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hibernate 数据库查询
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 :name11 ]]>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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