DATACASS-297 - Add support for streaming queries to select entities.

We now support java.util.stream.Stream as return type on query methods. Streaming queries wrap the iterator returned by Cassandra's ResultSet. Elements are fetched in batches (see QueryOptions that can be set in CassandraCqlClusterFactoryBean) and processed element by element instead of reading all results. Streaming queries are supported only for select queries.

Original pull request: #65.

spring-cql/src/main/java/org/springframework/cassandra/core/CqlTemplate.java

@@ -52,6 +52,7 @@ import org.springframework.dao.DataAccessException;
 import org.springframework.dao.IncorrectResultSizeDataAccessException;
 import org.springframework.dao.InvalidDataAccessApiUsageException;
 import org.springframework.dao.QueryTimeoutException;
+import org.springframework.dao.support.PersistenceExceptionTranslator;
 import org.springframework.util.Assert;
 
 import com.datastax.driver.core.BoundStatement;
@@ -854,19 +855,49 @@ public class CqlTemplate extends CassandraAccessor implements CqlOperations {
 	}
 
 	/**
-	 * Attempts to translate the {@link RuntimeException} into a Spring Data {@link Exception}.
+	 * Attempts to translate the {@link Exception} into a Spring Data {@link Exception}.
+	 * @param ex the Exception
+	 * @return the translated {@link RuntimeException}
 	 */
 	@SuppressWarnings("all")
-	protected RuntimeException translateExceptionIfPossible(RuntimeException e) {
-
-		RuntimeException resolved = getExceptionTranslator().translateExceptionIfPossible(e);
-		return (resolved != null ? resolved : e);
+	protected RuntimeException translateExceptionIfPossible(Exception ex) {
+		return translateExceptionIfPossible(ex, getExceptionTranslator());
 	}
 
+	/**
+	 * Tries to convert the given {@link RuntimeException} into a {@link DataAccessException} but returns the original
+	 * exception if the conversation failed. Thus allows safe re-throwing of the return value.
+	 *
+	 * @param ex the exception to translate
+	 * @param exceptionTranslator the {@link PersistenceExceptionTranslator} to be used for translation
+	 * @return
+	 */
 	@SuppressWarnings("all")
-	protected RuntimeException translateExceptionIfPossible(Exception e) {
-		return (e instanceof RuntimeException ? translateExceptionIfPossible((RuntimeException) e)
-				: new CassandraUncategorizedDataAccessException("Caught Uncategorized Exception", e));
+	protected static RuntimeException translateExceptionIfPossible(Exception ex, PersistenceExceptionTranslator exceptionTranslator) {
+
+		Assert.notNull(ex, "Exception must not be null");
+		Assert.notNull(exceptionTranslator, "PersistenceExceptionTranslator must not be null");
+
+		if (ex instanceof RuntimeException) {
+			return potentiallyConvertRuntimeException((RuntimeException) ex, exceptionTranslator);
+		}
+
+		return new CassandraUncategorizedDataAccessException("Caught Uncategorized Exception", ex);
+	}
+
+	/**
+	 * Tries to convert the given {@link RuntimeException} into a {@link DataAccessException} but returns the original
+	 * exception if the conversation failed. Thus allows safe re-throwing of the return value.
+	 *
+	 * @param ex the exception to translate
+	 * @param exceptionTranslator the {@link PersistenceExceptionTranslator} to be used for translation
+	 * @return
+	 */
+	private static RuntimeException potentiallyConvertRuntimeException(RuntimeException ex,
+			PersistenceExceptionTranslator exceptionTranslator) {
+
+		RuntimeException resolved = exceptionTranslator.translateExceptionIfPossible(ex);
+		return resolved == null ? ex : resolved;
 	}
 
 	@Override