Unleash Client doesn't handle Spring Boot dev tools hot reloading

[espada-edalex]

Describe the bug

I've configured a basic Unleash client in a Spring Boot application and when the hot reload triggers the background process crashes. The Configuration class is very simple and just setting the parameters (I'm using GitLab feature flags, so there is not api key needed).

I was able to stop the crashing by building our own UnleashScheduledExecutor, but then we had multiple clients running in the background because they weren't being shutdown during the reload.

Steps to reproduce the bug

No response

Expected behavior

No response

Logs, error output, etc.

2025-07-18T05:41:11.031Z  INFO 189 --- [example] [  restartedMain] i.g.repository.FeatureBackupHandlerFile  : Unleash will try to load feature toggle states from temporary backup
2025-07-18T05:41:11.034Z ERROR 189 --- [example] [  restartedMain] i.g.util.UnleashScheduledExecutorImpl    : Unleash background task crashed

java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask@70ae5d0a[Not completed, task = java.util.concurrent.Executors$RunnableAdapter@32432edc[Wrapped task = io.getunleash.repository.FeatureRepositoryImpl$$Lambda/0x000071686cb72198@344d5c9c]] rejected from java.util.concurrent.ScheduledThreadPoolExecutor@261362f3[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 1]
   at java.base/java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2081) ~[na:na]
   at java.base/java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:841) ~[na:na]
   at java.base/java.util.concurrent.ScheduledThreadPoolExecutor.delayedExecute(ScheduledThreadPoolExecutor.java:340) ~[na:na]
   at java.base/java.util.concurrent.ScheduledThreadPoolExecutor.scheduleAtFixedRate(ScheduledThreadPoolExecutor.java:632) ~[na:na]
   at io.getunleash.util.UnleashScheduledExecutorImpl.setInterval(UnleashScheduledExecutorImpl.java:43) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at io.getunleash.repository.FeatureRepositoryImpl.initCollections(FeatureRepositoryImpl.java:125) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at io.getunleash.repository.FeatureRepositoryImpl.<init>(FeatureRepositoryImpl.java:92) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at io.getunleash.repository.FeatureRepositoryImpl.<init>(FeatureRepositoryImpl.java:65) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at io.getunleash.repository.FeatureRepositoryImpl.<init>(FeatureRepositoryImpl.java:51) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at io.getunleash.repository.FeatureRepositoryImpl.<init>(FeatureRepositoryImpl.java:39) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at io.getunleash.repository.FeatureRepositoryImpl.<init>(FeatureRepositoryImpl.java:34) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at io.getunleash.EngineProxyImpl.<init>(EngineProxyImpl.java:38) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at io.getunleash.DefaultUnleash.defaultToggleRepository(DefaultUnleash.java:38) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at io.getunleash.DefaultUnleash.<init>(DefaultUnleash.java:42) ~[unleash-client-java-11.0.2.jar:11.0.2]
   at com.example.config.UnleashConfiguration.unleash(UnleashConfiguration.java:39) ~[classes/:na]
   at com.example.config.UnleashConfiguration$$SpringCGLIB$$0.CGLIB$unleash$0(<generated>) ~[classes/:na]
   at com.example.config.UnleashConfiguration$$SpringCGLIB$$FastClass$$1.invoke(<generated>) ~[classes/:na]
   at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:258) ~[spring-core-6.2.8.jar:6.2.8]
   at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:400) ~[spring-context-6.2.8.jar:6.2.8]
   at com.example.config.UnleashConfiguration$$SpringCGLIB$$0.unleash(<generated>) ~[classes/:na]
   at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[na:na]
   at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[na:na]
   at org.springframework.beans.factory.support.SimpleInstantiationStrategy.lambda$instantiate$0(SimpleInstantiationStrategy.java:171) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiateWithFactoryMethod(SimpleInstantiationStrategy.java:88) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:168) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:653) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:489) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1375) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1205) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:569) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:529) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:339) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:373) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:337) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1683) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1628) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:913) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:791) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:240) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1395) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1232) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:569) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:529) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:339) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:373) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:337) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.DefaultListableBeanFactory.instantiateSingleton(DefaultListableBeanFactory.java:1222) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingleton(DefaultListableBeanFactory.java:1188) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:1123) ~[spring-beans-6.2.8.jar:6.2.8]
   at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:987) ~[spring-context-6.2.8.jar:6.2.8]
   at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:627) ~[spring-context-6.2.8.jar:6.2.8]
   at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:146) ~[spring-boot-3.5.3.jar:3.5.3]
   at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:752) ~[spring-boot-3.5.3.jar:3.5.3]
   at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:439) ~[spring-boot-3.5.3.jar:3.5.3]
   at org.springframework.boot.SpringApplication.run(SpringApplication.java:318) ~[spring-boot-3.5.3.jar:3.5.3]
   at org.springframework.boot.SpringApplication.run(SpringApplication.java:1361) ~[spring-boot-3.5.3.jar:3.5.3]
   at org.springframework.boot.SpringApplication.run(SpringApplication.java:1350) ~[spring-boot-3.5.3.jar:3.5.3]
   at com.example.Application.main(Application.java:10) ~[classes/:na]
   at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[na:na]
   at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[na:na]
   at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:50) ~[spring-boot-devtools-3.5.3.jar:3.5.3]

Screenshots

No response

Additional context

package com.example.config;

import io.getunleash.DefaultUnleash;
import io.getunleash.util.UnleashConfig;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
@Slf4j
public class UnleashConfiguration {

  @Value("${unleash.api-url}")
  private String apiUrl;

  @Value("${unleash.app-name}")
  private String appName;

  @Value("${unleash.instance-id}")
  private String instanceId;

  @Bean
  public DefaultUnleash unleash() {
    UnleashConfig config = UnleashConfig.builder()
      .appName(appName)
      .instanceId(instanceId)
      .unleashAPI(apiUrl)
      .synchronousFetchOnInitialisation(false)
      .build();

    return new DefaultUnleash(config);
  }
}

Unleash version

GitLab feature flags feature - https://docs.gitlab.com/operations/feature_flags/

Subscription type

None

Hosting type

None

SDK information (language and version)

11.0.2

[chriswk]

Hi @espada-edalex

So this doesn't work, because the built-in scheduled executor implements shutdown() shutting down the executor, so when the hot reloader comes and wants to reuse it, it's no longer alive.

You're the first to report this, so I'm assuming this only happens during development.

I have to admit, I'm unsure how Spring hot reload works behind the scenes, but if you have an idea on how you want it to work, feel free to start a PR, discussing how we'd make it work with reloading without getting multiple instances.

[espada-edalex]

Hi @chriswk 👋

I wish I could help with this, unfortunately I know little Spring or Java, and wouldn't even know where to start with something like this (is it possible not to shutdown the executor/client so it can be reused after the reload?).

You are correct, too, this is happening in development. In production a whole new instance would be started, so it wouldn't have this issue.

[espada-edalex]

Despite myself, I went to have a look at the LaunchDarkly SDK (that doesn't have an issue with hot reloading), and they seem to be using the same kind of Executor

• https://github.com/Unleash/unleash-java-sdk/blob/main/src/main/java/io/getunleash/util/UnleashScheduledExecutorImpl.java#L17
• https://github.com/launchdarkly/java-core/blob/main/lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/LDClient.java#L554

Except Unleash also wraps it in a ScheduledThreadPoolExecutor (not sure if that would make any difference).

One other difference I noticed is that the LaunchDarkly client also calls shutdownNow rather than shutdown (also not sure if that would make a difference)

• https://github.com/launchdarkly/java-core/blob/main/lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/LDClient.java#L509