Avoid exception in ConversionService.canConvert() for Enum targets

Prior to this commit, ConversionService#canConvert(Class, Class)
threw an IllegalArgumentException when invoked with Enum.class as the
target type (i.e., `canConvert(String.class, Enum.class)`), because
ConverterFactory#getConverter() in StringToEnumConverterFactory and
IntegerToEnumConverterFactory eagerly resolved the concrete enum type.

To address that, StringToEnumConverterFactory and
IntegerToEnumConverterFactory now implement ConditionalConverter so
that matches() can reject non-concrete-enum targets before
getConverter() is ever invoked.

Closes gh-34532

Signed-off-by: MoonFruit <dkmoonfruit@gmail.com>

Co-authored-by: Sam Brannen <104798+sbrannen@users.noreply.github.com>

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

@@ -22,7 +22,6 @@ import org.springframework.core.convert.ConversionFailedException;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.TypeDescriptor;
 import org.springframework.core.convert.converter.GenericConverter;
-import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -71,12 +70,17 @@ abstract class ConversionUtils {
 		return false;
 	}
 
-	public static Class<?> getEnumType(Class<?> targetType) {
+	/**
+	 * Resolve the enum type for the supplied target type.
+	 * @param targetType the target type for which to resolve the enum type
+	 * @return the resolved enum type, or {@code null} if the supplied target type
+	 * does not refer to an enum
+	 */
+	public static @Nullable Class<?> resolveEnumType(Class<?> targetType) {
 		Class<?> enumType = targetType;
 		while (enumType != null && !enumType.isEnum()) {
 			enumType = enumType.getSuperclass();
 		}
-		Assert.notNull(enumType, () -> "The target type " + targetType.getName() + " does not refer to an enum");
 		return enumType;
 	}
 

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

@@ -18,8 +18,11 @@ package org.springframework.core.convert.support;
 
 import org.jspecify.annotations.Nullable;
 
+import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.core.convert.converter.ConditionalConverter;
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.ConverterFactory;
+import org.springframework.util.Assert;
 
 /**
  * Converts from an Integer to a {@link java.lang.Enum} by calling {@link Class#getEnumConstants()}.
@@ -29,11 +32,18 @@ import org.springframework.core.convert.converter.ConverterFactory;
  * @since 4.3
  */
 @SuppressWarnings({"rawtypes", "unchecked"})
-final class IntegerToEnumConverterFactory implements ConverterFactory<Integer, Enum> {
+final class IntegerToEnumConverterFactory implements ConverterFactory<Integer, Enum>, ConditionalConverter {
 
 	@Override
 	public <T extends Enum> Converter<Integer, @Nullable T> getConverter(Class<T> targetType) {
-		return new IntegerToEnum(ConversionUtils.getEnumType(targetType));
+		Class<?> enumType = ConversionUtils.resolveEnumType(targetType);
+		Assert.notNull(enumType, () -> "The target type " + targetType.getName() + " does not refer to an enum");
+		return new IntegerToEnum(enumType);
+	}
+
+	@Override
+	public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
+		return (ConversionUtils.resolveEnumType(targetType.getType()) != null);
 	}
 
 

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

@@ -18,8 +18,11 @@ package org.springframework.core.convert.support;
 
 import org.jspecify.annotations.Nullable;
 
+import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.core.convert.converter.ConditionalConverter;
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.ConverterFactory;
+import org.springframework.util.Assert;
 
 /**
  * Converts from a String to a {@link java.lang.Enum} by calling {@link Enum#valueOf(Class, String)}.
@@ -29,11 +32,18 @@ import org.springframework.core.convert.converter.ConverterFactory;
  * @since 3.0
  */
 @SuppressWarnings({"rawtypes", "unchecked"})
-final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {
+final class StringToEnumConverterFactory implements ConverterFactory<String, Enum>, ConditionalConverter {
 
 	@Override
 	public <T extends Enum> Converter<String, @Nullable T> getConverter(Class<T> targetType) {
-		return new StringToEnum(ConversionUtils.getEnumType(targetType));
+		Class<?> enumType = ConversionUtils.resolveEnumType(targetType);
+		Assert.notNull(enumType, () -> "The target type " + targetType.getName() + " does not refer to an enum");
+		return new StringToEnum(enumType);
+	}
+
+	@Override
+	public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
+		return (ConversionUtils.resolveEnumType(targetType.getType()) != null);
 	}
 
 

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

@@ -503,6 +503,20 @@ class GenericConversionServiceTests {
 		assertThat(conversionService.convert("base1", MyEnum.class)).isEqualTo(MyEnum.A);
 	}
 
+	@Test  // gh-34532
+	void canConvertToEnumDoesNotThrowForNonEnumTargetType() {
+		conversionService.addConverterFactory(new StringToEnumConverterFactory());
+		conversionService.addConverterFactory(new IntegerToEnumConverterFactory());
+
+		assertThat(conversionService.canConvert(String.class, Enum.class)).isFalse();
+		assertThat(conversionService.canConvert(Integer.class, Enum.class)).isFalse();
+
+		assertThat(conversionService.canConvert(String.class, MyEnum.class)).isTrue();
+		assertThat(conversionService.canConvert(Integer.class, MyEnum.class)).isTrue();
+		assertThat(conversionService.convert("A", MyEnum.class)).isEqualTo(MyEnum.A);
+		assertThat(conversionService.convert(0, MyEnum.class)).isEqualTo(MyEnum.A);
+	}
+
 	@Test
 	void convertNullAnnotatedStringToString() throws Exception {
 		String source = null;