Improve GenericTypeResolver to resolve type variable recursively

Fix GH-24963

spring-core/src/test/java/org/springframework/core/GenericTypeResolverTests.java

@@ -39,6 +39,7 @@ import static org.springframework.util.ReflectionUtils.findMethod;
  * @author Sam Brannen
  * @author Sebastien Deleuze
  * @author Stephane Nicoll
+ * @author Yanming Zhou
  */
 @SuppressWarnings({"unchecked", "rawtypes"})
 class GenericTypeResolverTests {
@@ -203,6 +204,13 @@ class GenericTypeResolverTests {
 		assertThat(resolvedType).isEqualTo(reference.getType());
 	}
 
+	@Test
+	void resolveNestedTypeVariable() throws Exception {
+		Type resolved = resolveType(ListOfListSupplier.class.getMethod("get").getGenericReturnType(),
+				StringListOfListSupplier.class);
+		assertThat(ResolvableType.forType(resolved).getGeneric(0).getGeneric(0).resolve()).isEqualTo(String.class);
+	}
+
 	private static Method method(Class<?> target, String methodName, Class<?>... parameterTypes) {
 		Method method = findMethod(target, methodName, parameterTypes);
 		assertThat(method).describedAs(target.getName() + "#" + methodName).isNotNull();
@@ -382,5 +390,14 @@ class GenericTypeResolverTests {
 		}
 	}
 
+	public interface ListOfListSupplier<T> {
+
+		List<List<T>> get();
+
+	}
+
+	public interface StringListOfListSupplier extends ListOfListSupplier<String> {
+
+	}
 
 }

spring-core/src/test/java/org/springframework/core/ResolvableTypeTests.java

@@ -66,6 +66,7 @@ import static org.mockito.Mockito.verify;
  * @author Phillip Webb
  * @author Juergen Hoeller
  * @author Sebastien Deleuze
+ * @author Yanming Zhou
  */
 @SuppressWarnings("rawtypes")
 @ExtendWith(MockitoExtension.class)
@@ -1314,6 +1315,12 @@ class ResolvableTypeTests {
 		assertThat(type.hasUnresolvableGenerics()).isTrue();
 	}
 
+	@Test
+	void hasUnresolvableGenericsWhenNested() throws Exception {
+		ResolvableType type = ResolvableType.forMethodReturnType(ListOfListSupplier.class.getMethod("get"));
+		assertThat(type.hasUnresolvableGenerics()).isTrue();
+	}
+
 	@Test
 	void spr11219() throws Exception {
 		ResolvableType type = ResolvableType.forField(BaseProvider.class.getField("stuff"), BaseProvider.class);
@@ -1617,6 +1624,12 @@ class ResolvableTypeTests {
 	}
 
 
+	public interface ListOfListSupplier<T> {
+
+		List<List<T>> get();
+
+	}
+
 	static class EnclosedInParameterizedType<T> {
 
 		static class InnerRaw {