How to use multi-thread consumer in kafka 0.9.0?

Question

The doc of kafka give an approach about with following describes:

One Consumer Per Thread:A simple option is to give each thread its own consumer > in

Answer 1

Kafka consumer is not thread safe. As you pointed out in your question, the document stated that

A simple option is to give each thread its own consumer instance

But in your code, you have the same consumer instance wrapped by different KafkaConsumerRunner instances. Thus multiple threads are accessing the same consumer instance. The kafka documentation clearly stated

The Kafka consumer is NOT thread-safe. All network I/O happens in the thread of the application making the call. It is the responsibility of the user to ensure that multi-threaded access is properly synchronized. Un-synchronized access will result in ConcurrentModificationException.

That's exactly the exception you received.