title: spring batch itemreader read multiple lines and dynamic chunk size
body: <p>there is one case to handle more than 3M records which in the flat file, and we want to read 3k records one time and then process it to writer, is there any better way to process that? beside that, we have some other conditions, since our flat file format like below:</p>
  
  <p>userid1, transaction1
  userid1, transaction2
  userid1, transaction3
  userid2, transaction1
  userid2, transaction3</p>
  
  <p>one user may have many transactions, the item reader will read the transaction for one user to different threads, we want to put all the transaction for this user in the same thread, so we want to dynamically change the chunksize, so is there any better way for that?</p>