Java-无限递归与Jackson JSON和Hibernate JPA问题

Question

尝试将具有双向关联的JPA对象转换为JSON时，我不断

org.codehaus.jackson.map.JsonMappingException: Infinite recursion (StackOverflowError)

我所发现的只是该线程，基本上以建议避免双向关联为结尾。有谁知道这个Spring错误的解决方法？

代码段：

业务对象1：

@Entity
@Table(name = "ta_trainee", uniqueConstraints = {@UniqueConstraint(columnNames = {"id"})})
public class Trainee extends BusinessObject {

    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    @Column(name = "id", nullable = false)
    private Integer id;

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

    @Column(name = "surname", nullable = true)
    private String surname;

    @OneToMany(mappedBy = "trainee", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @Column(nullable = true)
    private Set<BodyStat> bodyStats;

    @OneToMany(mappedBy = "trainee", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @Column(nullable = true)
    private Set<Training> trainings;

    @OneToMany(mappedBy = "trainee", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @Column(nullable = true)
    private Set<ExerciseType> exerciseTypes;

    public Trainee() {
        super();
    }

    ... getters/setters ...

业务对象2：

import javax.persistence.*;
import java.util.Date;

@Entity
@Table(name = "ta_bodystat", uniqueConstraints = {@UniqueConstraint(columnNames = {"id"})})
public class BodyStat extends BusinessObject {

    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    @Column(name = "id", nullable = false)
    private Integer id;

    @Column(name = "height", nullable = true)
    private Float height;

    @Column(name = "measuretime", nullable = false)
    @Temporal(TemporalType.TIMESTAMP)
    private Date measureTime;

    @ManyToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @JoinColumn(name="trainee_fk")
    private Trainee trainee;

控制器：

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;

import javax.servlet.http.HttpServletResponse;
import javax.validation.ConstraintViolation;
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;

@Controller
@RequestMapping(value = "/trainees")
public class TraineesController {

    final Logger logger = LoggerFactory.getLogger(TraineesController.class);

    private Map<Long, Trainee> trainees = new ConcurrentHashMap<Long, Trainee>();

    @Autowired
    private ITraineeDAO traineeDAO;

    /**
     * Return json repres. of all trainees
     */
    @RequestMapping(value = "/getAllTrainees", method = RequestMethod.GET)
    @ResponseBody        
    public Collection getAllTrainees() {
        Collection allTrainees = this.traineeDAO.getAll();

        this.logger.debug("A total of " + allTrainees.size() + "  trainees was read from db");

        return allTrainees;
    }    
}

JPA实施学员DAO：

@Repository
@Transactional
public class TraineeDAO implements ITraineeDAO {

    @PersistenceContext
    private EntityManager em;

    @Transactional
    public Trainee save(Trainee trainee) {
        em.persist(trainee);
        return trainee;
    }

    @Transactional(readOnly = true)
    public Collection getAll() {
        return (Collection) em.createQuery("SELECT t FROM Trainee t").getResultList();
    }
}

persistence.xml

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"
             version="1.0">
    <persistence-unit name="RDBMS" transaction-type="RESOURCE_LOCAL">
        <exclude-unlisted-classes>false</exclude-unlisted-classes>
        <properties>
            <property name="hibernate.hbm2ddl.auto" value="validate"/>
            <property name="hibernate.archive.autodetection" value="class"/>
            <property name="dialect" value="org.hibernate.dialect.MySQL5InnoDBDialect"/>
            <!-- <property name="dialect" value="org.hibernate.dialect.HSQLDialect"/>         -->
        </properties>
    </persistence-unit>
</persistence>

Answer 1

你现在可以使用JsonIgnoreProperties到属性（序列化过程中）的抑制序列，或忽略JSON性能的处理读取（反序列化过程）。如果这不是你想要的，请继续阅读以下内容。

（感谢As Zammel AlaaEddine指出了这一点）。

JsonManagedReference和JsonBackReference

从Jackson 1.6开始，你可以使用两个注释来解决无限递归问题，而不必在序列化过程中忽略getter / setter方法：@JsonManagedReference和@JsonBackReference。

说明

为了使Jackson正常工作，不应将关系的两个方面之一进行序列化，以避免引起你stackoverflow错误的infite循环。

因此，Jackson接受了引用的前一部分（你Set<BodyStat> bodyStats在Trainee类中），并将其转换为类似json的存储格式；这就是所谓的编组过程。然后，Jackson查找引用的Trainee trainee后半部分（即，在BodyStat类中），并将其保留不变，而不对其进行序列化。关系的这一部分将在前向引用的反序列化（反编组）期间重新构建。

你可以这样更改代码（我跳过了无用的部分）：

业务对象1：

@Entity
@Table(name = "ta_trainee", uniqueConstraints = {@UniqueConstraint(columnNames = {"id"})})
public class Trainee extends BusinessObject {

    @OneToMany(mappedBy = "trainee", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @Column(nullable = true)
    @JsonManagedReference
    private Set<BodyStat> bodyStats;

业务对象2：

@Entity
@Table(name = "ta_bodystat", uniqueConstraints = {@UniqueConstraint(columnNames = {"id"})})
public class BodyStat extends BusinessObject {

    @ManyToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @JoinColumn(name="trainee_fk")
    @JsonBackReference
    private Trainee trainee;

现在一切都应该正常工作。

如果你需要更多信息，我在我的博客Keenformatics上写了一篇有关Json和Jackson Stackoverflow问题的文章。

编辑：

你可以检查的另一个有用的注释是@JsonIdentityInfo：使用它，每次Jackson序列化你的对象时，它将为它添加一个ID（或你选择的另一个属性），这样就不会每次都完全“扫描”它。当你在更多相互关联的对象之间形成链循环时（例如：Order-> OrderLine-> User-> Order and over over），这很有用。

在这种情况下，你必须要小心，因为你可能需要多次读取对象的属性（例如，在一个产品列表中有多个共享同一卖方的产品），并且此注释会阻止你这样做。我建议始终查看Firebug日志，以检查Json响应，并查看代码中发生了什么。