如何使用Hibernate定义友谊关系？

Question

我需要有"FriendRequest"和"ListOfFriends"功能.类似于facebook,它显示收到的朋友请求数和已批准的朋友数.

通过"FriendRequest",我的意思是拥有用户收到的朋友请求列表.

通过"ListOfFriends",我的意思是拥有用户的朋友列表.

为了实现这一点,我定义了以下类,但是当我尝试使用其用户名检索用户时,将抛出"Stackoverflow"异常消息.它似乎进入了一个无限循环.

当我从我的toString方法中删除"FriendRequests"和"Friends"时,它会停止抛出异常.

这个问题类似于我要实现的问题但不同之处在于我不知道要拥有单独的Credential类.

会员

@Entity
public class Member implements Serializable {
    @Id
    @GeneratedValue
    private long id;
    @Column(name = "username", nullable = false, unique = true)
    private String username;
    @Column(nullable = false)
    private String fname;
    @Column(nullable = false)
    private String lname;
    @OneToMany(fetch = FetchType.EAGER, mappedBy = "requester")
    private Set<Friendship> friendRequests = new HashSet<Friendship>();
    @OneToMany(fetch = FetchType.EAGER, mappedBy = "friend")
    private Set<Friendship> friends = new HashSet<Friendship>();

    getters and setters

    @Override
    public String toString() {
       return "Member [
                .....                 
                , fname=" + fname + ", lname=" + lname
                //  + friendRequests.size() + ", friends=" + friends.size() + 
       "]";
    }
}

友谊

@Entity
public class Friendship implements Serializable {
    @Id
    @ManyToOne
    @JoinColumn(referencedColumnName = "username")
    Member requester;
    @Id
    @ManyToOne
    @JoinColumn(referencedColumnName = "username")
    Member friend;
    @Temporal(javax.persistence.TemporalType.DATE)
    Date date;
    @Column(nullable = false)
    boolean active;

Answer 1

我不明白你遇到的问题，尽管我对你的代码做了很少的修改，所以它可以与我的设置一起工作。

/META-INF/persistence.xml

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
  <persistence-unit name="hibernate-entitymanager-demo" transaction-type="RESOURCE_LOCAL">
    <provider>org.hibernate.ejb.HibernatePersistence</provider>
    <properties>
      <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/hibernate4?zeroDateTimeBehavior=convertToNull"/>
      <property name="javax.persistence.jdbc.password" value="root"/>
      <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
      <property name="javax.persistence.jdbc.user" value="root"/>
      <property name="hibernate.cache.provider_class" value="org.hibernate.cache.NoCacheProvider"/>
      <!-- property name="javax.persistence.schema-generation.database.action" value="drop-and-create"/ -->
    </properties>
  </persistence-unit>
</persistence>

Main.java（这里有两个用例：createFriendships 来初始化一些数据，或者 findMembers 来解决有问题的用例。请注意javax.persistence.schema-generation.database.actionpersistence.xml 中的属性，因为您需要在第一个用例中创建数据库，而不是在后者）

public class Main {

    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("hibernate-entitymanager-demo");
        EntityManager em = emf.createEntityManager();

        EntityTransaction transaction = em.getTransaction();
        transaction.begin();

        // createFriendships(em);

        findMembers(em);

        transaction.commit();

        em.close();
        emf.close();
    }

    private static void findMembers(EntityManager em) {
        List<Member> list = em.createQuery("from Member").getResultList();
        for (Member m : list) {
            System.out.println(m);
        }
    }

    private static void createFriendships(EntityManager em) {
        List<Member> members = createMembers(em);

        for (int i = 0; i < members.size(); i++) {
            for (int j = 0; j < members.size(); j++) {
                if (i != j) {
                    createFriendship(em, members.get(i), members.get(j));
                }
            }
        }
    }

    private static List<Member> createMembers(EntityManager em) {
        List<Member> members = new ArrayList<>();
        members.add(createMember(em, "Roberta", "Williams", "rwilliams"));
        members.add(createMember(em, "Ken", "Williams", "kwilliams"));
        members.add(createMember(em, "Dave", "Grossman", "dgrossman"));
        members.add(createMember(em, "Tim", "Schafer", "tschafer"));
        members.add(createMember(em, "Ron", "Gilbert", "rgilbert"));
        return members;
    }

    private static Member createMember(EntityManager em, String fname, String lname, String username) {
        Member m = new Member();
        m.setFname(fname);
        m.setLname(lname);
        m.setUsername(username);
        em.persist(m);
        return m;
    }

    private static void createFriendship(EntityManager em, Member requester, Member friend) {
        Friendship f = new Friendship();
        f.setActive(true);
        f.setDate(new Date());
        f.setRequester(requester);
        f.setFriend(friend);
        em.persist(f);
    }

}

主要生产：

Member [fname = Roberta, lname = Williams, requests = 4, friends = 4]
Member [fname = Ken, lname = Williams, requests = 4, friends = 4]
Member [fname = Dave, lname = Grossman, requests = 4, friends = 4]
Member [fname = Tim, lname = Schafer, requests = 4, friends = 4]
Member [fname = Ron, lname = Gilbert, requests = 4, friends = 4]

Friendship.java真正的实际更改是引用的列名称，当我遇到某种异常时，我将其更改username为。除此之外，我认为在数据库规范化方面更好：idcould not get a field value by reflection

@Entity
@Table(name = "FRIENDSHIPS")
public class Friendship implements Serializable {

    @Id
    @ManyToOne
    @JoinColumn(referencedColumnName = "id")
    Member requester;

    @Id
    @ManyToOne
    @JoinColumn(referencedColumnName = "id")
    Member friend;

    @Temporal(javax.persistence.TemporalType.DATE)
    Date date;

    @Column(nullable = false)
    boolean active;

    // getters & setters

}

Member.java（此处没有更改，除了toString()）

@Entity
@Table(name = "MEMBERS")
public class Member implements Serializable {

    @Id
    @GeneratedValue
    private long id;

    @Column(name = "username", nullable = false, unique = true)
    private String username;

    @Column(nullable = false)
    private String fname;

    @Column(nullable = false)
    private String lname;

    @OneToMany(fetch = FetchType.EAGER, mappedBy = "requester")
    private Set<Friendship> friendRequests = new HashSet<>();

    @OneToMany(fetch = FetchType.EAGER, mappedBy = "friend")
    private Set<Friendship> friends = new HashSet<>();

    // getters & setters

    @Override
    public String toString() {
        return "Member [fname = " + fname + ", lname = " + lname
                + ", requests = " + friendRequests.size()
                + ", friends = " + friends.size() + "]";
    }

}