DATAJPA-949 - Added support for JPA 2.2-native stream execution.

We're now discovering JPA 2.2 on the classpath and invoke Query.getResultStream() directly in favor of our own implementation based on individual persistence provider APIs. We also use the method it it is already declared on their JPA 2.1 Query implementation. That allows us to use e.g. Hibernate 5.2's implementation of it, even if it is still compiled against JPA 2.1.

src/main/java/org/springframework/data/jpa/repository/query/JpaQueryExecution.java

@@ -15,6 +15,7 @@
  */
 package org.springframework.data.jpa.repository.query;
 
+import java.lang.reflect.Method;
 import java.util.Collection;
 import java.util.List;
 import java.util.Optional;
@@ -42,6 +43,7 @@ import org.springframework.data.util.StreamUtils;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.ReflectionUtils;
 
 /**
  * Set of classes to contain query execution strategies. Depending (mostly) on the return type of a
@@ -330,6 +332,9 @@ public abstract class JpaQueryExecution {
 
 		private static final String NO_SURROUNDING_TRANSACTION = "You're trying to execute a streaming query method without a surrounding transaction that keeps the connection open so that the Stream can actually be consumed. Make sure the code consuming the stream uses @Transactional or any other way of declaring a (read-only) transaction.";
 
+		private static Method streamMethod = ReflectionUtils.findMethod(Query.class, "getResultStream");
+		private static boolean dynamicCheck = streamMethod == null;
+
 		/*
 		 * (non-Javadoc)
 		 * @see org.springframework.data.jpa.repository.query.JpaQueryExecution#doExecute(org.springframework.data.jpa.repository.query.AbstractJpaQuery, java.lang.Object[])
@@ -342,6 +347,33 @@ public abstract class JpaQueryExecution {
 			}
 
 			Query jpaQuery = query.createQuery(values);
+
+			// JPA 2.2 on the classpath
+			if (streamMethod != null) {
+				return ReflectionUtils.invokeMethod(streamMethod, jpaQuery);
+			}
+
+			if (dynamicCheck) {
+
+				Method method = ReflectionUtils.findMethod(jpaQuery.getClass(), "getResultStream");
+
+				// Implementation available but on JPA 2.1
+				if (method != null) {
+
+					// Cache for subsequent reuse to prevent repeated reflection lookups
+					streamMethod = method;
+
+					return ReflectionUtils.invokeMethod(method, jpaQuery);
+
+				} else {
+
+					// Not available on implementation, skip further lookups
+					dynamicCheck = false;
+				}
+			}
+
+			// Fall back to legacy stream execution
+
 			PersistenceProvider persistenceProvider = PersistenceProvider.fromEntityManager(query.getEntityManager());
 			CloseableIterator<Object> iter = persistenceProvider.executeQueryWithResultStream(jpaQuery);