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
- 简单的添加@JsonIgnore之类，静态过滤属性，
- 对于Jackson使用
MixInAnnotation特性，加之AOP，实现动态过滤。 - 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
- 巧妙使用constructor设置不需要关联对象为null，
- 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
