Cannot the traceId be manually obtained in the service code?

[jarvis-ding]

jvm param
-javaagent:/skywalking-agent/skywalking-agent.jar=agent.service_name=service-a

springboot web request

@RestController
public class FrontController {
    @Resource
    ServiceA serviceA;
    @Trace
    @GetMapping("/demo")
    public String test() {
        return serviceA.a();
    }
}

serviceA

@DubboService(version = "1.0")
public class ServiceAImpl implements ServiceA {
    
    @Trace
    @Override
    public String a() {
        String traceId = TraceContext.traceId();
        int spanId = TraceContext.spanId();
        String s = TraceContext.segmentId();
        System.out.println(traceId + ":" + spanId + ":" + s);
        System.out.println("a()");
        return "a";
    }
}

print traceId Ignored_Trace，I did not get a normal traceId. What is the reason?