Fix potential NullPointerException in ProjectionSerializer.

Fixes GH-1947.

Author: odrotbohm

spring-data-rest-core/src/main/java/org/springframework/data/rest/core/mapping/ResourceMappings.java

@@ -16,6 +16,7 @@
 package org.springframework.data.rest.core.mapping;
 
 import org.springframework.data.util.Streamable;
+import org.springframework.lang.Nullable;
 
 /**
  * @author Oliver Gierke
@@ -28,6 +29,7 @@ public interface ResourceMappings extends Streamable<ResourceMetadata> {
 	 * @param type must not be {@literal null}.
 	 * @return the {@link ResourceMetadata} if available or {@literal null} otherwise.
 	 */
+	@Nullable
 	ResourceMetadata getMetadataFor(Class<?> type);
 
 	/**

spring-data-rest-webmvc/src/main/java/org/springframework/data/rest/webmvc/json/PersistentEntityJackson2Module.java

@@ -34,15 +34,11 @@
 import org.springframework.data.mapping.PersistentProperty;
 import org.springframework.data.mapping.context.PersistentEntities;
 import org.springframework.data.projection.TargetAware;
-import org.springframework.data.repository.support.Repositories;
 import org.springframework.data.repository.support.RepositoryInvoker;
 import org.springframework.data.repository.support.RepositoryInvokerFactory;
 import org.springframework.data.rest.core.UriToEntityConverter;
-import org.springframework.data.rest.core.config.RepositoryRestConfiguration;
-import org.springframework.data.rest.core.mapping.ResourceMappings;
 import org.springframework.data.rest.core.mapping.ResourceMetadata;
 import org.springframework.data.rest.core.support.EntityLookup;
-import org.springframework.data.rest.core.support.SelfLinkProvider;
 import org.springframework.data.rest.webmvc.EmbeddedResourcesAssembler;
 import org.springframework.data.rest.webmvc.PersistentEntityResource;
 import org.springframework.data.rest.webmvc.mapping.Associations;
@@ -649,7 +645,7 @@ static class ProjectionSerializer extends StdSerializer<TargetAware> {
 		 * @param invoker must not be {@literal null}.
 		 * @param unwrapping
 		 */
-		private ProjectionSerializer(LinkCollector collector, Associations mappings,
+		ProjectionSerializer(LinkCollector collector, Associations mappings,
 				RepresentationModelProcessorInvoker invoker, boolean unwrapping) {
 
 			super(TargetAware.class);
@@ -694,11 +690,11 @@ public JsonSerializer<TargetAware> unwrappingSerializer(NameTransformer unwrappe
 		 * @param value must not be {@literal null}.
 		 * @return
 		 */
-		private ProjectionResource toModel(TargetAware value) {
+		ProjectionResource toModel(TargetAware value) {
 
 			Object target = value.getTarget();
 			ResourceMetadata metadata = associations.getMetadataFor(value.getTargetClass());
-			Links links = metadata.isExported() ? collector.getLinksFor(target) : Links.NONE;
+			Links links = metadata != null && metadata.isExported() ? collector.getLinksFor(target) : Links.NONE;
 
 			EntityModel<TargetAware> resource = invoker.invokeProcessorsFor(EntityModel.of(value, links));
 
@@ -708,7 +704,6 @@ private ProjectionResource toModel(TargetAware value) {
 
 	static class ProjectionResource extends EntityModel<ProjectionResourceContent> {
 
-		@SuppressWarnings("deprecation")
 		ProjectionResource(TargetAware projection, Iterable<Link> links) {
 			super(new ProjectionResourceContent(projection, projection.getClass().getInterfaces()[0]), links);
 		}

spring-data-rest-webmvc/src/main/java/org/springframework/data/rest/webmvc/mapping/Associations.java

@@ -33,6 +33,7 @@
 import org.springframework.hateoas.TemplateVariable;
 import org.springframework.hateoas.TemplateVariables;
 import org.springframework.hateoas.UriTemplate;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -94,6 +95,7 @@ public List<Link> getLinksFor(Association<? extends PersistentProperty<?>> assoc
 	 * @param type must not be {@literal null}.
 	 * @return
 	 */
+	@Nullable
 	public ResourceMetadata getMetadataFor(Class<?> type) {
 
 		Assert.notNull(type, "Type must not be null");

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/json/PersistentEntityJackson2ModuleUnitTests.java

@@ -36,6 +36,7 @@
 import org.springframework.data.keyvalue.core.mapping.context.KeyValueMappingContext;
 import org.springframework.data.mapping.PersistentProperty;
 import org.springframework.data.mapping.context.PersistentEntities;
+import org.springframework.data.projection.TargetAware;
 import org.springframework.data.repository.support.RepositoryInvoker;
 import org.springframework.data.repository.support.RepositoryInvokerFactory;
 import org.springframework.data.rest.core.UriToEntityConverter;
@@ -49,8 +50,10 @@
 import org.springframework.data.rest.webmvc.json.PersistentEntityJackson2Module.LookupObjectSerializer;
 import org.springframework.data.rest.webmvc.json.PersistentEntityJackson2Module.NestedEntitySerializer;
 import org.springframework.data.rest.webmvc.mapping.Associations;
+import org.springframework.data.rest.webmvc.mapping.DefaultLinkCollector;
 import org.springframework.data.rest.webmvc.support.ExcerptProjector;
 import org.springframework.hateoas.EntityModel;
+import org.springframework.hateoas.RepresentationModel;
 import org.springframework.hateoas.UriTemplate;
 import org.springframework.hateoas.server.EntityLinks;
 import org.springframework.hateoas.server.mvc.RepresentationModelProcessorInvoker;
@@ -260,6 +263,19 @@ void doesNotWrapValuesWithoutUnwrappingSerializer() {
 		assertThatNoException().isThrownBy(() -> mapper.writeValueAsString(model));
 	}
 
+	@Test // GH-1947
+	void projectionsHandleMissingMetadata() {
+
+		var collector = new DefaultLinkCollector(persistentEntities, selfLinks, associations);
+		var invoker = mock(RepresentationModelProcessorInvoker.class);
+		when(invoker.invokeProcessorsFor(any(RepresentationModel.class)))
+				.then(invocation -> invocation.getArgument(0));
+
+		var serializer = new PersistentEntityJackson2Module.ProjectionSerializer(collector, associations, invoker, false);
+
+		assertThatNoException().isThrownBy(() -> serializer.toModel(mock(SampleProjection.class)));
+	}
+
 	/**
 	 * @author Oliver Gierke
 	 */
@@ -387,4 +403,6 @@ public void serialize(CustomType value, JsonGenerator gen, SerializerProvider pr
 			gen.writeEndObject();
 		}
 	}
+
+	interface SampleProjection extends TargetAware {}
 }