Displaying Huge Amount of Data

Issue of Displaying Huge Amount of Data

• time to query all data is unbearable long at one time.
  • MVVM. NotifyChange a property of ListModel triiggers high-cost re-query of all data which might lead to low performance.

Solution : Query One Page A Time

implement a custom ListModel. It only queries a page size of data during one request execution and store it as a cache.

package org.zkoss.reference.developer.mvvm.advance;

import java.util.*;
import org.zkoss.reference.developer.mvvm.advance.domain.*;
import org.zkoss.zk.ui.*;
import org.zkoss.zul.AbstractListModel;

public class LivePersonListModel extends AbstractListModel<Person>{

	private static final long serialVersionUID = -7982684413905984053L;
	
	private PersonDao personDao;
	private int pageSize = 10;
	private final String CACHE_KEY= LivePersonListModel.class+"_cache";
	
	public LivePersonListModel(PersonDao personDao){
		this.personDao = personDao;
	}
	
	public Person getElementAt(int index) {
		Map<Integer, Person> cache = getCache();

		Person targetPerson = cache.get(index);
		if (targetPerson == null){
			//if cache doesn't contain target object, query a page size of data starting from the index
			List<Person> pageResult = personDao.findAll(index, pageSize);
			int indexKey = index;
			for (Person o : pageResult ){
				cache.put(indexKey, o);
				indexKey++;
			}
		}else{
			return targetPerson;
		}

		//get the target after query from database
		targetPerson = cache.get(index);
		if (targetPerson == null){
			//if we still cannot find the target object from database, there is inconsistency between memory and the database
			throw new RuntimeException("Element at index "+index+" cannot be found in the database.");
		}else{
			return targetPerson;
		}
	}
	
	private Map<Integer, Person> getCache(){
		Execution execution = Executions.getCurrent();
		//we only reuse this cache in one execution to avoid accessing detached objects.
		//our filter opens a session during a HTTP request
		Map<Integer, Person> cache = (Map)execution.getAttribute(CACHE_KEY);
		if (cache == null){
			cache = new HashMap<Integer, Person>();
			execution.setAttribute(CACHE_KEY, cache);
		}
		return cache;
	}	

	public int getSize() {
		return personDao.findAllSize();
	}


	public int getPageSize() {
		return pageSize;
	}

	public void setPageSize(int pageSize) {
		this.pageSize = pageSize;
	}
}

It works fine under paging or default mold.