hibernate 數據庫查詢

hibernate提供了多種方式進行數據庫數據的查詢

HQL查詢

代碼如下

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();

        Query query = session.createQuery("from Student student where student.name like :name order by student.id asc");
        query.setParameter("name", "student%");
        //實現分頁
        query.setFirstResult(0);
        query.setMaxResults(2);
        List<Student> students = query.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

 

如果結果是單一的對象而不是集合，可以使用query. uniqueResult()

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();

        Query query = session.createQuery("select count(*) from Student");
        Long count = (Long)query.uniqueResult();

        tx.commit();
        HibernateFactory.closeSession();
    }
}

 

也可以直接訪問對象中的對象屬性（String hql = "from Student where grade.name= 'grade1'"；） 這樣相當於兩張表的聯合查詢

如果一次查詢多個對象，可以使用以下方式

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Query query = session.createQuery("from Student s,Grade g where s.id=g.id and s.name like :name");
        query.setParameter("name", "student%");
        List list = query.list();
        for(int i = 0;i < list.size();i++) {
            Object[] obj = (Object[]) list.get(i);
            Student student = (Student) obj[0];
            Grade grade = (Grade)obj[1];
            System.out.println(student.getName() + "|" + grade.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

 

Criteria方式查詢

代碼如下

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();

        Criteria criteria = session.createCriteria(Student.class);
        criteria.add(Restrictions.like("name", "student%"));
        
        criteria.setFirstResult(0);
        criteria.setMaxResults(2);
        criteria.addOrder(Order.desc("name"));
        
        List<Student> students = criteria.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

使用criteria方式查詢有以下三個步驟

1.使用Session實例 的createCriteria()方法創建Criteria對象

2.使用工具類Restrictions的方法為Criteria對象設置查詢條件，Order工具類的方法設置排序方式，Projections工具類的方法進行統計和分組

3.使用Criteria對象的list()方法進行查詢並返回結果

 

例子查詢

例子查詢將一個對象的非空屬性作為查詢條件進行查詢，代碼如下

public class HibernateTest {
    public static void main(String[] args) {
                //建立一個例子對象，它的非空屬性作為刪選條件
        Student studentExample = new Student();
        studentExample.setName("student1");
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Criteria criteria = session.createCriteria(Student.class);
        criteria.add(Example.create(studentExample));
        
        List<Student> students = criteria.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }

        tx.commit();
        HibernateFactory.closeSession();
    }
}

 

離線查詢

離線查詢建立一個DetachedCriteria對象，將查詢的條件等指定好，然后在session.beginTransaction()后將這個對象傳入，代碼如下

public class HibernateTest {
    public static void main(String[] args) {
        DetachedCriteria dc = DetachedCriteria.forClass(Student.class);
        dc.add(Restrictions.like("name", "student%"));
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Criteria criteria = dc.getExecutableCriteria(session);
        List<Student> students = criteria.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }

        tx.commit();
        HibernateFactory.closeSession();
    }
}

 

load/get方式

Student student = (Student) session.get(Student.class, 1);

Student student = (Student) session.load(Student.class, 1);

兩者區別

1.get()采用立即加載方式,而load()采用延遲加載;get()方法執行的時候,會立即向數據庫發出查詢語句, 而load()方法返回的是一個代理(此代理中只有一個id屬性),只有等真正使用該對象屬性的時候,才會發出sql語句

2.如果數據庫中沒有對應的記錄,get()方法返回的是null.而load()方法出現異常ObjectNotFoundException

 

命名查詢

1.配置hbm文件，將sql或者hql語句寫入hbm，並給與一個唯一標示這個查詢語句的name

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping >
    <class>...</class>
    
    <query name="hqlquery">
        <![CDATA[
            from Student where name like :name
        ]]>
    </query>
    
    <query name="sqlquery">
        <![CDATA[
            select s from Student s where name like ?
        ]]>
    </query>
</hibernate-mapping>

 

2.通過name在代碼中調用事先寫好的查詢語句

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Query query = session.getNamedQuery("hqlquery");
        query.setParameter("name", "student%");
        List<Student> students = query.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }
        
        Query sqlquery = session.getNamedQuery("sqlquery");
        sqlquery.setParameter(0, "student%");
        List<Student> students2 = query.list();
        for(Student student : students2) {
            System.out.println(student.getName());
        }
        
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

 

SQL查詢

hibernate支持原生的sql查詢語句，具體的代碼如下

返回單表的查詢

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();

        List<Student> students = session.createSQLQuery("select * from student").addEntity(Student.class).list();
        //或者采用以下語句
        // List<Student> students = session.createSQLQuery("select s.* from student as s").addEntity("s",Student.class).list();

        for(Student student : students) {
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

 

返回多個表的查詢

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        SQLQuery query = session.createSQLQuery("select s.*,g.* from student as s,grade as g where s.gradeid=g.id");
        query.addEntity("s",Student.class).addEntity("g",Grade.class);
        
        List list = query.list();
        //list的每一條數據都由Object數組組成，數組的每個約束對應一個對象
        for (int i = 0; i < list.size(); i++) {
            Object[] obj = (Object[]) list.get(i);
            Student student = (Student) obj[0];
            Grade grade = (Grade) obj[1];
        }

        tx.commit();
        HibernateFactory.closeSession();
    }
}

 

使用jdbc

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        session.doWork(new Work() {
            public void execute(Connection connection) throws SQLException {  
                // 這里已經得到connection了，可以繼續你的JDBC代碼。
                PreparedStatement ps = connection.prepareStatement("insert into student(id,name) values(?,?)");
                for(int i = 0;i < 500;i++) {
                    ps.setInt(1,i);
                    ps.setString(2,"student" + i);
                    ps.addBatch();    //添加到批處理命令中
                }
                ps.executeBatch();     //執行批處理命令
                // 注意不要close了這個connection。  
            }
        });
        
        //事務提交后才會執行插入操作
        tx.commit();
        HibernateFactory.closeSession();
    }
}

使用原生的jdbc操作，hibernate4之前可以用session.connection()獲取Connection對象，Hibernate4之后這個方法已經廢棄

 

Query.iterator與Query.list的比較

iterator的使用方法如下

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Query query = session.createQuery("from Student");
        Iterator iterator = query.iterate();
        while(iterator.hasNext()) {
            Student student = (Student)iterator.next();
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

 

iterator方式查詢產生的sql語句如下

Hibernate: select student0_.id as col_0_0_ from student student0_

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=?

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=?

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=?

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=?

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=?

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=?

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=?

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=?

通過list方式查詢產生的sql語句如下

Hibernate: select student0_.id as id1_1_, student0_.name as name2_1_, student0_.gradeid as gradeid3_1_ from student student0_

兩者相比較，list()只發一條語句將符合條件的數據全部查出，而iterator()卻現將id查出來，然后根據id再將符合條件的數據查出，這就構成了N＋1的問題；既然list更高效，為什么hibernate還將iterator存在呢？

執行以下程序

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        String hql = "from Student";
        
        Query listQuery = session.createQuery(hql);
        List<Student> list = listQuery.list();
        for(Student student : list) {
            System.out.println(student.getName());
        }
        
        Query iteratorQuery = session.createQuery(hql);
        Iterator iterator = iteratorQuery.iterate();
        while(iterator.hasNext()) {
            Student student = (Student)iterator.next();
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

最終產生的sql語句如下

Hibernate: select student0_.id as id1_1_, student0_.name as name2_1_, student0_.gradeid as gradeid3_1_ from student student0_
Hibernate: select student0_.id as col_0_0_ from student student0_

將list()和iterator()放在一起使用，這時的iterator只執行了一條SQL，原因在於hibernate的緩存機制

list()方法將執行Select SQL從數據庫中獲取所有符合滿足條件的記錄並構造相應的實體對象，實體對象構建完畢后，就將其納入緩存；

這樣等到iterator()執行時，首先會執行一條SQL來查詢符合條件數據的id，隨即，iterator方法首先在本地緩存內根據id查找對應的實體對象是否存在，如果緩存中已經存在對應的數據，則直接以此數據對象作為查詢結果；如果沒有找到，則再次執行Select語句獲得對應數據庫中的表記錄（如果iterator在數據庫中查到並構建了完整的數據對象，也會將其納入緩存中）;

list()方法無法讀取緩存，但它可以寫入緩存，在上面這個實例中，list()將讀取的數據放入緩存中，iterator()直接可以用於是出現了以上的結果；

如果目標數據只讀或者讀取相當頻繁，可以使用iterator()來減少性能上的消耗；