Consider return type of static methods in ObjectToObjectConverter

Prior to this commit, ObjectToObjectConverter considered the return
type of non-static `to[targetType.simpleName]()` methods but did not
consider the return type of static `valueOf(sourceType)`,
`of(sourceType)`, and `from(sourceType)` methods.

This led to scenarios in which `canConvert()` returned `true`, but a
subsequent `convert()` invocation resulted in a
ConverterNotFoundException, which violates the contract of the
converter.

This commit addresses this issue by taking into account the return type
of a static valueOf/of/from factory method when determining if the
ObjectToObjectConverter supports a particular conversion. Whereas the
existing check in `determineToMethod()` ensures that the method return
type is assignable to the `targetType`, the new check in
`determineFactoryMethod()` leniently ensures that the method return
type and `targetType` are "related" (i.e., reside in the same type
hierarchy).

Closes gh-28609

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -41,15 +41,21 @@
  * <h3>Conversion Algorithm</h3>
  * <ol>
  * <li>Invoke a non-static {@code to[targetType.simpleName]()} method on the
- * source object that has a return type equal to {@code targetType}, if such
+ * source object that has a return type assignable to {@code targetType}, if such
  * a method exists. For example, {@code org.example.Bar Foo#toBar()} is a
  * method that follows this convention.
  * <li>Otherwise invoke a <em>static</em> {@code valueOf(sourceType)} or Java
  * 8 style <em>static</em> {@code of(sourceType)} or {@code from(sourceType)}
- * method on the {@code targetType}, if such a method exists.
+ * method on the {@code targetType} that has a return type <em>related</em> to
+ * {@code targetType}, if such a method exists. For example, a static
+ * {@code Foo.of(sourceType)} method that returns a {@code Foo},
+ * {@code SuperFooType}, or {@code SubFooType} is a method that follows this
+ * convention. {@link java.time.ZoneId#of(String)} is a concrete example of
+ * such a static factory method which returns a subtype of {@code ZoneId}.
  * <li>Otherwise invoke a constructor on the {@code targetType} that accepts
  * a single {@code sourceType} argument, if such a constructor exists.
- * <li>Otherwise throw a {@link ConversionFailedException}.
+ * <li>Otherwise throw a {@link ConversionFailedException} or
+ * {@link IllegalStateException}.
  * </ol>
  *
  * <p><strong>Warning</strong>: this converter does <em>not</em> support the
@@ -193,7 +199,18 @@ private static Method determineFactoryMethod(Class<?> targetClass, Class<?> sour
 				method = ClassUtils.getStaticMethod(targetClass, "from", sourceClass);
 			}
 		}
-		return method;
+
+		return (method != null && areRelatedTypes(targetClass, method.getReturnType()) ? method : null);
+	}
+
+	/**
+	 * Determine if the two types reside in the same type hierarchy (i.e., type 1
+	 * is assignable to type 2 or vice versa).
+	 * @since 5.3.21
+	 * @see ClassUtils#isAssignable(Class, Class)
+	 */
+	private static boolean areRelatedTypes(Class<?> type1, Class<?> type2) {
+		return (ClassUtils.isAssignable(type1, type2) || ClassUtils.isAssignable(type2, type1));
 	}
 
 	@Nullable

spring-core/src/test/java/org/springframework/core/convert/converter/DefaultConversionServiceTests.java

@@ -824,6 +824,9 @@ void convertObjectToStringWithValueOfMethodPresentUsingToString() {
 		assertThat(ISBN.toStringCount).as("toString() invocations").isEqualTo(1);
 	}
 
+	/**
+	 * @see org.springframework.core.convert.support.ObjectToObjectConverterTests
+	 */
 	@Test
 	void convertObjectToObjectUsingValueOfMethod() {
 		ISBN.reset();

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

@@ -0,0 +1,123 @@
+/*
+ * Copyright 2002-2022 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.core.convert.support;
+
+import java.util.Optional;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.core.convert.ConverterNotFoundException;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+
+/**
+ * Unit tests for {@link ObjectToObjectConverter}.
+ *
+ * @author Sam Brannen
+ * @author Phil Webb
+ * @since 5.3.21
+ * @see org.springframework.core.convert.converter.DefaultConversionServiceTests#convertObjectToObjectUsingValueOfMethod()
+ */
+class ObjectToObjectConverterTests {
+
+	private final GenericConversionService conversionService = new GenericConversionService() {{
+		addConverter(new ObjectToObjectConverter());
+	}};
+
+
+	/**
+	 * This test effectively verifies that the {@link ObjectToObjectConverter}
+	 * was properly registered with the {@link GenericConversionService}.
+	 */
+	@Test
+	void nonStaticToTargetTypeSimpleNameMethodWithMatchingReturnType() {
+		assertThat(conversionService.canConvert(Source.class, Data.class))
+			.as("can convert Source to Data").isTrue();
+		Data data = conversionService.convert(new Source("test"), Data.class);
+		assertThat(data).asString().isEqualTo("test");
+	}
+
+	@Test
+	void nonStaticToTargetTypeSimpleNameMethodWithDifferentReturnType() {
+		assertThat(conversionService.canConvert(Text.class, Data.class))
+			.as("can convert Text to Data").isFalse();
+		assertThat(conversionService.canConvert(Text.class, Optional.class))
+			.as("can convert Text to Optional").isFalse();
+		assertThatExceptionOfType(ConverterNotFoundException.class)
+			.as("convert Text to Data")
+			.isThrownBy(() -> conversionService.convert(new Text("test"), Data.class));
+	}
+
+	@Test
+	void staticValueOfFactoryMethodWithDifferentReturnType() {
+		assertThat(conversionService.canConvert(String.class, Data.class))
+			.as("can convert String to Data").isFalse();
+		assertThatExceptionOfType(ConverterNotFoundException.class)
+			.as("convert String to Data")
+			.isThrownBy(() -> conversionService.convert("test", Data.class));
+	}
+
+
+	static class Source {
+
+		private final String value;
+
+		private Source(String value) {
+			this.value = value;
+		}
+
+		public Data toData() {
+			return new Data(this.value);
+		}
+
+	}
+
+	static class Text {
+
+		private final String value;
+
+		private Text(String value) {
+			this.value = value;
+		}
+
+		public Optional<Data> toData() {
+			return Optional.of(new Data(this.value));
+		}
+
+	}
+
+	static class Data {
+
+		private final String value;
+
+		private Data(String value) {
+			this.value = value;
+		}
+
+		@Override
+		public String toString() {
+			return this.value;
+		}
+
+		public static Optional<Data> valueOf(String string) {
+			return (string != null) ? Optional.of(new Data(string)) : Optional.empty();
+		}
+
+	}
+
+}