The following Environment Variable have to be set for user assigned identity to access key vault. By default the key testKey will be loaded from the Key Vault and printed in /key endpoint

APPLICATIONINSIGHTS_CONNECTION_STRING=InstrumentationnKey...
SPRING_CLOUD_AZURE_KEYVAULT_SECRET_PROPERTYSOURCES_0_CREDENTIAL_CLIENTID=<uai-clientd>
SPRING_CLOUD_AZURE_KEYVAULT_SECRET_PROPERTYSOURCES_0_CREDENTIAL_MANAGEDIDENTITYENABLED=true
SPRING_CLOUD_AZURE_KEYVAULT_SECRET_PROPERTYSOURCES_0_PROFILE_TENANTID=<tenantid>
SPRING_CLOUD_AZURE_KEYVAULT_SECRET_PROPERTYSOURCEENABLED=true
SPRING_CLOUD_AZURE_KEYVAULT_SECRET_PROPERTYSOURCES_0_ENDPOINT=https://my-kv-endpoint

Throws the following exception in container apps, see microsoft/azure-container-apps#341

8/5/2022, 7:03:26.363 PM,"        at io.opentelemetry.javaagent.shaded.instrumentation.reactor.TracingSubscriber.lambda$onNext$1(TracingSubscriber.java:58)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.loader.Launcher.launch(Launcher.java:108)"
8/5/2022, 7:03:26.363 PM,"        at io.opentelemetry.javaagent.shaded.instrumentation.reactor.TracingSubscriber.onNext(TracingSubscriber.java:58)"
8/5/2022, 7:03:26.363 PM,"        at com.azure.core.util.paging.ContinuablePagedByPageIterable.iterator(ContinuablePagedByPageIterable.java:41)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)"
8/5/2022, 7:03:26.363 PM,"        at com.azure.spring.cloud.autoconfigure.keyvault.environment.KeyVaultEnvironmentPostProcessor.buildKeyVaultPropertySource(KeyVaultEnvironmentPostProcessor.java:127)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.MonoDelaySubscription.accept(MonoDelaySubscription.java:53)"
8/5/2022, 7:03:26.363 PM,"Caused by: java.io.IOException: Server returned HTTP response code: 400 for URL: http://localhost:42356/msi/token?resource=https%3A%2F%2Fvault.azure.net&api-version=2019-08-01&client_id=xxxxx"       
8/5/2022, 7:03:26.363 PM,"        at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1520)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.SpringApplicationRunListeners.doWithListeners(SpringApplicationRunListeners.java:120)"
8/5/2022, 7:03:26.363 PM,"        at java.base/java.lang.Thread.run(Thread.java:829)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.loader.Launcher.launch(Launcher.java:58)"
8/5/2022, 7:03:26.363 PM,"        at java.base/java.lang.reflect.Method.invoke(Method.java:566)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.FluxDelaySubscription$DelaySubscriptionOtherSubscriber.onNext(FluxDelaySubscription.java:131)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.MonoDelay$MonoDelayRunnable.run(MonoDelay.java:286)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:143)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.MonoDelay$MonoDelayRunnable.propagateDelay(MonoDelay.java:271)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.SpringApplicationRunListeners.doWithListeners(SpringApplicationRunListeners.java:114)"
8/5/2022, 7:03:26.363 PM,"        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)"
8/5/2022, 7:03:26.363 PM,"        at com.azure.core.util.paging.ContinuablePagedByIteratorBase.requestPage(ContinuablePagedByIteratorBase.java:94)"
8/5/2022, 7:03:26.363 PM,"        ... 27 common frames omitted"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:169)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.loader.JarLauncher.main(JarLauncher.java:88)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.SpringApplication.prepareEnvironment(SpringApplication.java:339)"
8/5/2022, 7:03:26.363 PM,"        at com.azure.spring.cloud.autoconfigure.keyvault.environment.KeyVaultEnvironmentPostProcessor.buildKeyVaultPropertySource(KeyVaultEnvironmentPostProcessor.java:130)"
8/5/2022, 7:03:26.363 PM,"        Suppressed: java.lang.Exception: #block terminated with an error"
8/5/2022, 7:03:26.363 PM,"        at com.azure.spring.cloud.autoconfigure.keyvault.environment.KeyVaultOperation.<init>(KeyVaultOperation.java:77)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.MonoCallable.subscribe(MonoCallable.java:57)"
8/5/2022, 7:03:26.363 PM,"        at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.env.EnvironmentPostProcessorApplicationListener.onApplicationEnvironmentPreparedEvent(EnvironmentPostProcessorApplicationListener.java:102)"
8/5/2022, 7:03:26.363 PM,"        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)"
8/5/2022, 7:03:26.363 PM,"        at com.azure.spring.cloud.autoconfigure.keyvault.environment.KeyVaultOperation.refreshProperties(KeyVaultOperation.java:140)"
8/5/2022, 7:03:26.363 PM,"       
8/5/2022, 7:03:26.363 PM,"        at com.azure.spring.cloud.autoconfigure.keyvault.environment.KeyVaultOperation.lambda$refreshProperties$2(KeyVaultOperation.java:140)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:131)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.context.event.EventPublishingRunListener.environmentPrepared(EventPublishingRunListener.java:85)"
8/5/2022, 7:03:26.363 PM,"        at com.azure.identity.implementation.IdentityClient.lambda$authenticateToManagedIdentityEndpoint$55(IdentityClient.java:1322)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.SpringApplication.run(SpringApplication.java:297)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1312)"
8/5/2022, 7:03:26.363 PM,"Caused by: reactor.core.Exceptions$ReactiveException: java.io.IOException: Server returned HTTP response code: 400 for URL: http://localhost:42356/msi/token?resource=https%3A%2F%2Fvault.azure.net&api-version=2019-08-01&client_id=XXXXX"    
8/5/2022, 7:03:26.363 PM,"       
8/5/2022, 7:03:26.363 PM,"        at java.base/java.lang.Iterable.spliterator(Iterable.java:101)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.env.EnvironmentPostProcessorApplicationListener.onApplicationEvent(EnvironmentPostProcessorApplicationListener.java:87)"
8/5/2022, 7:03:26.363 PM,"        at com.azure.spring.cloud.autoconfigure.keyvault.environment.KeyVaultEnvironmentPostProcessor.postProcessEnvironment(KeyVaultEnvironmentPostProcessor.java:86)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1301)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.Exceptions.propagate(Exceptions.java:392)"
8/5/2022, 7:03:26.363 PM,"19:03:24.268 [main] ERROR org.springframework.boot.SpringApplication - Application run failed"  
8/5/2022, 7:03:26.363 PM,"        at reactor.core.scheduler.SchedulerTask.call(SchedulerTask.java:28)"
8/5/2022, 7:03:26.363 PM,"        at com.example.demo.DemoApplicationKt.main(DemoApplication.kt:13)"
8/5/2022, 7:03:26.363 PM,"        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)"
8/5/2022, 7:03:26.363 PM,"        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)"
8/5/2022, 7:03:26.363 PM,"        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.MonoDelaySubscription.accept(MonoDelaySubscription.java:34)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.SpringApplicationRunListeners.environmentPrepared(SpringApplicationRunListeners.java:65)"
8/5/2022, 7:03:26.363 PM,"        at com.azure.spring.cloud.autoconfigure.keyvault.environment.KeyVaultEnvironmentPostProcessor.buildKeyVaultPropertySourceList(KeyVaultEnvironmentPostProcessor.java:115)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:176)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.loader.MainMethodRunner.run(MainMethodRunner.java:49)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:97)"
8/5/2022, 7:03:26.363 PM,"        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)"
8/5/2022, 7:03:26.363 PM,"        at com.azure.core.util.paging.ContinuablePagedByPageIterable$ContinuablePagedByPageIterator.<init>(ContinuablePagedByPageIterable.java:54)"
8/5/2022, 7:03:26.363 PM,"        at java.base/java.util.Optional.map(Optional.java:265)"
8/5/2022, 7:03:26.363 PM,"java.lang.IllegalStateException: Failed to configure KeyVault property source"             
8/5/2022, 7:03:26.363 PM,"        at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.scheduler.SchedulerTask.call(SchedulerTask.java:68)"
8/5/2022, 7:03:26.363 PM,"        at reactor.core.publisher.Flux.blockLast(Flux.java:2645)"
8/5/2022, 7:03:26.363 PM,"        at java.base/sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1924)"
8/5/2022, 7:03:26.363 PM,"        at org.springframework.boot.SpringApplicationRunListeners.lambda$environmentPrepared$2(SpringApplicationRunListeners.java:66)"
8/5/2022, 7:03:26.363 PM,"        at io.opentelemetry.javaagent.shaded.instrumentation.reactor.TracingSubscriber.withActiveSpan(TracingSubscriber.java:79)"