如何制作复合主键（java持久性注释）

你已经在这里得到了一些很好的答案，关于如何完全按照你的要求去做。

作为参考，让我只提到在Hibernate中执行此操作的推荐方法，即使用代理键作为主键，并将业务键标记为NaturalId：

尽管我们建议使用代理键作为主键，但应尝试标识所有实体的自然键。自然键是唯一且非空的属性或属性组合。它也是不可变的。映射元素内部自然键的属性。休眠将生成必要的唯一键和可空性约束，因此，您的映射将更加自我记录。

建议您实现 equals（） 和 hashCode（） 来比较实体的自然键属性。

在代码中，使用注释时，这将看起来像这样：

@Entity
public class UserRole {
  @Id
  @GeneratedValue
  private long id;

  @NaturalId
  private User user;
  @NaturalId
  private Role role;
}

使用此功能将为您节省很多麻烦，因为您会发现何时经常必须引用/映射组合的主键。

我艰难地发现了这一点，最终放弃了与Hibernate的战斗，而是决定顺其自然。我完全理解，这在你的情况下可能是不可能的，因为你可能正在处理遗留软件或依赖项，但我只想提到它以供将来参考。（如果你不能使用它，也许其他人可以！

为了满足您的要求，您可以将@ManyToMany映射为@OneToMany映射。这样，USER_ROLE将同时包含USER_ID和ROLE_ID作为复合主键

我将向您展示如何：

@Entity
@Table(name="USER")
public class User {

    @Id
    @GeneratedValue
    private Integer id;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="joinedUserRoleId.user")
    private List<JoinedUserRole> joinedUserRoleList = new ArrayList<JoinedUserRole>();

    // no-arg required constructor
    public User() {}

    public User(Integer id) {
        this.id = id;
    }

    // addRole sets up bidirectional relationship
    public void addRole(Role role) {
        // Notice a JoinedUserRole object
        JoinedUserRole joinedUserRole = new JoinedUserRole(new JoinedUserRole.JoinedUserRoleId(this, role));

        joinedUserRole.setUser(this);
        joinedUserRole.setRole(role);

        joinedUserRoleList.add(joinedUserRole);
    }

}

@Entity
@Table(name="USER_ROLE")
public class JoinedUserRole {

    public JoinedUserRole() {}

    public JoinedUserRole(JoinedUserRoleId joinedUserRoleId) {
        this.joinedUserRoleId = joinedUserRoleId;
    }

    @ManyToOne
    @JoinColumn(name="USER_ID", insertable=false, updatable=false)
    private User user;

    @ManyToOne
    @JoinColumn(name="ROLE_ID", insertable=false, updatable=false)
    private Role role;

    @EmbeddedId
    // Implemented as static class - see bellow
    private JoinedUserRoleId joinedUserRoleId;

    // required because JoinedUserRole contains composite id
    @Embeddable
    public static class JoinedUserRoleId implements Serializable {

        @ManyToOne
        @JoinColumn(name="USER_ID")
        private User user;

        @ManyToOne
        @JoinColumn(name="ROLE_ID")
        private Role role;

        // required no arg constructor
        public JoinedUserRoleId() {}

        public JoinedUserRoleId(User user, Role role) {
            this.user = user;
            this.role = role;
        }

        public JoinedUserRoleId(Integer userId, Integer roleId) {
            this(new User(userId), new Role(roleId));
        }

        @Override
        public boolean equals(Object instance) {
            if (instance == null)
                return false;

            if (!(instance instanceof JoinedUserRoleId))
                return false;

            final JoinedUserRoleId other = (JoinedUserRoleId) instance;
            if (!(user.getId().equals(other.getUser().getId())))
                return false;

            if (!(role.getId().equals(other.getRole().getId())))
                return false;

            return true;
        }

        @Override
        public int hashCode() {
            int hash = 7;
            hash = 47 * hash + (this.user != null ? this.user.hashCode() : 0);
            hash = 47 * hash + (this.role != null ? this.role.hashCode() : 0);
            return hash;
        }

    }

}

记得

如果对象具有分配的标识符或复合键，则应在调用 save（） 之前将标识符分配给对象实例。

因此，我们创建了一个像这样的JoinedUserRoleId构造函数，以便处理它

public JoinedUserRoleId(User user, Role role) {
    this.user = user;
    this.role = role;
}

最后是角色类

@Entity
@Table(name="ROLE")
public class Role {

    @Id
    @GeneratedValue
    private Integer id;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="JoinedUserRoleId.role")
    private List<JoinedUserRole> joinedUserRoleList = new ArrayList<JoinedUserRole>();

    // no-arg required constructor
    public Role() {}

    public Role(Integer id) {
        this.id = id;
    }

    // addUser sets up bidirectional relationship
    public void addUser(User user) {
        // Notice a JoinedUserRole object
        JoinedUserRole joinedUserRole = new JoinedUserRole(new JoinedUserRole.JoinedUserRoleId(user, this));

        joinedUserRole.setUser(user);
        joinedUserRole.setRole(this);

        joinedUserRoleList.add(joinedUserRole);
    }

}

根据测试，让我们写以下内容

User user = new User();
Role role = new Role();

// code in order to save a User and a Role
Session session = HibernateUtil.getSessionFactory().openSession();
session.beginTransaction();

Serializable userId  = session.save(user);
Serializable roleId = session.save(role);

session.getTransaction().commit();
session.clear();
session.close();

// code in order to set up bidirectional relationship
Session anotherSession = HibernateUtil.getSessionFactory().openSession();
anotherSession.beginTransaction();

User savedUser = (User) anotherSession.load(User.class, userId);
Role savedRole = (Role) anotherSession.load(Role.class, roleId);

// Automatic dirty checking
// It will set up bidirectional relationship
savedUser.addRole(savedRole);

anotherSession.getTransaction().commit();
anotherSession.clear();
anotherSession.close();

请注意，根据上面的代码，没有对 JoinedUserRole 类的引用。

如果要检索已加入的用户角色，请尝试以下操作

Session session = HibernateUtil.getSessionFactory().openSession();
session.beginTransaction();

Integer userId;
Integer roleId;

// Lets say you have set up both userId and roleId
JoinedUserRole joinedUserRole = (JoinedUserRole) session.get(JoinedUserRole.class, new JoinedUserRole.JoinedUserRoleId(userId, roleId));

// some operations

session.getTransaction().commit();
session.clear();
session.close();

问候