java.lang.ClassCastException: [B cannot be cast to java.util.List when using Redis

Question

I have the following code :

jedis.mget(objects.toArray(new String[objects.size()]));

where objects is a list of string. The code runs fine most of the time. But unexpectedly raises following exception.

java.lang.ClassCastException: [B cannot be cast to java.util.List
at redis.clients.jedis.Connection.getBinaryMultiBulkReply(Connection.java:221)
at redis.clients.jedis.Connection.getMultiBulkReply(Connection.java:214)
at redis.clients.jedis.Jedis.mget(Jedis.java:383)

If i run the same code again it runs fine with same data. Can't understand the reason for such issue.

Answer 1

TL;DR

Jedis instances are not thread-safe, calls made to a Jedis instance across multiple threads will result in these types of errors. Try switching to use a JedisPool to manage the creation of connections.

Explanation

I was seeing these exact same exceptions and while trying to figure out the problem I came across a similar issue on the Jedis project explaining Jedis is not thread-safe.

Since I found this question when searching for this same exception I will share some details of my particular case so it may help future readers.

In my particular case I calling the SCAN command using RedisTemplate from Spring Data Redis. I was making calls to the resulting Cursor outside of the connections lifecycle.

My original code looked like this.

public Long size() {
    Cursor<byte[]> scan =  redisTemplate.execute((RedisConnection connection) -> connection.scan(ScanOptions.scanOptions().match(prefix + "*").build()));
    AtomicLong count = new AtomicLong();
    scan.forEachRemaining(bytes -> count.getAndIncrement());
    return count.get();
}

After a minor change I found the correct way to do this is by simply moving the cursor interactions inside the execute lambda.

public Long size() {
    return redisTemplate.execute((RedisConnection connection) -> {
        Cursor<byte[]> scan = connection.scan(ScanOptions.scanOptions().match(prefix + "*").build());
        AtomicLong count = new AtomicLong();
        scan.forEachRemaining(bytes -> count.getAndIncrement());
        return count.get();
    });
}