hibernate提供了多种方式进行数据库数据的查询
HQL查询
代码如下
1 public class HibernateTest { 2 public static void main(String[] args) { 3 Session session = HibernateFactory.currentSession(); 4 Transaction tx = session.beginTransaction(); 5 6 Query query = session.createQuery("from Student student where student.name like :name order by student.id asc"); 7 query.setParameter("name", "student%"); 8 //实现分页 9 query.setFirstResult(0); 10 query.setMaxResults(2); 11 List<Student> students = query.list(); 12 for(Student student : students) { 13 System.out.println(student.getName()); 14 } 15 16 tx.commit(); 17 HibernateFactory.closeSession(); 18 } 19 }
如果结果是单一的对象而不是集合,可以使用query. uniqueResult()
1 public class HibernateTest { 2 public static void main(String[] args) { 3 Session session = HibernateFactory.currentSession(); 4 Transaction tx = session.beginTransaction(); 5 6 Query query = session.createQuery("select count(*) from Student"); 7 Long count = (Long)query.uniqueResult(); 8 9 tx.commit(); 10 HibernateFactory.closeSession(); 11 } 12 }
也可以直接访问对象中的对象属性(String hql = "from Student where grade.name= ‘grade1‘";) 这样相当于两张表的联合查询
如果一次查询多个对象,可以使用以下方式
1 public class HibernateTest { 2 public static void main(String[] args) { 3 4 Session session = HibernateFactory.currentSession(); 5 Transaction tx = session.beginTransaction(); 6 7 Query query = session.createQuery("from Student s,Grade g where s.id=g.id and s.name like :name"); 8 query.setParameter("name", "student%"); 9 List list = query.list(); 10 for(int i = 0;i < list.size();i++) { 11 Object[] obj = (Object[]) list.get(i); 12 Student student = (Student) obj[0]; 13 Grade grade = (Grade)obj[1]; 14 System.out.println(student.getName() + "|" + grade.getName()); 15 } 16 17 tx.commit(); 18 HibernateFactory.closeSession(); 19 } 20 }
Criteria方式查询
代码如下
1 public class HibernateTest { 2 public static void main(String[] args) { 3 Session session = HibernateFactory.currentSession(); 4 Transaction tx = session.beginTransaction(); 5 6 Criteria criteria = session.createCriteria(Student.class); 7 criteria.add(Restrictions.like("name", "student%")); 8 9 criteria.setFirstResult(0); 10 criteria.setMaxResults(2); 11 criteria.addOrder(Order.desc("name")); 12 13 List<Student> students = criteria.list(); 14 for(Student student : students) { 15 System.out.println(student.getName()); 16 } 17 18 tx.commit(); 19 HibernateFactory.closeSession(); 20 } 21 }
使用criteria方式查询有以下三个步骤
1.使用Session实例 的createCriteria()方法创建Criteria对象
2.使用工具类Restrictions的方法为Criteria对象设置查询条件,Order工具类的方法设置排序方式,Projections工具类的方法进行统计和分组
3.使用Criteria对象的list()方法进行查询并返回结果
例子查询
例子查询将一个对象的非空属性作为查询条件进行查询,代码如下
1 public class HibernateTest { 2 public static void main(String[] args) { 3 //建立一个例子对象,它的非空属性作为删选条件 4 Student studentExample = new Student(); 5 studentExample.setName("student1"); 6 7 Session session = HibernateFactory.currentSession(); 8 Transaction tx = session.beginTransaction(); 9 10 Criteria criteria = session.createCriteria(Student.class); 11 criteria.add(Example.create(studentExample)); 12 13 List<Student> students = criteria.list(); 14 for(Student student : students) { 15 System.out.println(student.getName()); 16 } 17 18 tx.commit(); 19 HibernateFactory.closeSession(); 20 } 21 }
离线查询
离线查询建立一个DetachedCriteria对象,将查询的条件等指定好,然后在session.beginTransaction()后将这个对象传入,代码如下
1 public class HibernateTest { 2 public static void main(String[] args) { 3 DetachedCriteria dc = DetachedCriteria.forClass(Student.class); 4 dc.add(Restrictions.like("name", "student%")); 5 6 Session session = HibernateFactory.currentSession(); 7 Transaction tx = session.beginTransaction(); 8 9 Criteria criteria = dc.getExecutableCriteria(session); 10 List<Student> students = criteria.list(); 11 for(Student student : students) { 12 System.out.println(student.getName()); 13 } 14 15 tx.commit(); 16 HibernateFactory.closeSession(); 17 } 18 }
load/get方式
Student student = (Student) session.get(Student.class, 1);
Student student = (Student) session.load(Student.class, 1);
两者区别
1.get()采用立即加载方式,而load()采用延迟加载;get()方法执行的时候,会立即向数据库发出查询语句, 而load()方法返回的是一个代理(此代理中只有一个id属性),只有等真正使用该对象属性的时候,才会发出sql语句
2.如果数据库中没有对应的记录,get()方法返回的是null.而load()方法出现异常ObjectNotFoundException
命名查询
1.配置hbm文件,将sql或者hql语句写入hbm,并给与一个唯一标示这个查询语句的name
1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE hibernate-mapping PUBLIC 3 "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 4 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 5 <hibernate-mapping > 6 <class>...</class> 7 8 <query name="hqlquery"> 9 <![CDATA[ 10 from Student where name like :name 11 ]]> 12 </query> 13 14 <query name="sqlquery"> 15 <![CDATA[ 16 select s from Student s where name like ? 17 ]]> 18 </query> 19 </hibernate-mapping>
2.通过name在代码中调用事先写好的查询语句
1 public class HibernateTest { 2 public static void main(String[] args) { 3 4 Session session = HibernateFactory.currentSession(); 5 Transaction tx = session.beginTransaction(); 6 7 Query query = session.getNamedQuery("hqlquery"); 8 query.setParameter("name", "student%"); 9 List<Student> students = query.list(); 10 for(Student student : students) { 11 System.out.println(student.getName()); 12 } 13 14 Query sqlquery = session.getNamedQuery("sqlquery"); 15 sqlquery.setParameter(0, "student%"); 16 List<Student> students2 = query.list(); 17 for(Student student : students2) { 18 System.out.println(student.getName()); 19 } 20 21 22 tx.commit(); 23 HibernateFactory.closeSession(); 24 } 25 }
SQL查询
hibernate支持原生的sql查询语句,具体的代码如下
返回单表的查询
1 public class HibernateTest { 2 public static void main(String[] args) { 3 Session session = HibernateFactory.currentSession(); 4 Transaction tx = session.beginTransaction(); 5 6 List<Student> students = session.createSQLQuery("select * from student").addEntity(Student.class).list(); 7 //或者采用以下语句 8 // List<Student> students = session.createSQLQuery("select s.* from student as s").addEntity("s",Student.class).list(); 9 10 for(Student student : students) { 11 System.out.println(student.getName()); 12 } 13 14 tx.commit(); 15 HibernateFactory.closeSession(); 16 } 17 }
返回多个表的查询
1 public class HibernateTest { 2 public static void main(String[] args) { 3 Session session = HibernateFactory.currentSession(); 4 Transaction tx = session.beginTransaction(); 5 6 SQLQuery query = session.createSQLQuery("select s.*,g.* from student as s,grade as g where s.gradeid=g.id"); 7 query.addEntity("s",Student.class).addEntity("g",Grade.class); 8 9 List list = query.list(); 10 //list的每一条数据都由Object数组组成,数组的每个约束对应一个对象 11 for (int i = 0; i < list.size(); i++) { 12 Object[] obj = (Object[]) list.get(i); 13 Student student = (Student) obj[0]; 14 Grade grade = (Grade) obj[1]; 15 } 16 17 tx.commit(); 18 HibernateFactory.closeSession(); 19 } 20 }
Query.iterator与Query.list的比较
iterator的使用方法如下
1 public class HibernateTest { 2 public static void main(String[] args) { 3 4 Session session = HibernateFactory.currentSession(); 5 Transaction tx = session.beginTransaction(); 6 7 Query query = session.createQuery("from Student"); 8 Iterator iterator = query.iterate(); 9 while(iterator.hasNext()) { 10 Student student = (Student)iterator.next(); 11 System.out.println(student.getName()); 12 } 13 14 tx.commit(); 15 HibernateFactory.closeSession(); 16 } 17 }
iterator方式查询产生的sql语句如下
1 Hibernate: select student0_.id as col_0_0_ from student student0_ 2 3 Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=? 4 5 Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=? 6 7 Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=? 8 9 Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=? 10 11 Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=? 12 13 Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=? 14 15 Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=? 16 17 Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=?
通过list方式查询产生的sql语句如下
1 Hibernate: select student0_.id as id1_1_, student0_.name as name2_1_, student0_.gradeid as gradeid3_1_ from student student0_
两者相比较,list()只发一条语句将符合条件的数据全部查出,而iterator()却现将id查出来,然后根据id再将符合条件的数据查出,这就构成了N+1的问题;既然list更高效,为什么hibernate还将iterator存在呢?
执行以下程序
1 public class HibernateTest { 2 public static void main(String[] args) { 3 4 Session session = HibernateFactory.currentSession(); 5 Transaction tx = session.beginTransaction(); 6 String hql = "from Student"; 7 8 Query listQuery = session.createQuery(hql); 9 List<Student> list = listQuery.list(); 10 for(Student student : list) { 11 System.out.println(student.getName()); 12 } 13 14 Query iteratorQuery = session.createQuery(hql); 15 Iterator iterator = iteratorQuery.iterate(); 16 while(iterator.hasNext()) { 17 Student student = (Student)iterator.next(); 18 System.out.println(student.getName()); 19 } 20 21 tx.commit(); 22 HibernateFactory.closeSession(); 23 } 24 }
最终产生的sql语句如下
1 Hibernate: select student0_.id as id1_1_, student0_.name as name2_1_, student0_.gradeid as gradeid3_1_ from student student0_ 2 Hibernate: select student0_.id as col_0_0_ from student student0_
将list()和iterator()放在一起使用,这时的iterator只执行了一条SQL,原因在于hibernate的缓存机制
list()方法将执行Select SQL从数据库中获取所有符合满足条件的记录并构造相应的实体对象,实体对象构建完毕后,就将其纳入缓存;
这样等到iterator()执行时,首先会执行一条SQL来查询符合条件数据的id,随即,iterator方法首先在本地缓存内根据id查找对应的实体对象是否存在,如果缓存中已经存在对应的数据,则直接以此数据对象作为查询结果;如果没有找到,则再次执行Select语句获得对应数据库中的表记录(如果iterator在数据库中查到并构建了完整的数据对象,也会将其纳入缓存中);
list()方法无法读取缓存,但它可以写入缓存,在上面这个实例中,list()将读取的数据放入缓存中,iterator()直接可以用于是出现了以上的结果;
如果目标数据只读或者读取相当频繁,可以使用iterator()来减少性能上的消耗;
hibernate 数据库查询