elkarel elkarel - 1 month ago 15
JSON Question

jackson self reference leading to cycle



I have a problem when converting hibernate objects to JSON with Jackson, because some objects have a self-reference in their definition.
The problem is that I don't control the code of those entities so I cannot put annotations in there.

Actually I would like to just limit the depth of the recursivity, for example to 5 levels. I need generic code that works with any entity object that contains self reference.. Is it possible?
I don't mind use another JSON library.

The following simple code

ObjectMapper mapper = new ObjectMapper();

Query q = session.createQuery("from Hazard ");
List<Hazard> hazards = q.list();

for (Hazard h : hazards) {
String hazardJson;
hazardJson = mapper.writeValueAsString(h);
}


Gives me exception:


org.codehaus.jackson.map.JsonMappingException: Direct self-reference leading to cycle (through reference chain: com.fgm.imsma.pojo.Hazard["location"]->com.fgm.imsma.pojo.Location["location"])
at org.codehaus.jackson.map.ser.BeanPropertyWriter._reportSelfReference(BeanPropertyWriter.java:473)
at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:411)
at org.codehaus.jackson.map.ser.BeanSerializer.serializeFields(BeanSerializer.java:245)
at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:212)
at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:428)
at org.codehaus.jackson.map.ser.BeanSerializer.serializeFields(BeanSerializer.java:245)
at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:212)
at org.codehaus.jackson.map.ser.StdSerializerProvider._serializeValue(StdSerializerProvider.java:587)
at org.codehaus.jackson.map.ser.StdSerializerProvider.serializeValue(StdSerializerProvider.java:245)
at org.codehaus.jackson.map.ObjectMapper._configAndWriteValue(ObjectMapper.java:1993)
at org.codehaus.jackson.map.ObjectMapper.writeValueAsString(ObjectMapper.java:1595)
at imsma.json.GetObjects.main(GetObjects.java:47)



Thanks in advance for any suggestions!

Answer

Use Jackson 2.0 - it handles cyclic references (with @JsonIdentityInfo annotation)

If you cannot add annotations directly to the class then use MixIn annotations. An example here: http://wiki.fasterxml.com/JacksonMixInAnnotations

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