How to convert beans with bi-directional reference?

Question

I wrote some generic bean conversion that used together to convert a complex bean. Sometimes the input bean has bi-directional reference, and the converter traps in an infinite loop. How can I manage that?

For example the input is A bean that has relation to B bean and B bean has relation to exactly the A instance. When converting the A object starts, the B converter invokes to convert the b field in A, and also on converting the B object, A converter invokes to convert the a field in the B class, and as you know, the converter traps on infinite loop. Think about a more complex relation between beans that the relation graph has more than two nodes.

How can I avoid this infinite recursion in the converters?

Use an `IdentityHashMap` to keep a reference to the objects already transformed, and check in the `IdentityHashMap` before transforming a new object if it's not already in it. — Florent Bayle, Jan 12 '15 at 14:26
@fardjad `IdentityHashMap` doesn't use `hashCode()` and `equals()`. — Florent Bayle, Jan 12 '15 at 14:30

score 1 · Answer 1 · edited May 23 '17 at 11:59

Object references form a net.

A -------> B
^--- C <---'

When serializing to XML, JSON or similar, you are building a tree. Since a tree is a simpler form of a net you can not simply copy one into the other.

A
'--> B
     '--> C
          '--> A (recursion alarm!)
               '--> ...

A common solution is to

either stop serializing, as soon as recursion is detected (just keep track of all objects that are already serialized)
```
A
'--> B
     '--> C
```
or, when detecting recursion, add a stub reference to another node in the serialized tree. A unique identifier of nodes is required.
```
A(1)
'--> B(2)
     '--> C(3)
          '--> Stub that links to (1)
```

For the technical detail of storing the already serialized objects:

using a HashSet might seem handy as it does not contain duplicates - but since it requires hashCode and equals methods for all items this is generally not a good idea
using the keys in IdentityHashMap can be used, it will compare keys using == and not contain duplicates
but since you usually check if the collection already contains the item before to add it, you can also use ArrayList with this code: collection.stream().anyMatch(x -> x == key)

How to convert beans with bi-directional reference?

1 Answers1