Feign client lazy initialization fails when using CompletableFuture.supplyAsync() to call the Feign client initially

[maverick1601]

Describe the bug
Feign client initialization is always lazy. As such, the first calling thread triggers the initialization, which may be another Spring component (e.g. REST controller) using CompletableFuture.supplyAsync() to do so. This uses ForkJoinWorkerThread underneath, resulting in a ClassNotFoundException, if and only if running in a dockerized environment based on Spring Boot Build Image plugin.

Using other JVM Executors(except newWorkStealingExecutor), e.g. fixedSizeThreadExecutor(), does not result in this behavior. Also eagerly initializing the Feign client by calling it synchronously, e.g. from within a CommandLineRunner, circumvents this. Once, the Feign client is properly initialized, there is no problem, however, Feign does not yet support eager initialization.

Sample
A sample project with instructions to reproduce is provided here: https://github.com/maverick1601/openfeign-build-image-error

[OlgaMaciaszek]

I was able to reproduce it with the sample. Example stacktrace:

 java.lang.ClassNotFoundException: org.springframework.boot.autoconfigure.condition.OnPropertyCondition
      at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(Unknown Source) ~[na:na]
      at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(Unknown Source) ~[na:na]
      at java.base/java.lang.ClassLoader.loadClass(Unknown Source) ~[na:na]
      at java.base/java.lang.Class.forName0(Native Method) ~[na:na]
      at java.base/java.lang.Class.forName(Unknown Source) ~[na:na]
      at org.springframework.util.ClassUtils.forName(ClassUtils.java:284) ~[spring-core-5.3.3.jar:5.3.3]
      at org.springframework.util.ClassUtils.resolveClassName(ClassUtils.java:324) ~[spring-core-5.3.3.jar:5.3.3]
      at org.springframework.context.annotation.ConditionEvaluator.getCondition(ConditionEvaluator.java:124) ~[spring-context-5.3.3.jar:5.3.3]
      at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:96) ~[spring-context-5.3.3.jar:5.3.3]
      at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:88) ~[spring-context-5.3.3.jar:5.3.3]
      at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:71) ~[spring-context-5.3.3.jar:5.3.3]
       at org.springframework.context.annotation.AnnotatedBeanDefinitionReader.doRegisterBean(AnnotatedBeanDefinitionReader.java:254) ~[spring-context-5.3.3.jar:5.3.3]
       at org.springframework.context.annotation.AnnotatedBeanDefinitionReader.registerBean(AnnotatedBeanDefinitionReader.java:147) ~[spring-context-5.3.3.jar:5.3.3]
       at org.springframework.context.annotation.AnnotatedBeanDefinitionReader.register(AnnotatedBeanDefinitionReader.java:137) ~[spring-context-5.3.3.jar:5.3.3]
      at org.springframework.context.annotation.AnnotationConfigApplicationContext.register(AnnotationConfigApplicationContext.java:168) ~[spring-context-5.3.3.jar:5.3.3]
      at org.springframework.cloud.context.named.NamedContextFactory.createContext(NamedContextFactory.java:122) ~[spring-cloud-context-3.0.1.jar:3.0.1]
      at org.springframework.cloud.context.named.NamedContextFactory.getContext(NamedContextFactory.java:101) ~[spring-cloud-context-3.0.1.jar:3.0.1]
       at org.springframework.cloud.context.named.NamedContextFactory.getInstances(NamedContextFactory.java:181) ~[spring-cloud-context-3.0.1.jar:3.0.1]
       at org.springframework.cloud.openfeign.loadbalancer.FeignBlockingLoadBalancerClient.execute(FeignBlockingLoadBalancerClient.java:85) ~[spring-cloud-openfeign-core-3.0.1.jar:3.0.1]
      at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:119) ~[feign-core-10.10.1.jar:na]
       at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:89) ~[feign-core-10.10.1.jar:na]
       at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:100) ~[feign-core-10.10.1.jar:na]
       at com.sun.proxy.$Proxy73.fetchIt(Unknown Source) ~[na:na]
       at de.digitalfrontiers.spring.build.image.SampleController$sampleApi$1.get(SampleController.kt:14) ~[classes/:na]
       at de.digitalfrontiers.spring.build.image.SampleController$sampleApi$1.get(SampleController.kt:9) ~[classes/:na]
      at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(Unknown Source) ~[na:na]
       at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(Unknown Source) ~[na:na]
       at java.base/java.util.concurrent.ForkJoinTask.doExec(Unknown Source) ~[na:na]
       at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(Unknown Source) ~[na:na]
       at java.base/java.util.concurrent.ForkJoinPool.scan(Unknown Source) ~[na:na]
       at java.base/java.util.concurrent.ForkJoinPool.runWorker(Unknown Source) ~[na:na]
       at java.base/java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source) ~[na:na]

[OlgaMaciaszek]

@maverick1601, thanks for reporting this. Will discuss it with the team and get back to you.

[OlgaMaciaszek]

@maverick1601 we don't recommend using CompletableFuture.supplyAsync(), however, an enhancement that could be helpful here would be to offer an opt-in eager-loading mechanism. We don't think it's a priority at this point, so we would not work on this right now, however, we would welcome pull requests. As a reference, here is a PR, that introduced eager-loading in SC Ribbon where child contexts were being lazily initialised, very similarly to what we have in OpenFeign: spring-cloud/spring-cloud-netflix@567a6c4.

[davgia]

Hi, is there any news about this issue and the associated PR? I encountered a similar issue when using openfeign client called within a method annotated as @Async that uses a ForkJoinPool executor (spring-boot: 2.5.5, spring-cloud: 2020.0.4). I have yet to find a solution or workaround.

P.S. I think it's related to #600

P.S.S. I've tried the solution described in #600 and I managed to make it working. I hope that it can be fixed asap because it's not only a openfeign issue but it does apply on LoadBalanced RestTemplates too

[OlgaMaciaszek]

This issue is not within the team's priorities right now, but is open to community contributions.

[mgrigis]

An alternative solution is to use rxjava,

mvn dep

io.reactivex rxjava