ConfigResource topicResource = new ConfigResource(Type.TOPIC, null);
AdminClient kafkaAdminClient = new KafkaAdminClient(...);
Map<ConfigResource, Config> configs =  kafkaAdminClient.describeConfigs(resources).all().get(15, TimeUnit.MILLISECONDS);
System.out.printlin(configs.toString());
...
{ConfigResource{type=TOPIC, name='null'}=Config(entries=[ConfigEntry(name=flush.messages, value=9223372036854775807, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=message.timestamp.type, value=CreateTime, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=preallocate, value=false, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=cleanup.policy, value=compact,delete, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=segment.bytes, value=1073741824, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=delete.retention.ms, value=86400000, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=segment.ms, value=604800000, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=min.insync.replicas, value=1, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=file.delete.delay.ms, value=60000, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=retention.ms, value=604800000, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=max.message.bytes, value=1000012, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=message.format.version, value=1.0-IV0, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=index.interval.bytes, value=4096, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=retention.bytes, value=-1, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=segment.index.bytes, value=10485760, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=segment.jitter.ms, value=0, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=compression.type, value=producer, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=min.cleanable.dirty.ratio, value=0.5, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=min.compaction.lag.ms, value=0, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=unclean.leader.election.enable, value=false, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=message.timestamp.difference.max.ms, value=9223372036854775807, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=flush.ms, value=9223372036854775807, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=follower.replication.throttled.replicas, value=, isDefault=true, isSensitive=false, isReadOnly=false), ConfigEntry(name=leader.replication.throttled.replicas, value=, isDefault=true, isSensitive=false, isReadOnly=false)])}