问题
I have two classes as shown below in a bi-directional Many to Many relationship:
Parent implements Serializable{
@ManytoMany(//declaration for join table)
@JsonBackReference
@com.fasterxml.jackson.annotation.JsonIgnore
Set <Child> childSet;
}
Child implements Serializable{
@ManytoMany(//declaration for join table)
@JsonManagedReference
@com.fasterxml.jackson.annotation.JsonIgnore
Set <Parent> parentSet;
// other getter and setters
}
I make a call in my DAO to get a particular parent. Alongwith the parent details I want to fetch the children of the parent. Something like this:
Hibernate.initialize(parent.getChildSet()); //this works perfectly
// and I get the details of parent along with the children in my DAO call.
But when I do the below in my business service while returning the data to the controller the children are omitted from the parent json string.
jacksonMapper.writeValueAsString(parent);
So i removed the @JsonIgnore on Child attribute inside Parent class thinking that jackson might understand that these fields are not to be ignored while writing to a string as shown below. But it still does ignore them! :(
Parent implements Serializable{
@ManytoMany(//declaration for join table)
@JsonBackReference
//@com.fasterxml.jackson.annotation.JsonIgnore
Set <Child> childSet;
}
Any idea where I might be going wrong?
回答1:
I have not been able to find out why this is happening. Meanwhile I have opted for a workaround. I am making two separate calls to DB. One to fetch the parent first and then second to fetch the child based on the fetched parentId.
Alternatively, I can make both the DB calls at the service same time and prepare the JSON as a complex string before sending it to the ui:
complex:{
parent:parent,
child:child
}
In either case, it is a workaround. The ideal solution is just remove@JsonIgnore in the mapping only from the parent side for the child class. But somehow that doesn't seem to work. I'll post in case I find why the "ideal" solution is not working!
Ideal Solution Updated as answer on 15 Aug 2016 The Independence Day of India:
The problem is in the mapping:
Parent implements Serializable{
@ManytoMany(//declaration for join table)
@JsonBackReference
@com.fasterxml.jackson.annotation.JsonIgnore
Set <Child> childSet;
}
When you say @JsonBackReference it actually means ignore this field while writing the Json, i.e. to say,
@JsonBackReference <-> @JsonIgnore
Hence the child is omitted when the parent is serialized. With ORM mappings it's always a best practice to have the annotations one sided rather than double sided. In that way, you can avoid a lot of unwanted exceptions while fetching the data and secondly, keep your business code clean.
JsonManagedReference vs JsonBackReference
来源：https://stackoverflow.com/questions/36809325/losing-the-child-data-in-a-bi-directional-relationship-when-using-jacksonmapper