Fix TypeInformation.OBJECT initialization.

We now create a decoupled instance of ClassTypeInformation to avoid initialization visibility cycle issues when TypeInformation.OBJECT (and other fields) are initialized with null because ClassTypeInformation.OBJECT hasn't been initialized yet.

Closes #3340

Author: mp911de

src/main/java/org/springframework/data/util/ClassTypeInformation.java

@@ -61,6 +61,10 @@ public class ClassTypeInformation<S> extends TypeDiscoverer<S> {
 
 	private final Class<S> type;
 
+	ClassTypeInformation(Class<?> type) {
+		this(ResolvableType.forType(type));
+	}
+
 	ClassTypeInformation(ResolvableType type) {
 		super(type);
 		this.type = (Class<S>) type.resolve(Object.class);

src/main/java/org/springframework/data/util/TypeInformation.java

@@ -42,11 +42,11 @@
 @SuppressWarnings({ "deprecation", "rawtypes" })
 public interface TypeInformation<S> {
 
-	TypeInformation<Collection> COLLECTION = ClassTypeInformation.COLLECTION;
-	TypeInformation<List> LIST = ClassTypeInformation.LIST;
-	TypeInformation<Set> SET = ClassTypeInformation.SET;
-	TypeInformation<Map> MAP = ClassTypeInformation.MAP;
-	TypeInformation<Object> OBJECT = ClassTypeInformation.OBJECT;
+	TypeInformation<Collection> COLLECTION = new ClassTypeInformation(Collection.class);
+	TypeInformation<List> LIST = new ClassTypeInformation(List.class);
+	TypeInformation<Set> SET = new ClassTypeInformation(Set.class);
+	TypeInformation<Map> MAP = new ClassTypeInformation(Map.class);
+	TypeInformation<Object> OBJECT = new ClassTypeInformation(Object.class);
 
 	/**
 	 * Creates a new {@link TypeInformation} from the given {@link ResolvableType}.

src/test/java/org/springframework/data/util/ClassTypeInformationUnitTests.java

@@ -45,6 +45,24 @@
  */
 public class ClassTypeInformationUnitTests {
 
+	@Test // GH-3340
+	void typeInformationConstantsShouldNotBeNull() {
+
+		assertThat(ClassTypeInformation.COLLECTION).isNotNull();
+		assertThat(TypeInformation.COLLECTION).isNotNull();
+		assertThat(TypeInformation.LIST).isNotNull();
+		assertThat(TypeInformation.SET).isNotNull();
+		assertThat(TypeInformation.MAP).isNotNull();
+		assertThat(TypeInformation.OBJECT).isNotNull();
+		assertThat(ClassTypeInformation.OBJECT).isNotNull();
+
+		assertThat(TypeInformation.COLLECTION).isEqualTo(ClassTypeInformation.COLLECTION);
+		assertThat(TypeInformation.LIST).isEqualTo(ClassTypeInformation.LIST);
+		assertThat(TypeInformation.SET).isEqualTo(ClassTypeInformation.SET);
+		assertThat(TypeInformation.MAP).isEqualTo(ClassTypeInformation.MAP);
+		assertThat(TypeInformation.OBJECT).isEqualTo(ClassTypeInformation.OBJECT);
+	}
+
 	@Test
 	public void discoversTypeForSimpleGenericField() {