hibernate 使用的集合类（list和bag）

最新推荐文章于 2018-07-27 16:49:30 发布

longyuan20102011

最新推荐文章于 2018-07-27 16:49:30 发布

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分类专栏： hibernate 文章标签： list hibernate string class encoding session

本文链接：https://blog.youkuaiyun.com/longyuan20102011/article/details/7722693

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本文探讨了在ORM框架中如何正确配置一对多及多对多关系，详细讲解了使用不同集合类型（如Set和Bag）对数据库的影响，并通过实例演示了如何避免配置错误。

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1 一对多关系在使用的时候一定要慎重，尽量保证在多的段下面不超过100。
2 多对多关系尽量不要使用。

3 组件关联，就是说，不是两张表，是一张表。

4 set这个类，没有顺序，也不允许重复。
5 list可以允许重复。

Employee.java

package cn.itcast.domain;

public class Employee {
	private int id;
	private String name;
	private Department depart;
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public Department getDepart() {
		return depart;
	}
	public void setDepart(Department depart) {
		this.depart = depart;
	}
}

Employee.hbm.xml

<?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 package="cn.itcast.domain">
	<class name="Employee">
		<id name="id">
			<generator class="native"></generator>
		</id>
		<property name="name"></property>
		<many-to-one name="depart" column="depart_id"></many-to-one>
	</class>
	
</hibernate-mapping>

Department.java

package cn.itcast.domain;

import java.util.List;
public class Department {
	private int id;
	private String name;
	private List<Employee> employees;

	public List<Employee> getEmployees() {
		return employees;
	}
	public void setEmployees(List<Employee> employees) {
		this.employees = employees;
	}
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}

}

Department.hbm.xml

package cn.itcast.domain;

import java.util.List;
public class Department {
	private int id;
	private String name;
	private List<Employee> employees;

	public List<Employee> getEmployees() {
		return employees;
	}
	public void setEmployees(List<Employee> employees) {
		this.employees = employees;
	}
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}

}

Many2One.java

package cn.itcast.dao.test;

import java.util.ArrayList;
import java.util.List;

import org.hibernate.Session;
import org.hibernate.Transaction;

import cn.itcast.dao.HibernateUtil;
import cn.itcast.domain.Department;
import cn.itcast.domain.Employee;

public class Many2One {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		add();
	}
	
	static Employee query(int empId)
	{
		Session s=null;
		Transaction tx=null;
		try {
			s=HibernateUtil.getSession();
			tx=s.beginTransaction();
			Employee employee=(Employee) s.get(Employee.class,empId);
			tx.commit();
			return employee;
			
		} finally {
			if (s!=null)
				s.close();
		}
	}
	
	static Department add()
	{
		Session s=null;
		Transaction tx=null;
		try {
			Department depart =new Department();
			depart.setName("depart");
			
			Employee emp1=new Employee();
			emp1.setDepart(depart);//建立两个对象之间的关联
			emp1.setName("emp1");			
				
			Employee emp2=new Employee();
			emp2.setDepart(depart);//建立两个对象之间的关联
			emp2.setName("emp2");	
			
			List<Employee> list=new ArrayList<Employee>();
			list.add(emp1);
			list.add(emp2);
			depart.setEmployees(list);
			
			
			s=HibernateUtil.getSession();
			tx=s.beginTransaction();
			s.save(emp1);
			s.save(emp2);			
			s.save(depart);
			tx.commit();
			return depart;
			
		} finally {
			if (s!=null)
				s.close();
		}
	}

}

但是如果用list的话其实在数据库的表中多出来一项，用来描述list的顺序，但是他也是会消耗资源的。

所以我们现在我们使用bag来做配置，就不用多出来一项，其他的配置与list完全相同，测试代码页完全相同。

Department.hbm.xml

<?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 package="cn.itcast.domain">
	<class name="Department">
		<id name="id">
			<generator class="native"></generator>
		</id>
		<property name="name"></property>
		<!--  
		<set name="employees">
			<key column="depart_id"/>
			<one-to-many class="Employee"/>
		</set>
		-->
		<!--  
		<list name="employees">
			<key column="depart_id"></key>
			<list-index column="index_col"></list-index>
			<one-to-many class="Employee"/>
		</list>
		--> 
		<bag name="employees">
			<key column="depart_id"/>
			<one-to-many class="Employee"/>
		</bag>
	</class>
</hibernate-mapping>

在这个程序运行的时候，有一个小插曲。

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
Exception in thread "main" java.lang.ExceptionInInitializerError
at cn.itcast.dao.test.Many2One.add(Many2One.java:61)
at cn.itcast.dao.test.Many2One.main(Many2One.java:19)
Caused by: org.hibernate.InvalidMappingException: Could not parse mapping document from resource cn/itcast/domain/Department.hbm.xml
at org.hibernate.cfg.Configuration$MetadataSourceQueue.processHbmXml(Configuration.java:4009)
at org.hibernate.cfg.Configuration$MetadataSourceQueue.processHbmXmlQueue(Configuration.java:3998)
at org.hibernate.cfg.Configuration$MetadataSourceQueue.processMetadata(Configuration.java:3986)
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1398)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1856)
at cn.itcast.dao.HibernateUtil.<clinit>(HibernateUtil.java:29)
... 2 more
Caused by: org.hibernate.DuplicateMappingException: Duplicate collection role mapping cn.itcast.domain.Department.employees
at org.hibernate.cfg.Configuration$MappingsImpl.addCollection(Configuration.java:3211)
at org.hibernate.cfg.HbmBinder.createClassProperties(HbmBinder.java:2189)
at org.hibernate.cfg.HbmBinder.createClassProperties(HbmBinder.java:2164)
at org.hibernate.cfg.HbmBinder.bindRootPersistentClassCommonValues(HbmBinder.java:412)
at org.hibernate.cfg.HbmBinder.bindRootClass(HbmBinder.java:326)
at org.hibernate.cfg.HbmBinder.bindRoot(HbmBinder.java:177)
at org.hibernate.cfg.Configuration$MetadataSourceQueue.processHbmXml(Configuration.java:4006)
... 7 more