Ensure TypeConverterConverter is thread safe

Update `TypeConverterConverter` do that a new `SimpleTypeConverter` is
obtained for each `convert` operation. Prior to this commit the same
`SimpleTypeConverter` could be accessed concurrently from multiple
threads which is not allowed.

Fixes gh-27829

Author: philwebb

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

@@ -185,18 +185,10 @@ public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor t
 	private static class TypeConverterConversionService extends GenericConversionService {
 
 		TypeConverterConversionService(Consumer<PropertyEditorRegistry> initializer) {
-			addConverter(new TypeConverterConverter(createTypeConverter(initializer)));
+			addConverter(new TypeConverterConverter(initializer));
 			ApplicationConversionService.addDelimitedStringConverters(this);
 		}
 
-		private SimpleTypeConverter createTypeConverter(Consumer<PropertyEditorRegistry> initializer) {
-			SimpleTypeConverter typeConverter = new SimpleTypeConverter();
-			if (initializer != null) {
-				initializer.accept(typeConverter);
-			}
-			return typeConverter;
-		}
-
 		@Override
 		public boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType) {
 			// Prefer conversion service to handle things like String to char[].
@@ -213,10 +205,15 @@ public boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType)
 	 */
 	private static class TypeConverterConverter implements ConditionalGenericConverter {
 
-		private final SimpleTypeConverter typeConverter;
+		private final Consumer<PropertyEditorRegistry> initializer;
+
+		// SimpleTypeConverter is not thread-safe to use for conversion but we can use it
+		// in a thread-safe way to check if conversion is possible.
+		private final SimpleTypeConverter matchesOnlyTypeConverter;
 
-		TypeConverterConverter(SimpleTypeConverter typeConverter) {
-			this.typeConverter = typeConverter;
+		TypeConverterConverter(Consumer<PropertyEditorRegistry> initializer) {
+			this.initializer = initializer;
+			this.matchesOnlyTypeConverter = createTypeConverter();
 		}
 
 		@Override
@@ -226,32 +223,32 @@ public Set<ConvertiblePair> getConvertibleTypes() {
 
 		@Override
 		public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
-			return getPropertyEditor(targetType.getType()) != null;
-		}
-
-		@Override
-		public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
-			SimpleTypeConverter typeConverter = this.typeConverter;
-			return typeConverter.convertIfNecessary(source, targetType.getType());
-		}
-
-		private PropertyEditor getPropertyEditor(Class<?> type) {
+			Class<?> type = targetType.getType();
 			if (type == null || type == Object.class || Collection.class.isAssignableFrom(type)
 					|| Map.class.isAssignableFrom(type)) {
-				return null;
+				return false;
 			}
-			SimpleTypeConverter typeConverter = this.typeConverter;
-			PropertyEditor editor = typeConverter.getDefaultEditor(type);
+			PropertyEditor editor = this.matchesOnlyTypeConverter.getDefaultEditor(type);
 			if (editor == null) {
-				editor = typeConverter.findCustomEditor(type, null);
+				editor = this.matchesOnlyTypeConverter.findCustomEditor(type, null);
 			}
 			if (editor == null && String.class != type) {
 				editor = BeanUtils.findEditorByConvention(type);
 			}
-			if (editor == null || EXCLUDED_EDITORS.contains(editor.getClass())) {
-				return null;
+			return (editor != null && !EXCLUDED_EDITORS.contains(editor.getClass()));
+		}
+
+		@Override
+		public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
+			return createTypeConverter().convertIfNecessary(source, targetType.getType());
+		}
+
+		private SimpleTypeConverter createTypeConverter() {
+			SimpleTypeConverter typeConverter = new SimpleTypeConverter();
+			if (this.initializer != null) {
+				this.initializer.accept(typeConverter);
 			}
-			return editor;
+			return typeConverter;
 		}
 
 	}

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

@@ -19,6 +19,8 @@
 import java.beans.PropertyEditorSupport;
 import java.io.File;
 import java.time.Duration;
+import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.function.Consumer;
 
@@ -192,6 +194,30 @@ void convertWhenConversionFailsThrowsConversionFailedExceptionRatherThanConverte
 				.withRootCauseInstanceOf(ClassNotFoundException.class);
 	}
 
+	@Test
+	void convertWhenUsingTypeConverterConversionServiceFromMultipleThreads() {
+		BindConverter bindConverter = getPropertyEditorOnlyBindConverter(this::registerSampleTypeEditor);
+		ResolvableType type = ResolvableType.forClass(SampleType.class);
+		List<Thread> threads = new ArrayList<>();
+		List<SampleType> results = Collections.synchronizedList(new ArrayList<>());
+		for (int i = 0; i < 40; i++) {
+			threads.add(new Thread(() -> {
+				for (int j = 0; j < 20; j++) {
+					results.add(bindConverter.convert("test", type));
+				}
+			}));
+		}
+		threads.forEach(Thread::start);
+		for (Thread thread : threads) {
+			try {
+				thread.join();
+			}
+			catch (InterruptedException ex) {
+			}
+		}
+		assertThat(results).isNotEmpty().doesNotContainNull();
+	}
+
 	private BindConverter getPropertyEditorOnlyBindConverter(
 			Consumer<PropertyEditorRegistry> propertyEditorInitializer) {
 		return BindConverter.get(new ThrowingConversionService(), propertyEditorInitializer);
@@ -221,9 +247,12 @@ static class SampleTypePropertyEditor extends PropertyEditorSupport {
 
 		@Override
 		public void setAsText(String text) throws IllegalArgumentException {
-			SampleType value = new SampleType();
-			value.text = text;
-			setValue(value);
+			setValue(null);
+			if (text != null) {
+				SampleType value = new SampleType();
+				value.text = text;
+				setValue(value);
+			}
 		}
 
 	}