ConversionFailedException: Persisting a DBObject b

问题:

I am persisting a object:

@Document
public class PotentialCandidates {

    @Id
    private String jobid;

    @CreatedDate
    private DateTime created;

    @LastModifiedDate
    private DateTime modified;

    private DBObject potentialcandidates;

    public String getJobid() {
        return this.jobid;
    }   
    public void setJobid(String jobid) {
        this.jobid = jobid;
    }

    public DBObject getPotentialcandidates() {
        return this.potentialcandidates;
    }   
    public void setPotentialcandidates(DBObject potentialcandidates) {
        this.potentialcandidates = potentialcandidates;
    }

}

where potentialCandidates are set from a JSON string as so:

potentialCandidatesObj.setPotentialcandidates((DBObject)JSON.parse(valStr));

This persists fine to my mongodb and gives me an object on the DB I can drill down into, however when I try to retrieve my db object:

    public PotentialCandidates getPotentialCandidatesByJobid(String jobid) throws NoSuchPotentialCandidatesException , SystemException{

    PotentialCandidates Jobid = null;
try {
            Query query = new Query();
            query.addCriteria(Criteria.where("_id").is(jobid));
            Jobid = mongoTemplateJobs.findOne(query, PotentialCandidates.class,
                    COLLECTION_NAME);

            return Jobid;
        } catch (Exception ex) {
            throw new SystemException(ex);
        } finally {
            if (Jobid == null) {
                throw new NoSuchPotentialCandidatesException("No User with jobid: "
                        + jobid + "found..");
            }
        }
}

I encounter the following error:

org.springframework.core.convert.ConversionFailedException: Failed to convert from type java.util.ArrayList<?> to type com.mongodb.DBObject for value 'myString'; nested exception is org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type java.util.LinkedHashMap<?, ?> to type com.mongodb.DBObject

So it would seem I need some sort of logic to handle retrieves from mongo. I could use a different return class in my findOne query but that seems a little messy. Is there a standard approach to dealing with this?

回答1:

your error is probably exactly what it says in your exception: a ConversionFailed Exception caused by someone/something trying to convert from ArrayList to a LinkedHashMap; but there is just no fitting converter for that (ConverterNotFoundException).

where exactly this is happening is impossible to say since you only posted very little code. i can not find the String "myString" in your code, yet it is mentioned in the error.

Is there a standard approach to dealing with this?

spring data usually uses converters in its mapping process. to have more control over the mapping process some people prefer to implement and register a custom converter for their classes.

you can read about converters here

http://docs.spring.io/spring-data/data-mongo/docs/current/reference/html/mongo.core.html#mongo.custom-converters

and here

http://docs.spring.io/spring/docs/current/spring-framework-reference/html/validation.html#core-convert

maybe this will already be enough for you to fix the error yourself.

Edit: a short comment about this line:

potentialCandidatesObj.setPotentialcandidates((DBObject)JSON.parse(valStr));

you are casting to DBObject before calling the setter, because the setter takes a DBObject. this is bad, you should create another setter for JSON and do the casting there, or you will end up doing that casting operation everywhere in your code; that's not very DRY.

there is also something called DBRefs in spring data: The mapping framework doesn't have to store child objects embedded within the document. You can also store them separately and use a DBRef to refer to that document. When the object is loaded from MongoDB, those references will be eagerly resolved and you will get back a mapped object that looks the same as if it had been stored embedded within your master document. you might prefer this over a embedded DBObject.