Support conversion from Enum Interface

EnumToStringConverter in now conditional and only matches Enums that
do not implement interfaces that can be converted.

Issue: SPR-9692

Author: philwebb

spring-core/src/main/java/org/springframework/core/convert/support/DefaultConversionService.java

@@ -59,6 +59,7 @@ public static void addDefaultConverters(ConverterRegistry converterRegistry) {
 	// internal helpers
 
 	private static void addScalarConverters(ConverterRegistry converterRegistry) {
+		ConversionService conversionService = (ConversionService) converterRegistry;
 		converterRegistry.addConverter(new StringToBooleanConverter());
 		converterRegistry.addConverter(Boolean.class, String.class, new ObjectToStringConverter());
 
@@ -74,7 +75,7 @@ private static void addScalarConverters(ConverterRegistry converterRegistry) {
 		converterRegistry.addConverterFactory(new CharacterToNumberFactory());
 
 		converterRegistry.addConverterFactory(new StringToEnumConverterFactory());
-		converterRegistry.addConverter(Enum.class, String.class, new EnumToStringConverter());
+		converterRegistry.addConverter(Enum.class, String.class, new EnumToStringConverter(conversionService));
 
 		converterRegistry.addConverter(new StringToLocaleConverter());
 		converterRegistry.addConverter(Locale.class, String.class, new ObjectToStringConverter());

spring-core/src/main/java/org/springframework/core/convert/support/EnumToStringConverter.java

@@ -16,14 +16,37 @@
 
 package org.springframework.core.convert.support;
 
+import org.springframework.core.convert.ConversionService;
+import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.core.convert.converter.ConditionalConversion;
 import org.springframework.core.convert.converter.Converter;
+import org.springframework.util.ClassUtils;
+import org.springframework.util.ReflectionUtils;
 
 /**
- * Simply calls {@link Enum#name()} to convert a source Enum to a String.
+ * Calls {@link Enum#name()} to convert a source Enum to a String.  This converter will
+ * not match enums with interfaces that can be converterd.
  * @author Keith Donald
+ * @author Phillip Webb
  * @since 3.0
  */
-final class EnumToStringConverter implements Converter<Enum<?>, String> {
+final class EnumToStringConverter implements Converter<Enum<?>, String>, ConditionalConversion {
+
+	private final ConversionService conversionService;
+
+	public EnumToStringConverter(ConversionService conversionService) {
+		this.conversionService = conversionService;
+	}
+
+	public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
+		for (Class<?> interfaceType : ClassUtils.getAllInterfacesForClass(sourceType.getType())) {
+			if (conversionService.canConvert(TypeDescriptor.valueOf(interfaceType),
+					targetType)) {
+				return false;
+			}
+		}
+		return true;
+	}
 
 	public String convert(Enum<?> source) {
 		return source.name();

spring-core/src/test/java/org/springframework/core/convert/support/GenericConversionServiceTests.java

@@ -734,6 +734,22 @@ public Byte convert(Byte source) {
 		assertTrue(Arrays.equals(new byte[] { 2, 3, 4 }, converted));
 	}
 
+	@Test
+	public void testEnumToStringConversion() {
+		conversionService.addConverter(new EnumToStringConverter(conversionService));
+		String result = conversionService.convert(MyEnum.A, String.class);
+		assertEquals("A", result);
+	}
+
+	@Test
+	public void testEnumWithInterfaceToStringConversion() {
+		// SPR-9692
+		conversionService.addConverter(new EnumToStringConverter(conversionService));
+		conversionService.addConverter(new MyEnumInterfaceToStringConverter<MyEnum>());
+		String result = conversionService.convert(MyEnum.A, String.class);
+		assertEquals("1", result);
+	}
+
 	private static class MyConditionalConverter implements Converter<String, Color>,
 			ConditionalConversion {
 
@@ -803,4 +819,22 @@ public int getNestedMatchAttempts() {
 		}
 	}
 
+	interface MyEnumInterface {
+		String getCode();
+	}
+
+	public static enum MyEnum implements MyEnumInterface {
+		A {
+			public String getCode() {
+				return "1";
+			}
+		};
+	}
+
+	private static class MyEnumInterfaceToStringConverter<T extends MyEnumInterface>
+			implements Converter<T, String> {
+		public String convert(T source) {
+			return source.getCode();
+		}
+	}
 }