Configuration properties containing empty arrays, collections or maps fail with ConverterNotFoundException

[akoessler]

Configuring a Map property with an empty collection is no longer possible in 4.1.0, was working in 4.0.6.

Instead of the application starting with an empty map for that property, the startup fails with an Failed to load ApplicationContext error caused by:

Caused by: org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.String] to type [java.util.Map<java.lang.String, java.lang.String>]

e.g. with the following config class:

@ConfigurationProperties(prefix = "test")
public class TestProperties {
    private Map<String, String> testmap = new HashMap<>();

    public Map<String, String> getTestmap() {
        return testmap;
    }

    public void setTestmap(final Map<String, String> testmap) {
        this.testmap = testmap;
    }
}

and this in application.yaml:

test:
  testmap: {}

The result is an error at startup:

java.lang.IllegalStateException: Failed to load ApplicationContext for [WebMergedContextConfiguration@629fc74d testClass = **redacted**, locations = [], classes = [**redacted**, TestConfiguration], contextInitializerClasses = [], activeProfiles = [], propertySourceDescriptors = [], propertySourceProperties = ["org.springframework.boot.test.context.SpringBootTestContextBootstrapper=true"], contextCustomizers = [org.springframework.boot.webmvc.test.autoconfigure.WebDriverContextCustomizer@5003041b, org.springframework.boot.test.autoconfigure.OnFailureConditionReportContextCustomizerFactory$OnFailureConditionReportContextCustomizer@6bc248ed, [ImportsContextCustomizer@4228a2f6 key = [@org.springframework.test.context.BootstrapWith(org.springframework.boot.test.context.SpringBootTestContextBootstrapper.class), @org.springframework.stereotype.Indexed(), @org.springframework.boot.autoconfigure.SpringBootApplication(exclude={}, excludeName={}, nameGenerator=org.springframework.beans.factory.support.BeanNameGenerator.class, proxyBeanMethods=true, scanBasePackageClasses={}, scanBasePackages={}), @org.springframework.context.annotation.Import({org.springframework.boot.autoconfigure.AutoConfigurationImportSelector.class}), @org.springframework.boot.autoconfigure.EnableAutoConfiguration(exclude={}, excludeName={}), @org.springframework.context.annotation.ComponentScan(basePackageClasses={}, basePackages={}, excludeFilters={@org.springframework.context.annotation.ComponentScan.Filter(classes={org.springframework.boot.context.TypeExcludeFilter.class}, pattern={}, type=CUSTOM, value={org.springframework.boot.context.TypeExcludeFilter.class}), @org.springframework.context.annotation.ComponentScan.Filter(classes={org.springframework.boot.autoconfigure.AutoConfigurationExcludeFilter.class}, pattern={}, type=CUSTOM, value={org.springframework.boot.autoconfigure.AutoConfigurationExcludeFilter.class})}, includeFilters={}, lazyInit=false, nameGenerator=org.springframework.beans.factory.support.BeanNameGenerator.class, resourcePattern="**/*.class", scopeResolver=org.springframework.context.annotation.AnnotationScopeMetadataResolver.class, scopedProxy=DEFAULT, useDefaultFilters=true, value={}), @org.springframework.boot.test.context.SpringBootTest(args={}, classes={AppTestConfiguration.class, TestConfiguration.class}, properties={}, useMainMethod=NEVER, value={}, webEnvironment=DEFINED_PORT), @org.springframework.context.annotation.Import({org.springframework.boot.autoconfigure.AutoConfigurationPackages.Registrar.class}), @org.springframework.context.annotation.Configuration(enforceUniqueMethods=true, proxyBeanMethods=true, value=""), @org.springframework.context.annotation.Import({**redacted**, **redacted**, **redacted**, **redacted**, **redacted**}), @org.springframework.stereotype.Component(""), @org.apiguardian.api.API(consumers={"*"}, since="5.0", status=STABLE), @org.springframework.boot.autoconfigure.AutoConfigurationPackage(basePackageClasses={}, basePackages={}), @org.mockito.junit.jupiter.MockitoSettings(strictness=LENIENT), @org.springframework.boot.SpringBootConfiguration(proxyBeanMethods=true)]], org.springframework.boot.test.context.PropertyMappingContextCustomizer@0, org.springframework.boot.test.context.filter.ExcludeFilterContextCustomizer@200a26bc, org.springframework.boot.test.http.client.DisableReactorResourceFactoryGlobalResourcesContextCustomizerFactory$DisableReactorResourceFactoryGlobalResourcesContextCustomizerCustomizer@2f66e802, org.springframework.boot.test.json.DuplicateJsonObjectContextCustomizerFactory$DuplicateJsonObjectContextCustomizer@2a492f2a, org.springframework.test.context.support.DynamicPropertiesContextCustomizer@0, org.springframework.boot.web.server.context.SpringBootTestRandomPortContextCustomizer@5a021cb9, org.springframework.boot.test.context.SpringBootTestAnnotation@50d6d588], resourceBasePath = "src/main/webapp", contextLoader = org.springframework.boot.test.context.SpringBootContextLoader, parent = null]

	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.lambda$loadContext$0(DefaultCacheAwareContextLoaderDelegate.java:195)
	at org.springframework.test.context.cache.DefaultContextCache.put(DefaultContextCache.java:214)
	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:160)
	at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:128)
	at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.injectDependencies(DependencyInjectionTestExecutionListener.java:156)
	at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.prepareTestInstance(DependencyInjectionTestExecutionListener.java:111)
	at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:260)
	at org.springframework.test.context.junit.jupiter.SpringExtension.postProcessTestInstance(SpringExtension.java:242)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:186)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:197)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:214)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1716)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:570)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:560)
	at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:153)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:176)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:265)
	at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:632)
	at java.base/java.util.Optional.orElseGet(Optional.java:364)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1604)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1604)
Caused by: org.springframework.boot.context.properties.ConfigurationPropertiesBindException: Error creating bean with name 'TestProperties': Could not bind properties to 'TestProperties' : prefix=test, ignoreInvalidFields=false, ignoreUnknownFields=true
	at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.bind(ConfigurationPropertiesBindingPostProcessor.java:104)
	at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.postProcessBeforeInitialization(ConfigurationPropertiesBindingPostProcessor.java:84)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization(AbstractAutowireCapableBeanFactory.java:426)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1807)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:603)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:525)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:371)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:331)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:196)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.instantiateSingleton(DefaultListableBeanFactory.java:1225)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingleton(DefaultListableBeanFactory.java:1191)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:1121)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:994)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:621)
	at org.springframework.boot.web.server.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:143)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:756)
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:445)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:321)
	at org.springframework.boot.test.context.SpringBootContextLoader.lambda$loadContext$2(SpringBootContextLoader.java:155)
	at org.springframework.util.function.ThrowingSupplier.get(ThrowingSupplier.java:58)
	at org.springframework.util.function.ThrowingSupplier.get(ThrowingSupplier.java:46)
	at org.springframework.boot.SpringApplication.withHook(SpringApplication.java:1465)
	at org.springframework.boot.test.context.SpringBootContextLoader$ContextLoaderHook.run(SpringBootContextLoader.java:600)
	at org.springframework.boot.test.context.SpringBootContextLoader.loadContext(SpringBootContextLoader.java:155)
	at org.springframework.boot.test.context.SpringBootContextLoader.loadContext(SpringBootContextLoader.java:114)
	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:247)
	at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.lambda$loadContext$0(DefaultCacheAwareContextLoaderDelegate.java:167)
	... 21 more
Caused by: org.springframework.boot.context.properties.bind.BindException: Failed to bind properties under 'test.testmap' to java.util.Map<java.lang.String, java.lang.String>
	at org.springframework.boot.context.properties.bind.Binder.handleBindError(Binder.java:419)
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:378)
	at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$0(Binder.java:508)
	at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:127)
	at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:115)
	at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:71)
	at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$2(Binder.java:511)
	at org.springframework.boot.context.properties.bind.Binder.fromDataObjectBinders(Binder.java:521)
	at org.springframework.boot.context.properties.bind.Binder.lambda$bindDataObject$1(Binder.java:510)
	at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:636)
	at org.springframework.boot.context.properties.bind.Binder$Context.withDataObject(Binder.java:622)
	at org.springframework.boot.context.properties.bind.Binder.bindDataObject(Binder.java:513)
	at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:448)
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:374)
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:361)
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:288)
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:275)
	at org.springframework.boot.context.properties.ConfigurationPropertiesBinder.bind(ConfigurationPropertiesBinder.java:96)
	at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.bind(ConfigurationPropertiesBindingPostProcessor.java:101)
	... 48 more
Caused by: org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.String] to type [java.util.Map<java.lang.String, java.lang.String>]
	at org.springframework.boot.context.properties.bind.BindConverter.convert(BindConverter.java:123)
	at org.springframework.boot.context.properties.bind.BindConverter.convert(BindConverter.java:102)
	at org.springframework.boot.context.properties.bind.BindConverter.convert(BindConverter.java:96)
	at org.springframework.boot.context.properties.bind.MapBinder.bindAggregate(MapBinder.java:67)
	at org.springframework.boot.context.properties.bind.AggregateBinder.bind(AggregateBinder.java:59)
	at org.springframework.boot.context.properties.bind.Binder.lambda$bindAggregate$2(Binder.java:472)
	at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:636)
	at org.springframework.boot.context.properties.bind.Binder.bindAggregate(Binder.java:473)
	at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:431)
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:374)
	... 65 more

[wilkinsona]

This is due to #48958. It removes a YAML-specific quirk and aligns the behavior with other configuration forms such as .properties.

I can't see a use case for this YAML in Spring Boot 4.0:

test:
  testmap: {}

No property is added to the environment for test.testmap so it isn't overriding another property or anything like that. In other words, you can achieve the same effect by removing it entirely from your YAML.

In Boot 4.1, a test.testmap property is added to the environment. Its value is an empty String. This aligns with property-based behavior in Boot 4.0, as you can see in this example:

package com.example;

import java.util.HashMap;
import java.util.Map;

import com.example.Example.TestProperties;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.boot.context.properties.EnableConfigurationProperties;

@SpringBootApplication
@EnableConfigurationProperties(TestProperties.class)
public class Example {

	public static void main(String[] args) {
		SpringApplication.run(Example.class, "--test.testmap");
	}

	@ConfigurationProperties(prefix = "test")
	public static class TestProperties {

		private Map<String, String> testmap = new HashMap<>();

		public Map<String, String> getTestmap() {
			return this.testmap;
		}

		public void setTestmap(final Map<String, String> testmap) {
			this.testmap = testmap;
		}

	}

}

If you run this with Boot 4.0, it'll fail as it's trying to convert an empty string to a map.

[akoessler]

Thanks @wilkinsona for the details.
It does indeed work in 4.0.6 to just remove the map property from the application.yaml.
So for that specific issue we have right now, we could probably remove the affected properties from the yaml config, we would need to look into that in more detail.

However, I do still see it as problematic to not support that syntax. It is quite commonly used in yaml files and other tools to refer to empty arrays/maps/properties as [] / {}. just a few examples, especially with helm charts.

And it is kind of a breaking change to not support this anymore, as it breaks existing configuration files with an unclear error message.

IMHO it would be good to keep that support.
Or if this is really intended to be breaking, then at least have a more meaningful error message.

[Hathoute]

I think we should allow explicitly specifying empty objects. In some cases, it's useful to indicate that an attribute is intentionally empty, rather than omitted by mistake.

[philwebb]

Closing in favor of PR #50773. Thanks @xfocus3!