Edit ref docs and Javadoc for connection classes to clearly state the Thread-safety guarantees.

Closes #2653
Original pull request: #2667

src/main/java/org/springframework/data/redis/connection/RedisConnection.java

@@ -27,6 +27,10 @@ import org.springframework.dao.DataAccessException;
  * <p>
  * Additionally, performs exception translation between the underlying Redis client library and Spring DAO exceptions.
  * The methods follow as much as possible the Redis names and conventions.
+ * <p>
+ * Spring Data Redis {@link RedisConnection connections}, unlike perhaps their underlying native connection (for example:
+ * the Lettuce {@literal StatefulRedisConnection}) are not Thread-safe. Please refer to the corresponding the Javadoc
+ * for Redis client library (driver) specific connections provided by Spring Data Redis for more details.
  *
  * @author Costin Leau
  * @author Christoph Strobl

src/main/java/org/springframework/data/redis/connection/jedis/JedisConnection.java

@@ -59,6 +59,11 @@ import org.springframework.util.CollectionUtils;
 
 /**
  * {@code RedisConnection} implementation on top of <a href="https://github.com/redis/jedis">Jedis</a> library.
+ * <p>
+ * WARNING: The {@link JedisConnection} class is not Thread-safe. This class requires and uses a
+ * {@literal Jedis} instance from the Jedis client library (driver), which is very clearly
+ * <a href="https://github.com/redis/jedis/wiki/Getting-started#using-jedis-in-a-multithreaded-environment">documented</a>
+ * as not Thread-safe.
  *
  * @author Costin Leau
  * @author Jennifer Hickey
@@ -72,6 +77,8 @@ import org.springframework.util.CollectionUtils;
  * @author Ninad Divadkar
  * @author Guy Korland
  * @author Dengliming
+ * @author John Blum
+ * @see redis.clients.jedis.Jedis
  */
 public class JedisConnection extends AbstractRedisConnection {
 

src/main/java/org/springframework/data/redis/connection/lettuce/LettuceConnection.java

@@ -79,6 +79,13 @@ import org.springframework.util.ObjectUtils;
 /**
  * {@code RedisConnection} implementation on top of <a href="https://github.com/mp911de/lettuce">Lettuce</a> Redis
  * client.
+ * <p>
+ * WARNING: While the underlying Lettuce {@literal RedisClient} and {@literal StatefulRedisConnection} instances used by
+ * {@link LettuceConnection} are Thread-safe, this class itself is not Thread-safe. Therefore, instances of {@link LettuceConnection}
+ * should not be shared across multiple Threads when executing Redis commands and other operations. If optimal performance
+ * is required by your application(s), then we recommend direct access to the low-level, API provided by the underlying
+ * Lettuce client library (driver), where such Thread-safety guarantees can be made. Simply call {@link #getNativeConnection()}
+ * and use the native resource as required.
  *
  * @author Costin Leau
  * @author Jennifer Hickey
@@ -89,6 +96,7 @@ import org.springframework.util.ObjectUtils;
  * @author Ninad Divadkar
  * @author Tamil Selvan
  * @author ihaohong
+ * @author John Blum
  */
 public class LettuceConnection extends AbstractRedisConnection {