Synchronize RedisCache.get(…) with ValueLoader only if value is absent.

RedisCache.get(…) now optimistically fetches the cache value before entering cache-wide synchronization. The previous version synchronized all calls to `get(key, valueLoader)`.

Closes #2079
Original pull request: #2082.

src/main/java/org/springframework/data/redis/cache/RedisCache.java

@@ -44,6 +44,7 @@ import org.springframework.util.ReflectionUtils;
  *
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Piotr Mionskowski
  * @see RedisCacheConfiguration
  * @see RedisCacheWriter
  * @since 2.0
@@ -118,7 +119,7 @@ public class RedisCache extends AbstractValueAdaptingCache {
 	 */
 	@Override
 	@SuppressWarnings("unchecked")
-	public synchronized <T> T get(Object key, Callable<T> valueLoader) {
+	public <T> T get(Object key, Callable<T> valueLoader) {
 
 		ValueWrapper result = get(key);
 
@@ -126,6 +127,17 @@ public class RedisCache extends AbstractValueAdaptingCache {
 			return (T) result.get();
 		}
 
+		return getSynchronized(key, valueLoader);
+	}
+
+	@SuppressWarnings("unchecked")
+	private synchronized <T> T getSynchronized(Object key, Callable<T> valueLoader) {
+		ValueWrapper result = get(key);
+
+		if (result != null) {
+			return (T) result.get();
+		}
+
 		T value = valueFromLoader(key, valueLoader);
 		put(key, value);
 		return value;