Java Hello World — Spring Boot REST API

目标

用 Spring Boot 3 创建一个 RESTful Web 服务，返回 JSON 格式的 "Hello World"。

完整代码

// HelloWorldApplication.java
package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.time.Instant;
import java.util.Map;

@SpringBootApplication
public class HelloWorldApplication {
    public static void main(String[] args) {
        SpringApplication.run(HelloWorldApplication.class, args);
    }
}

@RestController
class HelloController {

    @GetMapping("/")
    public Map<String, Object> hello() {
        return Map.of(
            "message", "Hello World",
            "timestamp", Instant.now().getEpochSecond()
        );
    }

    @GetMapping("/health")
    public Map<String, String> health() {
        return Map.of("status", "ok");
    }
}

运行步骤

方式一：Spring Initializr (推荐)

# 1. 访问 https://start.spring.io
# 2. 选 Maven / Java / Spring Boot 3.4+ / Java 21
# 3. 添加依赖: Spring Web
# 4. 下载解压，将上述代码复制到 src/main/java/com/example/demo/

# 5. 运行
./mvnw spring-boot:run     # macOS/Linux
mvnw.cmd spring-boot:run   # Windows

方式二：Maven 手动创建

<!-- pom.xml -->
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3.4.0</version>
</parent>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

测试

curl http://localhost:8080/
# 输出: {"message":"Hello World","timestamp":1717000000}

curl http://localhost:8080/health
# 输出: {"status":"ok"}

预期输出

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/

 :: Spring Boot :: (v3.4.0)
...
Tomcat started on port 8080

要点说明

• @SpringBootApplication 组合了 @Configuration、@EnableAutoConfiguration、@ComponentScan
• @RestController = @Controller + @ResponseBody（返回 JSON 而非视图）
• Map.of() 是 JDK 9+ 工厂方法，创建不可变 Map
• Spring Boot 内嵌 Tomcat，无需部署 war 包