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
时间： 2024-10-05 20:13:40