Page History

...

Lazy initialization for RecordHeader was introduced in KAFKA-10438, improving performance but also creating unexpected side effects.
Since the Consumer is not thread-safe, the same assumption naturally extends to ConsumerRecord. However, users often assume that read-only access across threads is safe.
With lazy initialization, this assumption no longer holds, which can lead to unexpected behaviorand users may encounter NullPointerException.
So far, three concurrency-related issues (KAFKA-12999, KAFKA-17725, KAFKA-18470) have been reported in connection with RecordHeader data access.
We can ensure that users have thread-safety guarantees when accessing RecordHeader in a read-only manner, rather than risking a eliminating the risk of NullPointerException.
(Note that because the Consumer is not thread-safe, if other threads can modify the RecordHeader through the Consumer, its thread-safety can no longer be guaranteed.)

...

Use double-checked locking and volatile to make RecordHeader thread-safe.
This ensures that synchronization happens only during initialization, and once initialized, subsequent accesses do not acquire any locks.
Note that valueBuffer should also be declared volatile to reduce unnecessary synchronization attempts.

...