HQL (二) 实体对象查询

实体对象查询（重要）  
   * N+1问题，在默认情况下使用query.iterate查询，有可能出现N+1问题，所谓的N+1
     是查询对象的时候发出了N+1条sql语句
    1:首先发出一条查询id列表的sql
    N:根据id列表发出n条查询语句

* list和iterate的区别?
   * list在默认情况下，只向缓存中放入数据，而不利用缓存中的数据
   * iterate在默认情况下有N+1问题，如果缓存中存在数据那么会根据id到缓存获取数据
     也就说iterate是利用缓存的
参见：SimpleObjectQueryTest1.java,SimpleObjectQueryTest2.java    

package com.wlh.hibernate;

import java.util.Iterator;
import java.util.List;

import junit.framework.TestCase;

import org.hibernate.Query;
import org.hibernate.Session;

public class SimpleObjectQueryTest1 extends TestCase{
  
	public void testQuery1(){
	  Session  session=null;
	  try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			//返回值为实体对象的集合
			//可以省略select语句
			Query query=session.createQuery("from Student");
			List students=query.list();
			for(Iterator iter=students.iterator();iter.hasNext();){
				Student student=(Student)iter.next();
				System.out.println("student="+student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
  }
	public void testQuery2(){
		  Session  session=null;
		  try {
				session = HibernateUtils.getSession();
				session.beginTransaction();
				//返回值为实体对象的集合
				//可以省略select语句,实体类可以加别名
				Query query=session.createQuery("from Student s");
				List students=query.list();
				for(Iterator iter=students.iterator();iter.hasNext();){
					Student student=(Student)iter.next();
					System.out.println("student="+student.getName());
				}
				session.getTransaction().commit();
			}catch(Exception e) {
				e.printStackTrace();
				session.getTransaction().rollback();
			}finally {
				HibernateUtils.closeSession(session);
			}
	  }
	
	public void testQuery3(){
		  Session  session=null;
		  try {
				session = HibernateUtils.getSession();
				session.beginTransaction();
				 //返回值为实体对象的集合
				//可以省略select语句,实体类可以用as加别名
				Query query=session.createQuery("from Student as s");
				List students=query.list();
				for(Iterator iter=students.iterator();iter.hasNext();){
					Student student=(Student)iter.next();
					System.out.println("student="+student.getName());
				}
				session.getTransaction().commit();
			}catch(Exception e) {
				e.printStackTrace();
				session.getTransaction().rollback();
			}finally {
				HibernateUtils.closeSession(session);
			}
	  }
	
	public void testQuery4(){
		  Session  session=null;
		  try {
				session = HibernateUtils.getSession();
				session.beginTransaction();
				//返回值为实体对象的集合
				//使用select查询实体对象，必须采用别名
				Query query=session.createQuery("select s from Student as s");
				List students=query.list();
				for(Iterator iter=students.iterator();iter.hasNext();){
					Student student=(Student)iter.next();
					System.out.println("student="+student.getName());
				}
				session.getTransaction().commit();
			}catch(Exception e) {
				e.printStackTrace();
				session.getTransaction().rollback();
			}finally {
				HibernateUtils.closeSession(session);
			}
	  }
}


package com.wlh.hibernate;

import java.util.Iterator;
import java.util.List;

import junit.framework.TestCase;

import org.hibernate.Query;
import org.hibernate.Session;

public class SimpleObjectQueryTest2 extends TestCase{
  
	/**
	 * 发出list查询
	 */
	public void testQuery1(){
	  Session  session=null;
	  try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			/**
			 * 采用list查询将会发出一条查询语句，取得Student数据
			 * select student0_.id as id1_, student0_.name as name1_, 
			 * student0_.createTime as createTime1_, student0_.classesid as classesid1_ 
			 * from t_student student0_
			 * 
			 */
			Query query=session.createQuery("from Student");
			List students=query.list();
			for(Iterator iter=students.iterator();iter.hasNext();){
				Student student=(Student)iter.next();
				System.out.println("student="+student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
  }
	
	
	/**
	 * 出现N+1问题
	 * 
	 * 发出查询id类表的sql语句：
	 *  select student0_.id as col_0_0_ from t_student student0_
	 *  
	 * 依次发出根据id查询Student的sql语句：
	 * select student0_.id as id1_0_, student0_.name as name1_0_, 
	 * student0_.createTime as createTime1_0_, student0_.classesid as classesid1_0_ 
	 * from t_student student0_ where student0_.id=?
	 * 
	 */
	public void testQuery2(){
		  Session  session=null;
		  try {
				session = HibernateUtils.getSession();
				session.beginTransaction();
				
				Query query=session.createQuery("from Student");
				Iterator iter=query.iterate();
				while(iter.hasNext()){
					Student student=(Student)iter.next();
					System.out.println("student="+student.getName());
				}
				session.getTransaction().commit();
			}catch(Exception e) {
				e.printStackTrace();
				session.getTransaction().rollback();
			}finally {
				HibernateUtils.closeSession(session);
			}
	  }
	  
	
	/**
	 * 先发出list查询，再发出iterate查询
	 */
	public void testQuery3(){
		Session  session=null;
		try {
				session = HibernateUtils.getSession();
				session.beginTransaction();
				Query query=null;
				
				 query=session.createQuery("from Student");
				List students=query.list();
				for(Iterator iter=students.iterator();iter.hasNext();){
					Student student=(Student)iter.next();
					System.out.println("student="+student.getName());
				}
				System.out.println("===============================");
				/**
				 * 不会出新N+1问题
				 * 
				 * 因为list操作已经将对象放到了一级缓存中（session级缓存），所以在使用Iterate操作的时候
				 * 它会发出根据id查询的语句，取得id列表，再根据id到缓存中获取数据，只有在缓存中找不到相应的数据
				 * 才会发出sql语句到数据库中查询
				 */
			    query=session.createQuery("from Student");
				Iterator iter=query.iterate();
				while(iter.hasNext()){
					Student student=(Student)iter.next();
					System.out.println("student="+student.getName());
				}
				session.getTransaction().commit();
			}catch(Exception e) {
				e.printStackTrace();
				session.getTransaction().rollback();
			}finally {
				HibernateUtils.closeSession(session);
			}

}
	
	/**
	 * 发出两次list查询
	 */
	public void testQuery4() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			List students = session.createQuery("from Student").list();
			
			for (Iterator iter =students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			System.out.println("------------------------------------------------");
			
			/**
			 * 再次发出查询语句
			 * 
			 * 在默认情况下list每次都会向数据库发出查询对象的sql，除非配置了查询缓存，所以下面的list
			 * 操作，虽然在一级缓存中已经有了对象数据，list不会利用缓存中的数据，而再次发出查询sql
			 * 
			 * list在默认情况下，只向缓存中放入数据，而不利用缓存中的数据
			 */
			students = session.createQuery("from Student").list();
			
			for (Iterator iter =students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}			
	
}