Convenient forType methods for ParameterizedTypeReference

Issue: SPR-16054

spring-core/src/main/java/org/springframework/core/ParameterizedTypeReference.java

@@ -53,6 +53,10 @@ public abstract class ParameterizedTypeReference<T> {
 		this.type = parameterizedType.getActualTypeArguments()[0];
 	}
 
+	private ParameterizedTypeReference(Type type) {
+		this.type = type;
+	}
+
 
 	public Type getType() {
 		return this.type;
@@ -75,6 +79,19 @@ public abstract class ParameterizedTypeReference<T> {
 	}
 
 
+	/**
+	 * Build a {@code ParameterizedTypeReference} wrapping the given type.
+	 * @param type a generic type (possibly obtained via reflection,
+	 * e.g. from {@link java.lang.reflect.Method#getGenericReturnType()})
+	 * @return a corresponding reference which may be passed into
+	 * {@code ParameterizedTypeReference}-accepting methods
+	 * @since 4.3.12
+	 */
+	public static <T> ParameterizedTypeReference<T> forType(Type type) {
+		return new ParameterizedTypeReference<T>(type) {
+		};
+	}
+
 	private static Class<?> findParameterizedTypeReferenceSubclass(Class<?> child) {
 		Class<?> parent = child.getSuperclass();
 		if (Object.class == parent) {

spring-core/src/main/java/org/springframework/core/ResolvableType.java

@@ -1340,6 +1340,19 @@ public class ResolvableType implements Serializable {
 		return forType(type, variableResolver);
 	}
 
+
+	/**
+	 * Return a {@link ResolvableType} for the specified {@link ParameterizedTypeReference}.
+	 * Note: The resulting {@link ResolvableType} may not be {@link Serializable}.
+	 * @param typeReference the reference to obtain the source type from
+	 * @return a {@link ResolvableType} for the specified {@link ParameterizedTypeReference}
+	 * @since 4.3.12
+	 * @see #forType(Type)
+	 */
+	public static ResolvableType forType(ParameterizedTypeReference<?> typeReference) {
+		return forType(typeReference.getType(), null ,null);
+	}
+
 	/**
 	 * Return a {@link ResolvableType} for the specified {@link Type} backed by a given
 	 * {@link VariableResolver}.

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -33,25 +33,40 @@ import static org.junit.Assert.*;
 public class ParameterizedTypeReferenceTests {
 
 	@Test
-	public void map() throws NoSuchMethodException {
+	public void stringTypeReference() {
-		Type mapType = getClass().getMethod("mapMethod").getGenericReturnType();
-		ParameterizedTypeReference<Map<Object,String>> mapTypeReference = new ParameterizedTypeReference<Map<Object,String>>() {};
-		assertEquals(mapType, mapTypeReference.getType());
-	}
-
-	@Test
-	public void list() throws NoSuchMethodException {
-		Type mapType = getClass().getMethod("listMethod").getGenericReturnType();
-		ParameterizedTypeReference<List<String>> mapTypeReference = new ParameterizedTypeReference<List<String>>() {};
-		assertEquals(mapType, mapTypeReference.getType());
-	}
-
-	@Test
-	public void string() {
 		ParameterizedTypeReference<String> typeReference = new ParameterizedTypeReference<String>() {};
 		assertEquals(String.class, typeReference.getType());
 	}
 
+	@Test
+	public void mapTypeReference() throws Exception {
+		Type mapType = getClass().getMethod("mapMethod").getGenericReturnType();
+		ParameterizedTypeReference<Map<Object,String>> typeReference = new ParameterizedTypeReference<Map<Object,String>>() {};
+		assertEquals(mapType, typeReference.getType());
+	}
+
+	@Test
+	public void listTypeReference() throws Exception {
+		Type listType = getClass().getMethod("listMethod").getGenericReturnType();
+		ParameterizedTypeReference<List<String>> typeReference = new ParameterizedTypeReference<List<String>>() {};
+		assertEquals(listType, typeReference.getType());
+	}
+
+	@Test
+	public void reflectiveTypeReferenceWithSpecificDeclaration() throws Exception{
+		Type listType = getClass().getMethod("listMethod").getGenericReturnType();
+		ParameterizedTypeReference<List<String>> typeReference = ParameterizedTypeReference.forType(listType);
+		assertEquals(listType, typeReference.getType());
+	}
+
+	@Test
+	public void reflectiveTypeReferenceWithGenericDeclaration() throws Exception{
+		Type listType = getClass().getMethod("listMethod").getGenericReturnType();
+		ParameterizedTypeReference<?> typeReference = ParameterizedTypeReference.forType(listType);
+		assertEquals(listType, typeReference.getType());
+	}
+
+
 	public static Map<Object, String> mapMethod() {
 		return null;
 	}

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

@@ -870,6 +870,22 @@ public class ResolvableTypeTests {
 		assertThat(this.typeVariableCaptor.getValue().getName(), equalTo("T"));
 	}
 
+	@Test
+	public void resolveTypeVariableFromReflectiveParameterizedTypeReference() throws Exception {
+		Type sourceType = Methods.class.getMethod("typedReturn").getGenericReturnType();
+		ResolvableType type = ResolvableType.forType(ParameterizedTypeReference.forType(sourceType));
+		assertThat(type.resolve(), nullValue());
+		assertThat(type.getType().toString(), equalTo("T"));
+	}
+
+	@Test
+	public void resolveTypeVariableFromDeclaredParameterizedTypeReference() throws Exception {
+		Type sourceType = Methods.class.getMethod("charSequenceReturn").getGenericReturnType();
+		ResolvableType reflectiveType = ResolvableType.forType(sourceType);
+		ResolvableType declaredType = ResolvableType.forType(new ParameterizedTypeReference<List<CharSequence>>() {});
+		assertEquals(reflectiveType, declaredType);
+	}
+
 	@Test
 	public void toStrings() throws Exception {
 		assertThat(ResolvableType.NONE.toString(), equalTo("?"));