hibernate無限遞歸問題

項目異常如下：

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"]->
......//無限遞歸



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
...
    
    
   
  
  
          

看異常的名字會發現是棧溢出，而且第一處后面出現大量的遞歸的提示，這邊的原因是因為使用了jpa的實體類注解@ManyToMany,@ManyToOne,@OneToOne並且是雙向表關聯

舉個例子：

@Entity
@Table(name = "t_student")
public class Student{
    @Id
    @GeneratedValue
    private Integer sId;
    @Column(length = 20)
    private String sName;
    //學生所修課程
    @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;
    //上課的學生
    @ManyToMany(cascade = CascadeType.ALL)
    private List<Student> students;
    //get、set 方法省略
}
    
    
   
  
  
          

一個student類和一個course類分別對應 t_student 表和 t_course表,它們是多對多的關系，一個學生可以修多個課程，一個課程可以有多個學生修

省略倉儲層代碼…

控制層代碼

@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;
    }
}
    
    
   
  
  
          

此時，程序運行便會報 java.lang.StackOverflowError: null 異常

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類中有一個字段List<Course> courses，在遍歷集合中，輸出一個student實例的時候，List<Course> courses 字段也將輸出，因為是雙向多對多的關聯查詢，每一個Course實例也會輸出List<Student> students 字段值，因此一直遞歸下去直到棧溢出報錯，若是單向多對多就不會發生這樣地遞歸

其實一開始報這個錯完全是因為對spring-data-jpa不熟悉，了解之后一想就通了。

解決辦法：

方法一：
此處，若不需要查看關聯表中的字段信息時，可以在遍歷List<Student> students集合時先將關聯對象設置為null,即：

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

這樣才能打印出Student 實例中除了courses 字段之外地所有數據

方法二：
取消使用雙向多對多關聯，改為使用單向多對多，這樣也可以同時將關聯表中所對應的數據查詢出來使用

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