問題來源

若是使用hibernate 建立entity，產(chǎn)生互相關(guān)聯(lián)的兩個類（ManytoOne, OnetoMany)

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

此時如果使用json轉(zhuǎn)換工具(fastjson,jackson等)會出現(xiàn)死循環(huán)。
網(wǎng)上很少說根本原因，絕大多數(shù)都在描述，因為json轉(zhuǎn)換B時，發(fā)現(xiàn)set<A>需要轉(zhuǎn)化，blabla之類。
但是根本原因在于，hibernate如果設置了fetch為lazy模式，每次訪問都會加載，如果按著網(wǎng)上說的原因，如果對應的值是null，是不會遞歸下去的。(待我明天求證一下)

網(wǎng)上流傳解決方法

1. 簡單的添加@JsonIgnore之類，靜態(tài)過濾屬性，
2. 對于Jackson使用MixInAnnotation特性，加之AOP，實現(xiàn)動態(tài)過濾。
3. 直接創(chuàng)建對應兩類集合類VO，不適用此類特性。

方法一簡單，但是不適用，每個業(yè)務上運用到的屬性可能都不一致，如果每個都建立對應類不如直接使用3
方法二首先感覺是繁瑣，再者就是性能問題（猜想，沒測試，未來添加測試，理由是有大量的反射，每次都需要動態(tài)解釋，如果可以在編譯階段生成，那性能不是問題）。
方法三簡單性能好，但是創(chuàng)建VO類太多，不適合維護。

我的解決方法

1. 巧妙使用constructor設置不需要關(guān)聯(lián)對象為null，
2. 配上@JsonIgnoreProperties({ "hibernateLazyInitializer", "handler" })

后者為了阻止Jackson在轉(zhuǎn)化時觸發(fā)hibernateLazyFetch機制
前者一是為了是在HQL語句里使用 select new A(B,AProperties) from ... 來實現(xiàn)直接注入，而是set null 可以使Jackson不會遞歸進行這個轉(zhuǎn)換。

這樣，我們只需要給每個類加上@JsonIgnoreProperties(value = { "hibernateLazyInitializer"})
為你每個需要的組合添加一個constructor便好（無需新建類）
很簡單可以實現(xiàn)多表聯(lián)查等復雜的映射問題

未來我會添加更加詳細的原理解釋。

實例

HQL語句

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

兩個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, 對應column映射
}

放一個關(guān)聯(lián)Entity類

@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, 對應column映射