DATAMONGO-535 - Fixed broken MongoDBUtils resource synchronization.

We now make sure we really re-use the database instance bound to the thread avoiding duplicate lookups of Mongo.getDb(…). This doesn't seem to gain a big performance benefit anymore as the Mongo instance caches the DB instances internally anyway.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoDbUtils.java

@@ -79,14 +79,12 @@ public abstract class MongoDbUtils {
 
 		DbHolder dbHolder = (DbHolder) TransactionSynchronizationManager.getResource(mongo);
 
-		if (dbHolder != null && !dbHolder.isEmpty()) {
+		// Do we have a populated holder and TX sync active?
+		if (dbHolder != null && !dbHolder.isEmpty() && TransactionSynchronizationManager.isSynchronizationActive()) {
 
-			DB db = null;
-
-			if (TransactionSynchronizationManager.isSynchronizationActive() && dbHolder.doesNotHoldNonDefaultDB()) {
-
-				db = dbHolder.getDB(databaseName);
+			DB db = dbHolder.getDB(databaseName);
 
+			// DB found but not yet synchronized
 			if (db != null && !dbHolder.isSynchronizedWithTransaction()) {
 
 				LOGGER.debug("Registering Spring transaction synchronization for existing MongoDB {}.", databaseName);
@@ -94,13 +92,13 @@ public abstract class MongoDbUtils {
 				TransactionSynchronizationManager.registerSynchronization(new MongoSynchronization(dbHolder, mongo));
 				dbHolder.setSynchronizedWithTransaction(true);
 			}
-			}
 
 			if (db != null) {
 				return db;
 			}
 		}
 
+		// Lookup fresh database instance
 		LOGGER.debug("Getting Mongo Database name=[{}]", databaseName);
 
 		DB db = mongo.getDB(databaseName);
@@ -117,8 +115,7 @@ public abstract class MongoDbUtils {
 			}
 		}
 
-		// Use same Session for further Mongo actions within the transaction.
-		// Thread object will get removed by synchronization at transaction completion.
+		// TX sync active, bind new database to thread
 		if (TransactionSynchronizationManager.isSynchronizationActive()) {
 
 			LOGGER.debug("Registering Spring transaction synchronization for MongoDB instance {}.", databaseName);

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/MongoDbUtilsUnitTests.java

@@ -17,10 +17,17 @@ package org.springframework.data.mongodb.core;
 
 import static org.hamcrest.CoreMatchers.*;
 import static org.junit.Assert.*;
+import static org.mockito.Matchers.*;
+import static org.mockito.Mockito.*;
 
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.invocation.InvocationOnMock;
+import org.mockito.runners.MockitoJUnitRunner;
+import org.mockito.stubbing.Answer;
 import org.springframework.transaction.support.TransactionSynchronizationManager;
 
 import com.mongodb.DB;
@@ -31,13 +38,21 @@ import com.mongodb.Mongo;
  * 
  * @author Oliver Gierke
  */
+@RunWith(MockitoJUnitRunner.class)
 public class MongoDbUtilsUnitTests {
 
+	@Mock
 	Mongo mongo;
 
 	@Before
 	public void setUp() throws Exception {
-		this.mongo = new Mongo();
+
+		when(mongo.getDB(anyString())).then(new Answer<DB>() {
+			public DB answer(InvocationOnMock invocation) throws Throwable {
+				return mock(DB.class);
+			}
+		});
+
 		TransactionSynchronizationManager.initSynchronization();
 	}
 
@@ -55,11 +70,15 @@ public class MongoDbUtilsUnitTests {
 	public void returnsNewInstanceForDifferentDatabaseName() {
 
 		DB first = MongoDbUtils.getDB(mongo, "first");
-		assertThat(first, is(notNullValue()));
-		assertThat(MongoDbUtils.getDB(mongo, "first"), is(first));
-
 		DB second = MongoDbUtils.getDB(mongo, "second");
 		assertThat(second, is(not(first)));
-		assertThat(MongoDbUtils.getDB(mongo, "second"), is(second));
+	}
+
+	@Test
+	public void returnsSameInstanceForSameDatabaseName() {
+
+		DB first = MongoDbUtils.getDB(mongo, "first");
+		assertThat(first, is(notNullValue()));
+		assertThat(MongoDbUtils.getDB(mongo, "first"), is(sameInstance(first)));
 	}
 }