JPA で大きなデータをループする。あとセッションキャッシュをなんとかする必要がある。

gistfile1.java

package akr.emp.service;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import javax.persistence.EntityManager;
import javax.persistence.TypedQuery;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

public class LazyFetchIterator<T> implements Iterator<T> {

    private static final Log LOG = LogFactory.getLog(LazyFetchIterator.class);

    private final EntityManager em;

    private final TypedQuery<T> query;

    private final long totalSize;

    private final int pageSize;

    private int currentIndex = 0;

    private List<T> fetched;

    private int fetchedListIndex;

    public LazyFetchIterator(EntityManager em, TypedQuery<T> query, long totalSize, int pageSize) {
        this.em = em;
        this.query = query;
        this.totalSize = totalSize;
        this.pageSize = pageSize;
    }

    @Override
    public boolean hasNext() {
        return currentIndex < totalSize - 1;
    }

    @Override
    public T next() {
        if (fetched == null || fetchedListIndex >= fetched.size() - 1) {
            fetchNext();
        }
        if (fetchedListIndex < fetched.size() - 1) {
            return getNext();
        }
        throw new NoSuchElementException();
    }

    private void fetchNext() {
        em.clear();
        fetched = query.setFirstResult(currentIndex).setMaxResults(pageSize).getResultList();
        fetchedListIndex = 0;
        currentIndex += fetched.size();
        if (currentIndex % 100000 == 0)
            LOG.debug("currentIndex=" + currentIndex);
    }

    private T getNext() {
        T e = fetched.get(fetchedListIndex);
        fetchedListIndex++;
        return e;
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException();
    }
}