把请求体 JSON 反序列化为对象
`@Service / @Repository / @Component`	把类注册成 Spring Bean

2. 单文件 Playground：迷你 Spring

为了在 Playground 里也能跑起来，我们用 JDK 自带的 HttpServer 写一个只有 80 行的「迷你 Spring」——保留 @RestController / @GetMapping / @PostMapping / @RequestBody / 依赖注入的概念：

迷你 Spring：注解 + DI + 路由

import com.sun.net.httpserver.*;
import java.io.*;
import java.lang.annotation.*;
import java.lang.reflect.*;
import java.net.*;
import java.nio.charset.StandardCharsets;
import java.util.*;
import java.util.concurrent.*;
 
public class Main {
    // ====== 注解（同名于 Spring） ======
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE)
    public @interface RestController {}
    @Retention(RetentionPolicy.RUNTIME) @Target({ElementType.TYPE, ElementType.METHOD})
    public @interface RequestMapping { String value() default ""; }
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
    public @interface GetMapping { String value() default ""; }
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
    public @interface PostMapping { String value() default ""; }
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.PARAMETER)
    public @interface RequestBody {}
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.PARAMETER)
    public @interface PathVariable {}
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
    public @interface ResponseBody {}
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE)
    public @interface Component {}
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD)
    public @interface Autowired {}
 
    // ====== 注解式 Controller（像 Spring 一样写） ======
    @RestController
    @RequestMapping("/users")
    static class UserController {
        @Autowired UserService service;   // 字段注入
 
        @GetMapping
        @ResponseBody
        public List<String> list() { return service.findAll(); }
 
        @GetMapping("/{id}")
        @ResponseBody
        public String get(@PathVariable("id") String id) {
            return service.findById(id);
        }
 
        @PostMapping
        @ResponseBody
        public String create(@RequestBody String name) {
            return service.create(name);
        }
    }
 
    @Component
    static class UserService {
        private final Map<String, String> store = new ConcurrentHashMap<>();
        public List<String> findAll() { return new ArrayList<>(store.values()); }
        public String findById(String id) {
            String u = store.get(id);
            if (u == null) throw new NoSuchElementException("user " + id);
            return u;
        }
        public String create(String name) {
            String id = String.valueOf(store.size() + 1);
            store.put(id, name);
            return "user#" + id + "=" + name;
        }
    }
 
    // ====== IoC 容器（依赖注入） ======
    static class AppContext {
        final Map<Class<?>, Object> beans = new HashMap<>();
        AppContext(Class<?>... components) throws Exception {
            // 1. 实例化所有 @Component / @RestController
            for (Class<?> c : components) {
                beans.put(c, c.getDeclaredConstructor().newInstance());
            }
            // 2. 注入 @Autowired 字段
            for (Object bean : beans.values()) {
                for (Field f : bean.getClass().getDeclaredFields()) {
                    if (f.isAnnotationPresent(Autowired.class)) {
                        f.setAccessible(true);
                        f.set(bean, beans.get(f.getType()));
                    }
                }
            }
        }
    }
 
    // ====== 路由注册（注解扫描） ======
    static class Router {
        record Handler(Object bean, Method m, String path, String httpMethod) {}
 
        void register(HttpServer server, AppContext ctx) {
            for (Object bean : ctx.beans.values()) {
                Class<?> c = bean.getClass();
                String classPath = "";
                if (c.isAnnotationPresent(RequestMapping.class)) {
                    classPath = c.getAnnotation(RequestMapping.class).value();
                }
                for (Method m : c.getDeclaredMethods()) {
                    String method = null, path = null;
                    if (m.isAnnotationPresent(GetMapping.class)) {
                        method = "GET";
                        path = m.getAnnotation(GetMapping.class).value();
                    } else if (m.isAnnotationPresent(PostMapping.class)) {
                        method = "POST";
                        path = m.getAnnotation(PostMapping.class).value();
                    }
                    if (method == null) continue;
                    final String fullPath = classPath + path;
                    Handler h = new Handler(bean, m, fullPath, method);
                    server.createContext(fullPath, exchange -> handle(exchange, h));
                }
            }
        }
 
        void handle(HttpServer.Exchange exchange, Handler h) throws IOException {
            try {
                String body = new String(exchange.getRequestBody().readAllBytes(), StandardCharsets.UTF_8);
                String path = exchange.getRequestURI().getPath();
                String[] segments = path.replace(h.path, "").split("/", -1);
                Object[] args = new Object[m(h).getParameterCount()];
                Parameter[] ps = m(h).getParameters();
                for (int i = 0; i < ps.length; i++) {
                    if (ps[i].isAnnotationPresent(RequestBody.class)) args[i] = body;
                    else if (ps[i].isAnnotationPresent(PathVariable.class)) {
                        String name = ps[i].getAnnotation(PathVariable.class).value();
                        args[i] = matchPathVar(h.path, path, name);
                    } else args[i] = null;
                }
                Object result = m(h).invoke(h.bean, args);
                byte[] out = (result == null ? "" : result.toString()).getBytes(StandardCharsets.UTF_8);
                exchange.sendResponseHeaders(200, out.length);
                exchange.getResponseBody().write(out);
            } catch (InvocationTargetException e) {
                String msg = e.getCause().getMessage();
                byte[] out = ("ERR: " + msg).getBytes(StandardCharsets.UTF_8);
                exchange.sendResponseHeaders(500, out.length);
                exchange.getResponseBody().write(out);
            } catch (Exception e) {
                String msg = e.getMessage();
                byte[] out = ("ERR: " + msg).getBytes(StandardCharsets.UTF_8);
                exchange.sendResponseHeaders(500, out.length);
                exchange.getResponseBody().write(out);
            } finally {
                exchange.close();
            }
        }
 
        Method m(Handler h) { return h.m; }
        String matchPathVar(String pattern, String actual, String name) {
            // 简单实现：/users/{id} -> 取 actual 里除了 pattern 以外的第一段
            String tail = actual.substring(pattern.length() + 1);
            return tail.split("/")[0];
        }
    }
 
    public static void main(String[] args) throws Exception {
        // 1. 启动容器
        AppContext ctx = new AppContext(UserController.class, UserService.class);
        HttpServer server = HttpServer.create(new InetSocketAddress("127.0.0.1", 0), 0);
        new Router().register(server, ctx);
        server.start();
        int port = server.getAddress().getPort();
        System.out.println("server up on http://127.0.0.1:" + port);
 
        // 2. 用 Java 自带 HttpClient 模拟 curl
        HttpClient client = HttpClient.newHttpClient();
        // POST /users
        HttpResponse<String> r1 = client.send(
            HttpRequest.newBuilder(URI.create("http://127.0.0.1:" + port + "/users"))
                       .header("Content-Type", "text/plain")
                       .POST(HttpRequest.BodyPublishers.ofString("Alice"))
                       .build(),
            HttpResponse.BodyHandlers.ofString());
        System.out.println("POST /users        -> " + r1.statusCode() + " " + r1.body());
 
        HttpResponse<String> r2 = client.send(
            HttpRequest.newBuilder(URI.create("http://127.0.0.1:" + port + "/users"))
                       .header("Content-Type", "text/plain")
                       .POST(HttpRequest.BodyPublishers.ofString("Bob"))
                       .build(),
            HttpResponse.BodyHandlers.ofString());
        System.out.println("POST /users        -> " + r2.statusCode() + " " + r2.body());
 
        // GET /users
        HttpResponse<String> r3 = client.send(
            HttpRequest.newBuilder(URI.create("http://127.0.0.1:" + port + "/users"))
                       .GET().build(),
            HttpResponse.BodyHandlers.ofString());
        System.out.println("GET  /users        -> " + r3.statusCode() + " " + r3.body());
 
        // GET /users/1
        HttpResponse<String> r4 = client.send(
            HttpRequest.newBuilder(URI.create("http://127.0.0.1:" + port + "/users/1"))
                       .GET().build(),
            HttpResponse.BodyHandlers.ofString());
        System.out.println("GET  /users/1      -> " + r4.statusCode() + " " + r4.body());
 
        // GET /users/999（业务异常）
        HttpResponse<String> r5 = client.send(
            HttpRequest.newBuilder(URI.create("http://127.0.0.1:" + port + "/users/999"))
                       .GET().build(),
            HttpResponse.BodyHandlers.ofString());
        System.out.println("GET  /users/999    -> " + r5.statusCode() + " " + r5.body());
 
        // 3. 关闭服务
        server.stop(0);
        System.out.println("server stopped");
    }
}

💡对照 Spring 看这个迷你实现

Spring	这里
`@SpringBootApplication` 启动容器	`new AppContext(...)`
组件扫描 `@Component`	手动 `AppContext` 列出类
字段注入 `@Autowired`	`AppContext` 反射写入字段
路由映射 `@GetMapping` 等	`Router.register` 扫注解
JSON 序列化	`toString()`（真实用 Jackson/Gson）
Tomcat 内嵌	`com.sun.net.httpserver.HttpServer`

3. 真实项目里的常见依赖注入方式

Spring 推荐构造器注入——更利于测试：

@Service
public class OrderService {
    private final UserService userService;
    private final PaymentService paymentService;
 
    public OrderService(UserService u, PaymentService p) {   // 构造器注入
        this.userService = u;
        this.paymentService = p;
    }
}

字段注入（上一节的 @Autowired 字段）虽然写法短，但不利于单元测试——无法在没有 Spring 容器时手动传 mock。

4. 测试

@WebMvcTest(UserController.class)
class UserControllerTest {
    @Autowired MockMvc mvc;
    @MockBean  UserService service;
 
    @Test
    void get_returnsUser() throws Exception {
        when(service.findById(1L)).thenReturn(new User(1L, "Alice", "a@x.com"));
        mvc.perform(get("/users/1"))
           .andExpect(status().isOk())
           .andExpect(jsonPath("$.name").value("Alice"));
    }
}

🎯 练习

补全路由

// 任务：在迷你 Spring 框架下，新增一个 TodoController
// 包含 GET /todos（返回所有 todo）和 POST /todos（创建并返回 id）
// 提示：参考 UserController 的写法，注意 RequestMapping 与 @Autowired
// 期望输出：
//   POST /todos       -> 200 todo#1=买牛奶
//   GET  /todos       -> 200 [买牛奶, 写代码]
//   POST /todos       -> 200 todo#2=写代码
//   GET  /todos       -> 200 [买牛奶, 写代码]
//   GET  /todos/1     -> 200 买牛奶
 
import com.sun.net.httpserver.*;
import java.io.*;
import java.lang.annotation.*;
import java.lang.reflect.*;
import java.net.*;
import java.nio.charset.StandardCharsets;
import java.util.*;
import java.util.concurrent.*;
 
public class Main {
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE)
    public @interface RestController {}
    @Retention(RetentionPolicy.RUNTIME) @Target({ElementType.TYPE, ElementType.METHOD})
    public @interface RequestMapping { String value() default ""; }
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
    public @interface GetMapping { String value() default ""; }
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
    public @interface PostMapping { String value() default ""; }
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.PARAMETER)
    public @interface RequestBody {}
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.PARAMETER)
    public @interface PathVariable {}
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
    public @interface ResponseBody {}
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE)
    public @interface Component {}
    @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD)
    public @interface Autowired {}
 
    @Component
    static class TodoService {
        private final List<String> todos = new CopyOnWriteArrayList<>();
        public List<String> findAll() { return todos; }
        public String create(String text) {
            todos.add(text);
            return "todo#" + todos.size() + "=" + text;
        }
        public String findByIndex(int i) { return todos.get(i - 1); }
    }
 
    // 补全：TodoController（包含 @RestController、@RequestMapping("/todos"）、@Autowired）
    // 提供 list() 和 create(@RequestBody String) 两个方法
    static class TodoController {
        // ...
    }
 
    // --- 容器 / 路由 / 启动（与前一节相同的模板） ---
    static class AppContext {
        final Map<Class<?>, Object> beans = new HashMap<>();
        AppContext(Class<?>... cs) throws Exception {
            for (Class<?> c : cs) beans.put(c, c.getDeclaredConstructor().newInstance());
            for (Object bean : beans.values()) {
                for (Field f : bean.getClass().getDeclaredFields()) {
                    if (f.isAnnotationPresent(Autowired.class)) {
                        f.setAccessible(true);
                        f.set(bean, beans.get(f.getType()));
                    }
                }
            }
        }
    }
 
    static class Router {
        void register(HttpServer server, AppContext ctx) {
            for (Object bean : ctx.beans.values()) {
                Class<?> c = bean.getClass();
                String cls = c.isAnnotationPresent(RequestMapping.class)
                    ? c.getAnnotation(RequestMapping.class).value() : "";
                for (Method m : c.getDeclaredMethods()) {
                    String method = null, path = "";
                    if (m.isAnnotationPresent(GetMapping.class))  { method = "GET";  path = m.getAnnotation(GetMapping.class).value(); }
                    if (m.isAnnotationPresent(PostMapping.class)) { method = "POST"; path = m.getAnnotation(PostMapping.class).value(); }
                    if (method == null) continue;
                    final String fp = cls + path;
                    Method fm = m; Object fb = bean;
                    server.createContext(fp, exchange -> {
                        try {
                            String body = new String(exchange.getRequestBody().readAllBytes(), StandardCharsets.UTF_8);
                            String tail = exchange.getRequestURI().getPath().substring(fp.length() + 1);
                            Object[] args = new Object[fm.getParameterCount()];
                            Parameter[] ps = fm.getParameters();
                            for (int i = 0; i < ps.length; i++) {
                                if (ps[i].isAnnotationPresent(RequestBody.class)) args[i] = body;
                                else if (ps[i].isAnnotationPresent(PathVariable.class)) args[i] = tail.split("/")[0];
                            }
                            Object r = fm.invoke(fb, args);
                            byte[] out = (r == null ? "" : r.toString()).getBytes(StandardCharsets.UTF_8);
                            exchange.sendResponseHeaders(200, out.length);
                            exchange.getResponseBody().write(out);
                        } catch (Exception e) {
                            byte[] out = ("ERR: " + e.getCause().getMessage()).getBytes(StandardCharsets.UTF_8);
                            exchange.sendResponseHeaders(500, out.length);
                            exchange.getResponseBody().write(out);
                        } finally { exchange.close(); }
                    });
                }
            }
        }
    }
 
    public static void main(String[] args) throws Exception {
        AppContext ctx = new AppContext(TodoController.class, TodoService.class);
        HttpServer server = HttpServer.create(new InetSocketAddress("127.0.0.1", 0), 0);
        new Router().register(server, ctx);
        server.start();
        int p = server.getAddress().getPort();
        System.out.println("up on " + p);
 
        HttpClient c = HttpClient.newHttpClient();
        System.out.println("POST /todos -> " + c.send(HttpRequest.newBuilder(URI.create("http://127.0.0.1:"+p+"/todos"))
            .POST(HttpRequest.BodyPublishers.ofString("买牛奶")).build(),
            HttpResponse.BodyHandlers.ofString()).body());
        System.out.println("GET  /todos -> " + c.send(HttpRequest.newBuilder(URI.create("http://127.0.0.1:"+p+"/todos"))
            .GET().build(), HttpResponse.BodyHandlers.ofString()).body());
        System.out.println("POST /todos -> " + c.send(HttpRequest.newBuilder(URI.create("http://127.0.0.1:"+p+"/todos"))
            .POST(HttpRequest.BodyPublishers.ofString("写代码")).build(),
            HttpResponse.BodyHandlers.ofString()).body());
        System.out.println("GET  /todos -> " + c.send(HttpRequest.newBuilder(URI.create("http://127.0.0.1:"+p+"/todos"))
            .GET().build(), HttpResponse.BodyHandlers.ofString()).body());
        System.out.println("GET  /todos/1 -> " + c.send(HttpRequest.newBuilder(URI.create("http://127.0.0.1:"+p+"/todos/1"))
            .GET().build(), HttpResponse.BodyHandlers.ofString()).body());
 
        server.stop(0);
    }
}

小结

✅ Spring Boot 启动类标 @SpringBootApplication，main 一行启动
✅ @RestController + @GetMapping / @PostMapping 把方法变成 HTTP 端点
✅ @RequestBody 反序列化请求体；@PathVariable 取 URL 占位符
✅ 推荐构造器注入依赖，而非字段注入
✅ 没有 Spring 时也能用 com.sun.net.httpserver 写极简 HTTP 服务
✅ 本章 Playground 演示了「注解扫描 + 依赖注入 + 路由」三件套是怎么工作的

下一章 项目：多线程下载器——把并发与 Future 串起来。