DATAJPA-658 - Fixed potential NullPointerException in JpaMetamodel.

When iterating over EntityType<?> we need to guard against ….getJavaType() returning null. That seems to be the case for Hibernate Envers which apparently registers EntityType instances without a backing type.

Original pull request: #146.

src/main/java/org/springframework/data/jpa/util/JpaMetamodel.java

@@ -76,7 +76,7 @@ public class JpaMetamodel {
 	public boolean isSingleIdAttribute(Class<?> entity, String name, Class<?> attributeType) {
 
 		return metamodel.getEntities().stream() //
-				.filter(it -> it.getJavaType().equals(entity)) //
+				.filter(it -> entity.equals(it.getJavaType())) //
 				.findFirst() //
 				.flatMap(it -> getSingularIdAttribute(it)) //
 				.filter(it -> it.getJavaType().equals(attributeType)) //

src/test/java/org/springframework/data/jpa/util/JpaMetamodelUnitTests.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2018 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.jpa.util;
+
+import static org.assertj.core.api.Assertions.*;
+import static org.mockito.Mockito.*;
+
+import java.util.Collections;
+
+import javax.persistence.metamodel.EntityType;
+import javax.persistence.metamodel.Metamodel;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+
+/**
+ * Unit tests for {@link JpaMetamodel}.
+ * 
+ * @author Oliver Gierke
+ */
+@RunWith(MockitoJUnitRunner.class)
+public class JpaMetamodelUnitTests {
+
+	@Mock Metamodel metamodel;
+
+	@Mock EntityType<?> type;
+
+	@Test
+	public void skipsEntityTypesWithoutJavaTypeForIdentifierLookup() {
+
+		doReturn(Collections.singleton(type)).when(metamodel).getEntities();
+
+		assertThat(new JpaMetamodel(metamodel).isSingleIdAttribute(Object.class, "id", Object.class)).isFalse();
+	}
+}