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This article shares with you the content of a sample analysis of basic Criteria queries in Hibernate. The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

To operate the database management system, the most basic thing is to use SQL (Standard Query Language) statements. Most databases support standard SQL statements. However, there are also some database-specific SQL statements. If the application program uses the database-specific SQL statements to query the database with the SQL statements, the program itself will have the problem of relying on the specific database.

When using Hibernate, even if you do not know the use and writing of SQL, you can use the API it provides to query SQL statements. Org.hibernate.Criteria encapsulates SQL. You can combine various query conditions from the point of view of Java objects, and Hibernate automatically generates SQL statements for you, without having to specifically manage the dependence of SQL on the database. It's like checking and validating SQL syntax at compile time.

In the case of the most basic Criteria query, if you want to query all the contents of the table corresponding to an object, you can query it as follows:

Criteria criteria = session.createCriteria (User.class); List users = criteria.list (); for (Iterator it = users.iterator (); it.hasNext ();) {User user = (User) it.next (); System.out.println (user.getId () + "\ t" + user.getName () + "/" + user.getAge ()) }

After Criteria is established, if no condition is given, the default is to query all the data in the table corresponding to the object. If you execute the above program snippet and set the "show_sql" attribute of Hibernate in the configuration file, you can see the generation of the following SQL statement under master control:

Hibernate: select this_.id as id0_0_, this_.name as name0_0_, this_.age as age0_0_ from T_USER this_

Org.hibernate.Criteria is actually a container with conditions attached. If you want to set query conditions, you need to use various static methods of org.hibernate.criterion.Restrictions to return org.hibernate.criterion.Criteria instances. Each org.hibernate.criterion.Criteria instance returned represents a condition. You need to use the add () method of org.hibernate.Criteria to add these condition instances, such as querying data with "age" greater than 20 and less than 40:

Criteria criteria = session.createCriteria (User.class); criteria.add (Restrictions.gt ("age", new Integer (20)); criteria.add (Restrictions.lt ("age", new Integer (40)); List users = criteria.list (); for (Iterator it = users.iterator (); it.hasNext ();) {User user = (User) it.next () System.out.println (user.getId () + "\ t" + user.getName () + "/" + user.getAge ());}

The gt () method of Restrictions represents a condition greater than (great than), while lt represents a condition less than (less than). Execute the above program fragment, observe the resulting SQL statement, and use the where and and clause to complete the conditional query of SQL:

Hibernate: select this_.id as id0_0_, this_.name as name0_0_, this_.age as age0_0_ from T_USER this_ where this_.age >? And this_.age

< ? 使用add()方法加入条件时，预设是使用and来组合条件，如果要用or的方式来组合条件，则可以使用Restrictions.or()方法，例如结合age等于（eq）20或（or）age为空（isNull）的条件： Criteria criteria = session.createCriteria(User.class); criteria.add(Restrictions.or( Restrictions.eq("age", new Integer(20)), Restrictions.isNull("age") )); List users = criteria.list(); 观察所产生的SQL语句，将使用where与or子句完成SQL的条件查询： Hibernate: select this_.id as id0_0_, this_.name as name0_0_, this_.age as age0_0_ from T_USER this_ where (this_.age=? or this_.age is null) 您也可以使用Restrictions.like()方法来进行SQL中like子句的功能，例如查询"name"中名称为"just"开头的资料： Criteria criteria = session.createCriteria(User.class); criteria.add(Restrictions.like("name", "just%")); List users = criteria.list(); 观察所产生的SQL语句如下： Hibernate: select this_.id as id0_0_, this_.name as name0_0_, this_.age as age0_0_ from T_USER this_ where this_.name like ? Restrictions的几个常用限定查询方法如下表所示： 方法说明Restrictions.eq等于Restrictions.allEq使用Map，使用key/value进行多个等于的比对Restrictions.gt大于 >

Restrictions.ge greater than or equal to > = Restrictions.lt less than

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