hibernate无限递归问题

项目异常如下：

2018-01-26 17:12:38.162  WARN 3128 --- [nio-8080-exec-6] .w.s.m.s.DefaultHandlerExceptionResolver : Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Infinite recursion (StackOverflowError); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Infinite recursion (StackOverflowError) (through reference chain: demo.server.core.domain.User["userAddresses"]->
org.hibernate.collection.internal.PersistentBag[0]->
demo.server.core.domain.UserAddress["user"]->
demo.server.core.domain.User["userAddresses"]->
org.hibernate.collection.internal.PersistentBag[0]->
demo.server.core.domain.UserAddress["user"]->
......//无限递归
java.lang.StackOverflowError: null
    at java.lang.ClassLoader.defineClass1(Native Method) ~[na:1.8.0_111]
    at java.lang.ClassLoader.defineClass(Unknown Source) ~[na:1.8.0_111]
    at java.security.SecureClassLoader.defineClass(Unknown Source) ~[na:1.8.0_111]
    at java.net.URLClassLoader.defineClass(Unknown Source) ~[na:1.8.0_111]
    at java.net.URLClassLoader.access$100(Unknown Source) ~[na:1.8.0_111]
    at java.net.URLClassLoader$1.run(Unknown Source) ~[na:1.8.0_111]
    at java.net.URLClassLoader$1.run(Unknown Source) ~[na:1.8.0_111]
    at java.security.AccessController.doPrivileged(Native Method) ~[na:1.8.0_111]
    at java.net.URLClassLoader.findClass(Unknown Source) ~[na:1.8.0_111]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[na:1.8.0_111]
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) ~[na:1.8.0_111]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[na:1.8.0_111]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:709) ~[jackson-databind-2.8.1.jar:2.8.1]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155) ~[jackson-databind-2.8.1.jar:2.8.1]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:149) ~[jackson-databind-2.8.1.jar:2.8.1]
    at
...

看异常的名字会发现是栈溢出，而且第一处后面出现大量的递归的提示，这边的原因是因为使用了jpa的实体类注解@ManyToMany,@ManyToOne,@OneToOne并且是双向表关联

举个例子：

@Entity
@Table(name = "t_student")
public class Student{
    @Id
    @GeneratedValue
    private Integer sId;
    @Column(length = 20)
    private String sName;
    //学生所修课程
    @ManyToMany(mappedBy = "students")
    private List<Course> courses;
    //get、set 方法省略
}

@Entity
@Table(name = "t_course")
public class Course{
    @Id
    @GeneratedValue
    private Integer cId;
    @Column(length = 20)
    private String cName;
    //上课的学生
    @ManyToMany(cascade = CascadeType.ALL)
    private List<Student> students;
    //get、set 方法省略
}

一个student类和一个course类分别对应 t_student 表和 t_course表,它们是多对多的关系，一个学生可以修多个课程，一个课程可以有多个学生修

省略仓储层代码…

控制层代码

@RestController
public class TestController{
    @Autowired
    private StudentRepository studentRep;
    @RequestMapping("/test")
    public int test(){
        List<Student> students = stntRep.findAll();
        students.forEach(data->{
            System.out.println(data);
        });
        return 1;
    }
}

此时，程序运行便会报 java.lang.StackOverflowError: null 异常

java.lang.StackOverflowError: null
    at java.util.AbstractCollection.toString(Unknown Source) ~[na:1.8.0_111]
    at org.hibernate.collection.internal.PersistentBag.toString(PersistentBag.java:510) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
    at java.lang.String.valueOf(Unknown Source) ~[na:1.8.0_111]
    at java.lang.StringBuilder.append(Unknown Source) ~[na:1.8.0_111]
    at com.demo.domain.Course.toString(Course.java:38) ~[classes/:na]
    at java.lang.String.valueOf(Unknown Source) ~[na:1.8.0_111]
    at java.lang.StringBuilder.append(Unknown Source) ~[na:1.8.0_111]
    at java.util.AbstractCollection.toString(Unknown Source) ~[na:1.8.0_111]
    at org.hibernate.collection.internal.PersistentBag.toString(PersistentBag.java:510) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
    at java.lang.String.valueOf(Unknown Source) ~[na:1.8.0_111]
    at java.lang.StringBuilder.append(Unknown Source) ~[na:1.8.0_111]
    at com.demo.domain.Student.toString(Student.java:40) ~[classes/:na]
    ....

Student类中有一个字段List<Course> courses，在遍历集合中，输出一个student实例的时候，List<Course> courses 字段也将输出，因为是双向多对多的关联查询，每一个Course实例也会输出List<Student> students 字段值，因此一直递归下去直到栈溢出报错，若是单向多对多就不会发生这样地递归

其实一开始报这个错完全是因为对spring-data-jpa不熟悉，了解之后一想就通了。

解决办法：

方法一：

此处，若不需要查看关联表中的字段信息时，可以在遍历List<Student> students集合时先将关联对象设置为null,即：

students.forEach(data->{
    data.setCourses(null);
    System.out.println(data);
});

这样才能打印出Student 实例中除了courses 字段之外地所有数据

方法二：

取消使用双向多对多关联，改为使用单向多对多，这样也可以同时将关联表中所对应的数据查询出来使用

原文地址：https://blog.csdn.net/pbhLOVEpp/article/details/77945651