Allow @ConfigurationPropertiesBinding to work with lambdas

Update `ConversionServiceDeducer` so that lambdas can be used with
`@ConfigurationPropertiesBinding` annotated `@Bean` methods.

This commit also allows more converter types to be detected.

Closes gh-44018

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/ConversionServiceDeducer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,19 +19,13 @@ package org.springframework.boot.context.properties;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 
-import org.springframework.beans.factory.ListableBeanFactory;
-import org.springframework.beans.factory.annotation.BeanFactoryAnnotationUtils;
-import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.boot.convert.ApplicationConversionService;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.convert.ConversionService;
-import org.springframework.core.convert.converter.Converter;
-import org.springframework.core.convert.converter.GenericConverter;
 import org.springframework.core.convert.support.DefaultConversionService;
-import org.springframework.format.Formatter;
-import org.springframework.format.FormatterRegistry;
 import org.springframework.format.support.FormattingConversionService;
 
 /**
@@ -61,13 +55,9 @@ class ConversionServiceDeducer {
 
 	private List<ConversionService> getConversionServices(ConfigurableApplicationContext applicationContext) {
 		List<ConversionService> conversionServices = new ArrayList<>();
-		ConverterBeans converterBeans = new ConverterBeans(applicationContext);
+		FormattingConversionService beansConverterService = new FormattingConversionService();
+		Map<String, Object> converterBeans = addBeans(applicationContext, beansConverterService);
 		if (!converterBeans.isEmpty()) {
-			FormattingConversionService beansConverterService = new FormattingConversionService();
-			DefaultConversionService.addCollectionConverters(beansConverterService);
-			beansConverterService
-				.addConverter(new ConfigurationPropertiesCharSequenceToObjectConverter(beansConverterService));
-			converterBeans.addTo(beansConverterService);
 			conversionServices.add(beansConverterService);
 		}
 		if (applicationContext.getBeanFactory().getConversionService() != null) {
@@ -83,50 +73,18 @@ class ConversionServiceDeducer {
 		return conversionServices;
 	}
 
+	private Map<String, Object> addBeans(ConfigurableApplicationContext applicationContext,
+			FormattingConversionService converterService) {
+		DefaultConversionService.addCollectionConverters(converterService);
+		converterService.addConverter(new ConfigurationPropertiesCharSequenceToObjectConverter(converterService));
+		return ApplicationConversionService.addBeans(converterService, applicationContext.getBeanFactory(),
+				ConfigurationPropertiesBinding.VALUE);
+	}
+
 	private boolean hasUserDefinedConfigurationServiceBean() {
 		String beanName = ConfigurableApplicationContext.CONVERSION_SERVICE_BEAN_NAME;
 		return this.applicationContext.containsBean(beanName) && this.applicationContext.getAutowireCapableBeanFactory()
 			.isTypeMatch(beanName, ConversionService.class);
 	}
 
-	private static class ConverterBeans {
-
-		@SuppressWarnings("rawtypes")
-		private final List<Converter> converters;
-
-		private final List<GenericConverter> genericConverters;
-
-		@SuppressWarnings("rawtypes")
-		private final List<Formatter> formatters;
-
-		ConverterBeans(ConfigurableApplicationContext applicationContext) {
-			ConfigurableListableBeanFactory beanFactory = applicationContext.getBeanFactory();
-			this.converters = beans(Converter.class, ConfigurationPropertiesBinding.VALUE, beanFactory);
-			this.genericConverters = beans(GenericConverter.class, ConfigurationPropertiesBinding.VALUE, beanFactory);
-			this.formatters = beans(Formatter.class, ConfigurationPropertiesBinding.VALUE, beanFactory);
-		}
-
-		private <T> List<T> beans(Class<T> type, String qualifier, ListableBeanFactory beanFactory) {
-			return new ArrayList<>(
-					BeanFactoryAnnotationUtils.qualifiedBeansOfType(beanFactory, type, qualifier).values());
-		}
-
-		boolean isEmpty() {
-			return this.converters.isEmpty() && this.genericConverters.isEmpty() && this.formatters.isEmpty();
-		}
-
-		void addTo(FormatterRegistry registry) {
-			for (Converter<?, ?> converter : this.converters) {
-				registry.addConverter(converter);
-			}
-			for (GenericConverter genericConverter : this.genericConverters) {
-				registry.addConverter(genericConverter);
-			}
-			for (Formatter<?> formatter : this.formatters) {
-				registry.addFormatter(formatter);
-			}
-		}
-
-	}
-
 }

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/convert/ApplicationConversionService.java

@@ -26,6 +26,7 @@ import java.util.function.Consumer;
 import java.util.function.Supplier;
 
 import org.springframework.beans.factory.ListableBeanFactory;
+import org.springframework.beans.factory.annotation.BeanFactoryAnnotationUtils;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.context.ConfigurableApplicationContext;
@@ -290,13 +291,30 @@ public class ApplicationConversionService extends FormattingConversionService {
 	 * @since 2.2.0
 	 */
 	public static void addBeans(FormatterRegistry registry, ListableBeanFactory beanFactory) {
+		addBeans(registry, beanFactory, null);
+	}
+
+	/**
+	 * Add {@link Printer}, {@link Parser}, {@link Formatter}, {@link Converter},
+	 * {@link ConverterFactory}, {@link GenericConverter}, and beans from the specified
+	 * bean factory.
+	 * @param registry the service to register beans with
+	 * @param beanFactory the bean factory to get the beans from
+	 * @param qualifier the qualifier required on the beans or {@code null}
+	 * @return the beans that were added
+	 * @since 3.5.0
+	 */
+	public static Map<String, Object> addBeans(FormatterRegistry registry, ListableBeanFactory beanFactory,
+			String qualifier) {
 		ConfigurableListableBeanFactory configurableBeanFactory = getConfigurableListableBeanFactory(beanFactory);
-		getBeans(beanFactory).forEach((beanName, bean) -> {
+		Map<String, Object> beans = getBeans(beanFactory, qualifier);
+		beans.forEach((beanName, bean) -> {
 			BeanDefinition beanDefinition = (configurableBeanFactory != null)
 					? configurableBeanFactory.getMergedBeanDefinition(beanName) : null;
 			ResolvableType type = (beanDefinition != null) ? beanDefinition.getResolvableType() : null;
 			addBean(registry, bean, type);
 		});
+		return beans;
 	}
 
 	private static ConfigurableListableBeanFactory getConfigurableListableBeanFactory(ListableBeanFactory beanFactory) {
@@ -309,19 +327,20 @@ public class ApplicationConversionService extends FormattingConversionService {
 		return null;
 	}
 
-	private static Map<String, Object> getBeans(ListableBeanFactory beanFactory) {
+	private static Map<String, Object> getBeans(ListableBeanFactory beanFactory, String qualifier) {
 		Map<String, Object> beans = new LinkedHashMap<>();
-		beans.putAll(getBeans(beanFactory, Printer.class));
-		beans.putAll(getBeans(beanFactory, Parser.class));
-		beans.putAll(getBeans(beanFactory, Formatter.class));
-		beans.putAll(getBeans(beanFactory, Converter.class));
-		beans.putAll(getBeans(beanFactory, ConverterFactory.class));
-		beans.putAll(getBeans(beanFactory, GenericConverter.class));
+		beans.putAll(getBeans(beanFactory, Printer.class, qualifier));
+		beans.putAll(getBeans(beanFactory, Parser.class, qualifier));
+		beans.putAll(getBeans(beanFactory, Formatter.class, qualifier));
+		beans.putAll(getBeans(beanFactory, Converter.class, qualifier));
+		beans.putAll(getBeans(beanFactory, ConverterFactory.class, qualifier));
+		beans.putAll(getBeans(beanFactory, GenericConverter.class, qualifier));
 		return beans;
 	}
 
-	private static <T> Map<String, T> getBeans(ListableBeanFactory beanFactory, Class<T> type) {
-		return beanFactory.getBeansOfType(type);
+	private static <T> Map<String, T> getBeans(ListableBeanFactory beanFactory, Class<T> type, String qualifier) {
+		return (!StringUtils.hasLength(qualifier)) ? beanFactory.getBeansOfType(type)
+				: BeanFactoryAnnotationUtils.qualifiedBeansOfType(beanFactory, type, qualifier);
 	}
 
 	static void addBean(FormatterRegistry registry, Object bean, ResolvableType beanType) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/ConversionServiceDeducerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,8 +16,10 @@
 
 package org.springframework.boot.context.properties;
 
+import java.io.ByteArrayInputStream;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.nio.charset.StandardCharsets;
 import java.util.List;
 
 import org.junit.jupiter.api.Test;
@@ -30,7 +32,10 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.converter.Converter;
+import org.springframework.format.Printer;
 import org.springframework.format.support.FormattingConversionService;
+import org.springframework.util.StreamUtils;
+import org.springframework.util.function.ThrowingSupplier;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -82,6 +87,28 @@ class ConversionServiceDeducerTests {
 		assertThat(conversionServices.get(1)).isSameAs(ApplicationConversionService.getSharedInstance());
 	}
 
+	@Test
+	void getConversionServiceWhenHasQualifiedConverterLambdaBeansContainsCustomizedFormattingService() {
+		ApplicationContext applicationContext = new AnnotationConfigApplicationContext(
+				CustomLambdaConverterConfiguration.class);
+		ConversionServiceDeducer deducer = new ConversionServiceDeducer(applicationContext);
+		List<ConversionService> conversionServices = deducer.getConversionServices();
+		assertThat(conversionServices).hasSize(2);
+		assertThat(conversionServices.get(0)).isExactlyInstanceOf(FormattingConversionService.class);
+		assertThat(conversionServices.get(0).canConvert(InputStream.class, OutputStream.class)).isTrue();
+		assertThat(conversionServices.get(0).canConvert(CharSequence.class, InputStream.class)).isTrue();
+		assertThat(conversionServices.get(1)).isSameAs(ApplicationConversionService.getSharedInstance());
+	}
+
+	@Test
+	void getConversionServiceWhenHasPrinterBean() {
+		ApplicationContext applicationContext = new AnnotationConfigApplicationContext(PrinterConfiguration.class);
+		ConversionServiceDeducer deducer = new ConversionServiceDeducer(applicationContext);
+		List<ConversionService> conversionServices = deducer.getConversionServices();
+		InputStream inputStream = new ByteArrayInputStream("test".getBytes(StandardCharsets.UTF_8));
+		assertThat(conversionServices.get(0).convert(inputStream, String.class)).isEqualTo("test");
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class CustomConverterServiceConfiguration {
 
@@ -114,6 +141,36 @@ class ConversionServiceDeducerTests {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class CustomLambdaConverterConfiguration {
+
+		@Bean
+		@ConfigurationPropertiesBinding
+		Converter<InputStream, OutputStream> testConverter() {
+			return (source) -> new TestConverter().convert(source);
+		}
+
+		@Bean
+		@ConfigurationPropertiesBinding
+		Converter<String, InputStream> stringConverter() {
+			return (source) -> new StringConverter().convert(source);
+		}
+
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	static class PrinterConfiguration {
+
+		@Bean
+		@ConfigurationPropertiesBinding
+		Printer<InputStream> inputStreamPrinter() {
+			return (source, locale) -> ThrowingSupplier
+				.of(() -> StreamUtils.copyToString(source, StandardCharsets.UTF_8))
+				.get();
+		}
+
+	}
+
 	private static final class TestApplicationConversionService extends ApplicationConversionService {
 
 	}