任务清单：
1. api Completed ✅

You will build a service that will accept HTTP GET requests at http://localhost:8080/greeting.

It will respond with a JSON representation of a greeting, as the following listing shows:
{"id":1,"content":"Hello, World!"}

package org.example.demo2;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

import java.util.concurrent.atomic.AtomicLong;

@RestController
public class GreetingController {

    private static final String template = "Hello, %s!";
    private final AtomicLong counter = new AtomicLong();

    @GetMapping("/greeting")
    public Greeting greeting(@RequestParam(value = "name", defaultValue = "World") String name) {
        return new Greeting(counter.incrementAndGet(), String.format(template, name));
    }
}