hibernate 数据库查询

2023-12-20 23:02:09

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；

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