DATACMNS-1762 - Support Optional wrapping for projection getters.

We now support nullable wrappers for projection interfaces. Getters are inspected whether their return type is a supported nullable wrapper. If so, then the value can be wrapped into that type. Null values default in that case to their corresponding empty wrapper representation.

Original Pull Request: #459

src/main/asciidoc/repository-projections.adoc

@@ -197,6 +197,30 @@ interface NamesOnly {
 
 Again, for more complex expressions, you should use a Spring bean and let the expression invoke a method, as described  <<projections.interfaces.open.bean-reference,earlier>>.
 
+[[projections.interfaces.nullable-wrappers]]
+=== Nullable Wrappers
+
+Getters in projection interfaces can make use of nullable wrappers for improved null-safety. Currently supported wrapper types are:
+
+* `java.util.Optional`
+* `com.google.common.base.Optional`
+* `scala.Option`
+* `io.vavr.control.Option`
+
+.A projection interface using nullable wrappers
+====
+[source, java]
+----
+interface NamesOnly {
+
+  Optional<String> getFirstname();
+}
+----
+====
+
+If the underlying projection value is not `null`, then values are returned using the present-representation of the wrapper type.
+In case the backing value is `null`, then the getter method returns the empty representation of the used wrapper type.
+
 [[projections.dtos]]
 == Class-based Projections (DTOs)
 

src/main/java/org/springframework/data/projection/ProjectingMethodInterceptor.java

@@ -31,6 +31,8 @@ import org.aopalliance.intercept.MethodInvocation;
 import org.springframework.core.CollectionFactory;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.data.util.ClassTypeInformation;
+import org.springframework.data.util.NullableWrapper;
+import org.springframework.data.util.NullableWrapperConverters;
 import org.springframework.data.util.TypeInformation;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -67,23 +69,43 @@ class ProjectingMethodInterceptor implements MethodInterceptor {
 	@Override
 	public Object invoke(@SuppressWarnings("null") @Nonnull MethodInvocation invocation) throws Throwable {
 
+		TypeInformation<?> type = ClassTypeInformation.fromReturnTypeOf(invocation.getMethod());
+		TypeInformation<?> resultType = type;
+		TypeInformation<?> typeToReturn = type;
+
 		Object result = delegate.invoke(invocation);
+		boolean applyWrapper = false;
+
+		if (NullableWrapperConverters.supports(type.getType())
+				&& (result == null || !NullableWrapperConverters.supports(result.getClass()))) {
+			resultType = NullableWrapperConverters.unwrapActualType(typeToReturn);
+			applyWrapper = true;
+		}
+
+		result = potentiallyConvertResult(resultType, result);
+
+		if (applyWrapper) {
+			return conversionService.convert(new NullableWrapper(result), typeToReturn.getType());
+		}
+
+		return result;
+	}
+
+	@Nullable
+	protected Object potentiallyConvertResult(TypeInformation<?> type, @Nullable Object result) {
 
 		if (result == null) {
 			return null;
 		}
 
-		TypeInformation<?> type = ClassTypeInformation.fromReturnTypeOf(invocation.getMethod());
-		Class<?> rawType = type.getType();
-
-		if (type.isCollectionLike() && !ClassUtils.isPrimitiveArray(rawType)) {
+		if (type.isCollectionLike() && !ClassUtils.isPrimitiveArray(type.getType())) {
 			return projectCollectionElements(asCollection(result), type);
 		} else if (type.isMap()) {
 			return projectMapValues((Map<?, ?>) result, type);
-		} else if (conversionRequiredAndPossible(result, rawType)) {
-			return conversionService.convert(result, rawType);
+		} else if (conversionRequiredAndPossible(result, type.getType())) {
+			return conversionService.convert(result, type.getType());
 		} else {
-			return getProjection(result, rawType);
+			return getProjection(result, type.getType());
 		}
 	}
 

src/main/java/org/springframework/data/projection/ProxyProjectionFactory.java

@@ -26,8 +26,9 @@ import org.aopalliance.intercept.MethodInvocation;
 import org.springframework.aop.framework.Advised;
 import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.beans.factory.BeanClassLoaderAware;
-import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.support.DefaultConversionService;
+import org.springframework.core.convert.support.GenericConversionService;
+import org.springframework.data.util.NullableWrapperConverters;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
@@ -47,8 +48,14 @@ import org.springframework.util.ConcurrentReferenceHashMap;
  */
 class ProxyProjectionFactory implements ProjectionFactory, BeanClassLoaderAware {
 
+	final static GenericConversionService CONVERSION_SERVICE = new DefaultConversionService();
+
+	static {
+		NullableWrapperConverters.registerConvertersIn(CONVERSION_SERVICE);
+		CONVERSION_SERVICE.removeConvertible(Object.class, Object.class);
+	}
+
 	private final List<MethodInterceptorFactory> factories;
-	private final ConversionService conversionService;
 	private final Map<Class<?>, ProjectionInformation> projectionInformationCache = new ConcurrentReferenceHashMap<>();
 	private @Nullable ClassLoader classLoader;
 
@@ -60,8 +67,6 @@ class ProxyProjectionFactory implements ProjectionFactory, BeanClassLoaderAware
 		this.factories = new ArrayList<>();
 		this.factories.add(MapAccessingMethodInterceptorFactory.INSTANCE);
 		this.factories.add(PropertyAccessingMethodInvokerFactory.INSTANCE);
-
-		this.conversionService = DefaultConversionService.getSharedInstance();
 	}
 
 	/*
@@ -174,7 +179,7 @@ class ProxyProjectionFactory implements ProjectionFactory, BeanClassLoaderAware
 				.createMethodInterceptor(source, projectionType);
 
 		return new ProjectingMethodInterceptor(this,
-				postProcessAccessorInterceptor(propertyInvocationInterceptor, source, projectionType), conversionService);
+				postProcessAccessorInterceptor(propertyInvocationInterceptor, source, projectionType), CONVERSION_SERVICE);
 	}
 
 	/**

src/test/java/org/springframework/data/projection/ProjectingMethodInterceptorUnitTests.java

@@ -74,12 +74,11 @@ class ProjectingMethodInterceptorUnitTests {
 		assertThat(methodInterceptor.invoke(invocation)).isEqualTo("Foo");
 	}
 
-	@Test // DATAREST-221
+	@Test // DATAREST-221, DATACMNS-1762
 	void returnsNullAsIs() throws Throwable {
 
 		MethodInterceptor methodInterceptor = new ProjectingMethodInterceptor(factory, interceptor, conversionService);
-
-		when(interceptor.invoke(invocation)).thenReturn(null);
+		mockInvocationOf("getString", null);
 
 		assertThat(methodInterceptor.invoke(invocation)).isNull();
 	}