This blog series is summarized here: Hibernate summary

Fetching strategies

Select different strategies to query according to different needs, and the sql generated in the background is different

1. Multi to one single end grabbing

The first way

/**
* Many to one single end grab fetch=join on many to one
 */
@Test
public void fetch2()
{
	Session session = HibernateUtils.getSession();
	try
	{
		Emp emp = (Emp) session.load(Emp.class, 1);
		// Send a join query of two tables to query the data of emp and team
		System.out.println(emp);
		Team team = emp.getTeam();
		// No hair sql
		System.out.println(team);
	} catch (Exception e)
	{
		e.printStackTrace();
	} finally
	{
		HibernateUtils.closeResource(session);
	}
}

The second way

/**
* fetch=select on many to one
 */
@Test
public void fetch1()
{
	Session session = HibernateUtils.getSession();
	try
	{
		Emp emp = (Emp) session.load(Emp.class, 1);
		// Issue an sql to query emp according to emp ﹣ no
		System.out.println(emp);
		// No hair sql
		Team team = emp.getTeam();
		// Issue a query to query the team's sql according to t_id
		System.out.println(team);
	} catch (Exception e)
	{
		e.printStackTrace();
	} finally
	{
		HibernateUtils.closeResource(session);
	}
}

2. One to many grabs

One end of one is a single object query

/**
 * One to many single object fetching set fetch=select (default)
 */
@Test
public void fetch1()
{
	Session session = HibernateUtils.getSession();
	try
	{
		Team team = (Team) session.load(Team.class, 1);
		// Send out the sql to query the team according to t ﹣ ID
		System.out.println(team);
		Set<Emp> emps = team.getSet();
		// Send out the sql to query emp employees according to team's t'id
		for (Emp e : emps)
		{
			System.out.println(e);
		}

	} catch (Exception e)
	{
		e.printStackTrace();
	} finally
	{
		HibernateUtils.closeResource(session);
	}
}

/**
 * One to many single object grabs fetch=join on set, no delay loading
 */
@Test
public void fetch2()
{
	Session session = HibernateUtils.getSession();
	try
	{
		Team team = (Team) session.load(Team.class, 1);
		// Issue the sql to query the connection between the team and the employee according to a t_id
		System.out.println(team);
		Set<Emp> emps = team.getSet();
		// No hair sql
		for (Emp e : emps)
		{
			System.out.println(e);
		}

	} catch (Exception e)
	{
		e.printStackTrace();
	} finally
	{
		HibernateUtils.closeResource(session);
	}
}

/**
 * One to many single object grabs the fetch=subselect on the set. If it is a single object query, it is exactly the same as select
 */
@Test
public void fetch3()
{
	Session session = HibernateUtils.getSession();
	try
	{
		Team team = (Team) session.load(Team.class, 1);
		// Issue an sql to query the connection between the team and the employees according to the T ID
		System.out.println(team);
		Set<Emp> emps = team.getSet();
		// No hair sql
		for (Emp e : emps)
		{
			System.out.println(e);
		}

	} catch (Exception e)
	{
		e.printStackTrace();
	} finally
	{
		HibernateUtils.closeResource(session);
	}
}

One end is multiple object queries

/**
* One end is multiple object queries, fetch=select
 */
@Test
public void fetch4()
{
	Session session = HibernateUtils.getSession();
	try
	{
		String hql = "from Team";
		Query query = session.createQuery(hql);
		// Issue sql to query all teams
		List<Team> teams = query.list();
		for (Team team : teams)
		{
			System.out.println(team);
			Set<Emp> emps = team.getSet();
			// Send out the sql to query the employees under the team according to the t_id
			for (Emp e : emps)
			{
				System.out.println(e);
			}
		}
	} catch (Exception e)
	{
		e.printStackTrace();
	} finally
	{
		HibernateUtils.closeResource(session);
	}
}

/**
 * One end is multiple object queries, fetch=join, consistent with select
 */
@Test
public void fetch5()
{
	Session session = HibernateUtils.getSession();
	try
	{
		String hql = "from Team";
		Query query = session.createQuery(hql);
		// Issue sql to query all teams
		List<Team> teams = query.list();
		for (Team team : teams)
		{
			System.out.println(team);
			Set<Emp> emps = team.getSet();
			// Send out the sql to query the employees under the team according to the t_id
			for (Emp e : emps)
			{
				System.out.println(e);
			}
		}
	} catch (Exception e)
	{
		e.printStackTrace();
	} finally
	{
		HibernateUtils.closeResource(session);
	}
}

/**
 * One end is multiple object queries, fetch=subselect
 */
@Test
public void fetch6()
{
	Session session = HibernateUtils.getSession();
	try
	{
		String hql = "from Team";
		Query query = session.createQuery(hql);
		// Issue sql to query all teams
		List<Team> teams = query.list();
		for (Team team : teams)
		{
			System.out.println(team);
			Set<Emp> emps = team.getSet();
			// The sql of a sub query queries all the players of all teams
			for (Emp e : emps)
			{
				System.out.println(e);
			}
		}
	} catch (Exception e)
	{
		e.printStackTrace();
	} finally
	{
		HibernateUtils.closeResource(session);
	}
}

If there is any mistake, please correct it!