I am using a DAO framework named "QueryDSL". When I launch a query, QueryDSL fetches all results together that isnot desired. I am looking for a soultion that lets me to load the results in lazy form.
3 Answers
I think you need kind of pagination there.
Read here how to organize it. You need something like this to load part of your data
Page<T> findAll(com.mysema.query.types.Predicate predicate,
Pageable pageable)
Comments
I eventually implemented a lazy list. This works well. :)
public class LazyList<T> implements List<T> {
private SQLQuery query;
private long size = 0;
private int limit = 100;
private Expression<T> exprsn;
private int offset = 0;
private List<T> cache = new ArrayList<>();
public LazyList(SQLQuery query, Expression<T> exprsn) {
this.query = query;
this.exprsn = exprsn;
size = query.count();
cache = query.limit(limit).offset(offset).list(exprsn);
}
@Override
public int size() {
return (int) size;
}
@Override
public boolean isEmpty() {
return size == 0;
}
@Override
public T get(int index) {
if (index < offset || index > offset + limit) {
cache = query.limit(limit).offset(offset).list(exprsn);
}
return cache.get(index - offset);
}
@Override
public Iterator<T> iterator() {
return new Iterator<T>() {
private int index = 0;
private int offset = 0;
private List<T> cache = new ArrayList<>();
{
cache = query.limit(limit).offset(offset).list(exprsn);
}
@Override
public boolean hasNext() {
return index < size;
}
@Override
public T next() {
if (index < offset || index > offset + limit) {
cache = query.limit(limit).offset(offset).list(exprsn);
}
return cache.get(index - offset);
}
};
}
@Override
public boolean contains(Object o) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public Object[] toArray() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public <T> T[] toArray(T[] a) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public boolean add(T e) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public boolean remove(Object o) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public boolean containsAll(Collection<?> c) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public boolean addAll(Collection<? extends T> c) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public boolean addAll(int index, Collection<? extends T> c) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public boolean removeAll(Collection<?> c) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public boolean retainAll(Collection<?> c) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void clear() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public T set(int index, T element) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void add(int index, T element) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public T remove(int index) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public int indexOf(Object o) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public int lastIndexOf(Object o) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public ListIterator<T> listIterator() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public ListIterator<T> listIterator(int index) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public List<T> subList(int fromIndex, int toIndex) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
}
4 Comments
Martin Petkov
What if
size overflows?
user3359139
This is a too simple implementation that just presents idea and needs to be completed for a real production. However, this satisfies my simple requirements. :)
Martin Petkov
Okay, but you shouldn't do such casts. Why do you need it long if you're only going to work with the 32 bits...
user3359139
Because query.count return a long value whereas list interface needs a integer. So, casting is a straightforward way. What is ur solution?
Querydsl SQL doesn't support lazy loading, in Querydsl JPA lazy loading is handled by the JPA provider.
