public class HeaderRequestInterceptor implements ClientHttpRequestInterceptor {

    private final String headerName;

    private final String headerValue;

    public HeaderRequestInterceptor(String headerName, String headerValue) {
        this.headerName = headerName;
        this.headerValue = headerValue;
    }

    @Override
    public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {
        request.getHeaders().set(headerName, headerValue);
        return execution.execute(request, body);
    }
}

@Configuration
public class RestTemplateConfig {

    @Bean
    public RestTemplate restTemplate() {
        List<ClientHttpRequestInterceptor> interceptors = new ArrayList<>();
        interceptors.add(new HeaderRequestInterceptor("token", "123"));

        RestTemplate restTemplate = new RestTemplate();
        restTemplate.setInterceptors(interceptors);

        return restTemplate;
    }
}

@Autowired private RestTemplate restTemplate;