巧妙解决Hibernate Jackson 双向关联循环

问题来源

若是使用hibernate 建立entity，产生互相关联的两个类（ManytoOne, OnetoMany)

class A{
private B b;
private String AProperties;
}
class B{
private set<A> a;
private String BProperties;
}

此时如果使用json转换工具(fastjson,jackson等)会出现死循环。
网上很少说根本原因，绝大多数都在描述，因为json转换B时，发现set<A>需要转化，blabla之类。
但是根本原因在于，hibernate如果设置了fetch为lazy模式，每次访问都会加载，如果按着网上说的原因，如果对应的值是null，是不会递归下去的。(待我明天求证一下)

网上流传解决方法

1. 简单的添加@JsonIgnore之类，静态过滤属性，
2. 对于Jackson使用MixInAnnotation特性，加之AOP，实现动态过滤。
3. 直接创建对应两类集合类VO，不适用此类特性。

方法一简单，但是不适用，每个业务上运用到的属性可能都不一致，如果每个都建立对应类不如直接使用3
方法二首先感觉是繁琐，再者就是性能问题（猜想，没测试，未来添加测试，理由是有大量的反射，每次都需要动态解释，如果可以在编译阶段生成，那性能不是问题）。
方法三简单性能好，但是创建VO类太多，不适合维护。

我的解决方法

1. 巧妙使用constructor设置不需要关联对象为null，
2. 配上@JsonIgnoreProperties({ "hibernateLazyInitializer", "handler" })

后者为了阻止Jackson在转化时触发hibernateLazyFetch机制
前者一是为了是在HQL语句里使用 select new A(B,AProperties) from ... 来实现直接注入，而是set null 可以使Jackson不会递归进行这个转换。

这样，我们只需要给每个类加上@JsonIgnoreProperties(value = { "hibernateLazyInitializer"})
为你每个需要的组合添加一个constructor便好（无需新建类）
很简单可以实现多表联查等复杂的映射问题

未来我会添加更加详细的原理解释。

实例

HQL语句

String hql="select new UserAccount(uau,ua.username) from UserAccount as ua left join ua.user where ua.userAccountId=?0";

两个Entity

@Entity
@Table(name = "user_account", catalog = "yunshen", uniqueConstraints = { @UniqueConstraint(columnNames = "telephone"),
        @UniqueConstraint(columnNames = "email"), @UniqueConstraint(columnNames = "username") })
@JsonIgnoreProperties(value = { "hibernateLazyInitializer", "handler"})
public class UserAccount implements java.io.Serializable {
    private Integer userAccountId;
    private User user;
    private String username;
    private String password;
    private String salt;
    private String telephone;
    private String email;
    private Date addTime;
    private Date loginTime;
    private int status;

    public UserAccount() {
    }

    public UserAccount(User user, String username,String password) {
        user.setDepartment(null);
        user.setUserAccounts(null);
        user.setUserAuthorities(null);
        this.password=password;
        this.user = user;
        this.username = username;
    }

    
    public UserAccount(User user, String username, String password, String salt, Date addTime, int status) {
        this.user = user;
        this.username = username;
        this.password = password;
        this.salt = salt;
        this.addTime = addTime;
        this.status = status;
    }

    public UserAccount(User user, String username, String password, String salt, String telephone, String email,
            Date addTime, Date loginTime, int status) {
        this.user = user;
        this.username = username;
        this.password = password;
        this.salt = salt;
        this.telephone = telephone;
        this.email = email;
        this.addTime = addTime;
        this.loginTime = loginTime;
        this.status = status;
    }
//省略掉getter and setter, 对应column映射
}

放一个关联Entity类

@Entity
@Table(name = "user")
@JsonIgnoreProperties(value = { "hibernateLazyInitializer", "handler"})
public class User implements java.io.Serializable {
    private Integer userId;
    private Department department;  
    private String name;
    private String sex;
    private Date birthday;
    private String remark;
    private Set<UserAccount> userAccounts = new HashSet<UserAccount>(0);
    private Set<UserAuthority> userAuthorities = new HashSet<UserAuthority>(0);

    public User() {
    }
    public User( String name, String sex) {
        this.name = name;
        this.sex = sex;
    }

    @Override
    public String toString() {
        return "User [userId=" + userId + ", name=" + name + ", sex=" + sex
                + ", birthday=" + birthday + ", remark=" + remark ;
    }

    public User(Department department, String name, String sex) {
        this.department = department;
        this.name = name;
        this.sex = sex;
    }

    public User(Department department, String name, String sex, Date birthday, String remark,
            Set<UserAccount> userAccounts, Set<UserAuthority> userAuthorities) {
        this.department = department;
        this.name = name;
        this.sex = sex;
        this.birthday = birthday;
        this.remark = remark;
        this.userAccounts = userAccounts;
        this.userAuthorities = userAuthorities;
    }
//省略掉getter and setter, 对应column映射