Can't call AppEngine Image API getServingUrl method when using SpringBoot stack - spring-boot

Question

Our project needs to serve scaled images. AppEngine ImageApi has a wonderful method called getServingUrl. But this API is only accessible from AppEngine.
Problem
I've created a PoC project with SpringBoot and gradle for AppEngine. And whenever I call a simple endpoint that should call getServingUrl I get the following error:
Can’t make API call blobstore.CreateEncodedGoogleStorageKey in a thread that is neither the original request thread nor a thread created by ThreadManage
But when I do a similar setup that is based on Servlets, it works as expected.
SpringBoot Setup
Here is an example of some files from my setup.
build.gradle:
;; https://github.com/GoogleCloudPlatform/app-gradle-plugin
buildscript {
repositories {
mavenCentral()
}
dependencies {
classpath 'com.google.cloud.tools:appengine-gradle-plugin:2.2.0'
}
}
plugins {
id 'org.springframework.boot' version '2.2.6.RELEASE'
id 'io.spring.dependency-management' version '1.0.9.RELEASE'
id 'java'
}
group = 'com.example'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '11'
apply plugin: 'com.google.cloud.tools.appengine' // App Engine tasks
repositories {
mavenCentral()
}
dependencies {
implementation 'org.springframework.boot:spring-boot-starter-web'
implementation "com.google.appengine:appengine-api-1.0-sdk:2.0.4"
testImplementation('org.springframework.boot:spring-boot-starter-test') {
exclude group: 'org.junit.vintage', module: 'junit-vintage-engine'
}
}
test {
useJUnitPlatform()
}
appengine { // App Engine tasks configuration
deploy { // deploy configuration
projectId = 'GCLOUD_CONFIG' // delegate to project in gcloud config
version = 'GCLOUD_CONFIG' // delegate to gcloud to generate a version
}
}
appengine/app.yml
runtime: java11
Controller
package com.example.demo.controller;
import com.google.appengine.api.images.ImagesServiceFactory;
import com.google.appengine.api.images.ServingUrlOptions;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
#RestController
public class ImageController {
#RequestMapping("/image-url")
public ImageUrlDto hello(#RequestParam String bucket, #RequestParam String image) {
ServingUrlOptions options = ServingUrlOptions.Builder
.withGoogleStorageFileName(String.format("/gs/%s/%s", bucket, image));
return new ImageUrlDto(ImagesServiceFactory.getImagesService().getServingUrl(options));
}
}
class ImageUrlDto {
String url;
public ImageUrlDto(String url) {
this.url = url;
}
}
Question
Servlet setup is working correctly, but spring setup no. I've read that Image API is accessible only within AppEngine standart environemnt and request thread/ThreadManager spawned thread.
Am I missing something for that Spring setup? Probably that requires some another form of deployment or thread configuration in order to work with Image API?