When using spring cloud like:

dependencyManagement {
  imports {
    mavenBom("org.springframework.cloud:spring-cloud-dependencies:2023.0.3")
  }
}

dependencies {
  ...
  implementation("org.springframework.cloud:spring-cloud-starter-kubernetes-client-config")
  ...
}

You can retrieve a bean of KubernetesClientPodUtils with:

@Autowired
KubernetesClientPodUtils k8sClient;

Then the namespace with:

String namespace = k8sClient.currentPod().get().getMetadata().getNamespace();