DATACMNS-390 - Create defensive copy for persistent entities in AbstractMappingContext.

To prevent ConcurrentModificationExceptions when iterating over PersistentEntities while further entity types are potentially added to the MappingContext we now return a immutable defensive copy of the current entity set.

src/main/java/org/springframework/data/mapping/context/AbstractMappingContext.java

@@ -124,7 +124,7 @@ public abstract class AbstractMappingContext<E extends MutablePersistentEntity<?
 	public Collection<E> getPersistentEntities() {
 		try {
 			read.lock();
-			return persistentEntities.values();
+			return Collections.unmodifiableSet(new HashSet<E>(persistentEntities.values()));
 		} finally {
 			read.unlock();
 		}

src/test/java/org/springframework/data/mapping/context/AbstractMappingContextUnitTests.java

@@ -21,6 +21,7 @@ import static org.mockito.Mockito.*;
 import groovy.lang.MetaClass;
 
 import java.util.Collections;
+import java.util.Iterator;
 import java.util.List;
 
 import org.junit.Before;
@@ -34,6 +35,7 @@ import org.springframework.data.mapping.PropertyPath;
 import org.springframework.data.mapping.model.BasicPersistentEntity;
 import org.springframework.data.mapping.model.MappingException;
 import org.springframework.data.mapping.model.SimpleTypeHolder;
+import org.springframework.data.util.ClassTypeInformation;
 import org.springframework.data.util.TypeInformation;
 
 /**
@@ -193,6 +195,24 @@ public class AbstractMappingContextUnitTests {
 		context.getPersistentPropertyPath("foo", Sample.class);
 	}
 
+	/**
+	 * @see DATACMNS-390
+	 */
+	@Test
+	public void exposesCopyOfPersistentEntitiesToAvoidConcurrentModificationException() {
+
+		SampleMappingContext context = new SampleMappingContext();
+		context.getPersistentEntity(ClassTypeInformation.MAP);
+
+		Iterator<BasicPersistentEntity<Object, SamplePersistentProperty>> iterator = context.getPersistentEntities()
+				.iterator();
+
+		while (iterator.hasNext()) {
+			context.getPersistentEntity(ClassTypeInformation.SET);
+			iterator.next();
+		}
+	}
+
 	class Person {
 		String name;
 	}