IllegalStateException问题

package dao;

import com.opensymphony.xwork2.ActionSupport;
import java.io.PrintWriter;
import java.util.Iterator;
import java.util.List;
import javax.swing.JOptionPane;
import org.apache.struts2.ServletActionContext;
import org.hibernate.Criteria;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.criterion.Order;
import persistence.HibernateUtil;
import pojo.Photo;

public class GetImages extends ActionSupport{
    int limit;

    public int getLimit() {
        return limit;
    }

    public void setLimit(int limit) {
        this.limit = limit;
    }

    @Override
    public String execute() throws Exception{
        ServletActionContext.getResponse().setContentType(\"text/html\");
        PrintWriter pw = ServletActionContext.getResponse().getWriter();
        SessionFactory sf = HibernateUtil.getSf();
        Session ss = sf.openSession();
        String returnText = \"\";

        Criteria cr = ss.createCriteria(Photo.class);
        cr.addOrder(Order.desc(\"uploadTime\"));
        cr.setFirstResult(0);
        cr.setMaxResults(getLimit());
        List<Photo> li = cr.list();
        Iterator<Photo> it = li.iterator();

        if(!(it.hasNext())){
            return ERROR;
        }
        while(it.hasNext()){
            Photo p = it.next();
            returnText = returnText + p.getPhotoName().trim()+\"\\\"\\\"\"+(String.valueOf(p.getUploadTime())).trim()+\"\\\"\\\"\"+p.getTitle().trim()+\"\\\"\\\"\"+p.getDescription().trim()+ \"\\\'\\\'\\n\";
        }

        returnText = returnText.substring(0, returnText.length()-2);
        JOptionPane.showMessageDialog(null, returnText);
        pw.println(returnText);
        pw.flush();

        pw.close();
        ss.close();
        sf.close();

        return SUCCESS;
    }

}