JPA ---- EntityManager使用

　　使用@PersistenceContext声明：

@PersistenceContext
private EntityManager entityManager;

　　

　　1、查询数据获取ResultSet结果集：

SessionImplementor session = entityManager.unwrap(SessionImplementor.class);
Connection connection = session.connection();
PreparedStatement ps = connection.prepareStatement(sql,ResultSet.TYPE_FORWARD_ONLY,
		              ResultSet.CONCUR_READ_ONLY);
ps.setFetchSize(Integer.MIN_VALUE);
ps.setFetchDirection(ResultSet.FETCH_REVERSE);
ResultSet rs = ps.executeQuery();
while (rs.next()) {
    Object value = rs.getObject(name);
}