批处理10条记录Java，JPA

Question

I am trying to fetch10 records at a time . The problem is the code just fetches first 10 records and last 4 records. It skips the in-between 10 records.
Considering that DB has 24 records in the table. The following program should fetch 24 records

SelectQuery: SELECT l FROM Object l where l.processed='N' fetchObjectsCount query : Select count(*) from Object obj where l.processed='N'

   private static Integer MAX_RESULT = 10;
     private DataExtraction objectExtraction(){
        int count = leadRepo.fetchObjectsCount();
        Query query = null;
        for (int i = 0; i < count; i++){
          if (i % MAX_RESULT == 0 ){
           query=entityManager.createNamedQuery("SelectQuery").setFirstResult(i).setMaxResults(MAX_RESULT);
           List<Object> tempList = (List<Object>) query.getResultList();
           entityManager.getTransaction().begin();

           for (Object ob : tempList){
              ob .setProcessed("Y");
              entityManager.persist(ob );                   
             }
             entityManager.getTransaction().commit();
             i = i+9;
        }

        if (i % MAX_RESULT < 1 && count - i <= MAX_RESULT){
        query = entityManager.createNamedQuery("SelectQuery").setFirstResult(i).setMaxResults(count-i);
        List<Object> tempList = (List<Object>) query.getResultList();
        entityManager.getTransaction().begin();

        for (Object ob : tempList){
        ob .setProcessed("Y");
        entityManager.persist(ob );                 
        }

        entityManager.getTransaction().commit();
      }
     }
     return this;

     }  

Answer 1

Given that you're fetching 10 at a time and you have 24 items in the db that code will only fetch two out of the required 3 times.

First iteration: retrieve 10 rows, i is set to 10
Second iteration: retrieve 10 rows, i is set to > 10
Third iteration: won't happen

Here is how I would go about this:

int count = leadRepo.fetchObjectsCount();
int numberProcessed = 0;
Integer MAX_RESULT = 10;
query = entityManager.createNamedQuery("SelectQuery");


while(numberProcessed < count) {

    query.setFirstResult(numberProcessed).setMaxResults(MAX_RESULT);
    List<Object> tempList = (List<Object>) query.getResultList();
    entityManager.getTransaction().begin();

    for (Object ob : tempList) {
        ob.setProcessed("Y");
        entityManager.merge(ob);  
    }

    entityManager.getTransaction().commit();
    objectList.addAll(tempList);
    numberProcessed += tempList.size();
}

This code assumes that you want to process all rows retrieved. If your named query is pulling out a different number of rows than fetchObjectsCount(); this should be written differently.

Answer 2

I think the problem is that you're processing the first ten records on your first iteration (because 0 % 10 == 0), then at the bottom of that loop you're incrementing i by 9. The loops starts again, incrementing i to 10, which again triggers the record processing loop. Now you're selecting the unprocessed records (only 14 now, since you've updated the first ten), skipping the first ten, and getting the last four. You might try starting your loop with 1 and see if that gives the results you expect.