spring-data-jpa双向表关联查询时引发异常：java.lang.StackOverflowError: null

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 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 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DAO层代码…

控制层代码

@RestController
public class TestController{
	@Autowired
	private StudentRepository studentRep;
	
	@RequestMapping("/test")
	public int test(){
		List 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 courses，在遍历集合中，输出一个student实例的时候，List 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 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 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