一个Hibernate多对多实例

更新：2015-02-11

	@ManyToMany(targetEntity = Role.class, fetch = FetchType.EAGER)
	@JoinTable(name = "T_USERS_ROLES", joinColumns = @JoinColumn(name = "USER_ID"), inverseJoinColumns = @JoinColumn(name = "ROLE_ID"))
	private Set roles = new HashSet();

	@ManyToMany(mappedBy = "roles", targetEntity = User.class , fetch = FetchType.LAZY)
	private Set users = new HashSet();

双向多对多，上面是在user中声明，下面是在role中声明。

如果单向多对多，则删除不要的那侧域声明。比如不需要通过role去查询拥有的user，可删除下面role侧的users域声明
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下面贴上经典的多对多实例：用户与角色。直接上代码。

用户User：

package cn.qeli.ums.entity;

import java.util.HashSet;
import java.util.Set;

public class User {

	private String userid;
	private String username;
	private String password;
	private Set roles = new HashSet();

	public User() {
		super();
	}

	public String getUserid() {
		return userid;
	}

	public void setUserid(String userid) {
		this.userid = userid;
	}

	public String getUsername() {
		return username;
	}

	public void setUsername(String username) {
		this.username = username;
	}

	public String getPassword() {
		return password;
	}

	public void setPassword(String password) {
		this.password = password;
	}

	public Set getRoles() {
		return roles;
	}

	public void setRoles(Set roles) {
		this.roles = roles;
	}
}

User.hbm.xml

角色Role：

package cn.qeli.ums.entity;

import java.util.HashSet;
import java.util.Set;

public class Role {

	private String roleid;
	private String rolename;
	private Integer ordernum;
	private String description;
	private Set users = new HashSet();

	public Role() {
		super();
	}

	public String getRoleid() {
		return roleid;
	}

	public void setRoleid(String roleid) {
		this.roleid = roleid;
	}

	public String getRolename() {
		return rolename;
	}

	public void setRolename(String rolename) {
		this.rolename = rolename;
	}

	public Integer getOrdernum() {
		return ordernum;
	}

	public void setOrdernum(Integer ordernum) {
		this.ordernum = ordernum;
	}

	public String getDescription() {
		return description;
	}

	public void setDescription(String description) {
		this.description = description;
	}

	public Set getUsers() {
		return users;
	}

	public void setUsers(Set users) {
		this.users = users;
	}
}

Role.hbm.xml

测试类：在测试前假设已经存在userid为402881e7394e8f4501394e8f476d0000的用户，存在roleid为402881e4393a6f3a01393a6f3c1a0000的角色。

package cn.qeli.ums.UTest;

import org.hibernate.Session;

import cn.qeli.ums.HibernateSessionFactory;
import cn.qeli.ums.entity.Role;
import cn.qeli.ums.entity.User;

public class UserTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		UserTest test = new UserTest();
		test.addUserToRole();
	}

	public void addUserToRole() {
		Session session = HibernateSessionFactory.getSession();
		session.beginTransaction();
		User xhg = (User) session.get("cn.qeli.ums.entity.User",
				"402881e7394e8f4501394e8f476d0000");
		Role sys = (Role) session.get("cn.qeli.ums.entity.Role",
				"402881e4393a6f3a01393a6f3c1a0000");
		xhg.getRoles().add(sys);
		session.getTransaction().commit();
	}

	public void addRole() {
		Session session = HibernateSessionFactory.getSession();
		session.beginTransaction();
		Role role = new Role();
		role.setRolename("系统管理员");
		role.setOrdernum(1);
		role.setDescription("进行系统维护的角色");
		session.save(role);
		session.getTransaction().commit();
		session.close();
	}

	public void addUser() {
		Session session = HibernateSessionFactory.getSession();
		session.beginTransaction();
		User user = new User();
		user.setUsername("晋江古");
		user.setPassword("1984");
		session.save(user);
		session.getTransaction().commit();
		session.close();
	}

}

执行测试类的功能，也就是给用户赋予一个角色。会输出以下sql语句：

select user0_.userid as userid0_0_, user0_.username as username0_0_, user0_.password as password0_0_ from Users user0_ where user0_.userid=?

select role0_.roleid as roleid2_0_, role0_.rolename as rolename2_0_, role0_.ordernum as ordernum2_0_, role0_.description as descript4_2_0_ from Roles role0_ where role0_.roleid=?

select roles0_.userid as userid0_1_, roles0_.roleid as roleid1_, role1_.roleid as roleid2_0_, role1_.rolename as rolename2_0_, role1_.ordernum as ordernum2_0_, role1_.description as descript4_2_0_ from user_role roles0_ inner join Roles role1_ on roles0_.roleid=role1_.roleid where roles0_.userid=?

insert into user_role (userid, roleid) values (?, ?)

执行后，数据库的结果是在中间表user_role中插入一条记录，将userid和roleid对应起来。

测试类要注意xhg.getRoles().add(sys);这行语句。细心的读者可能会发现，这行是不是也可以用sys.getUsers().add(xhg);来代替呢？答案是不能。因为我们在Role.hbm.xml中使用了【inverse="true"】。这属性的意思是，Role实体将不维护Role与其他实体的主外键关系。此例中，就是说让User来维护这种关系。其实不维护这种关系，说白了就是表明，凡用Role实体对象来执行set对象中的添加、更新、删除等语句，都无效。即输出的sql不会有最后一条insert语句。