Cannot read or serialize POJO with enumerations using Java MongoDB driver

Question

I have an existing object that I want to serialize in MongoDB using Java + POJO codec. For some reason the driver tries to create an instance of an enum instead of using valueOF:

org.bson.codecs.configuration.CodecConfigurationException: Failed to decode 'phase'. Failed to decode 'value'. Cannot find a public constructor for 'SimplePhaseEnumType'.
at org.bson.codecs.pojo.PojoCodecImpl.decodePropertyModel(PojoCodecImpl.java:192)
at org.bson.codecs.pojo.PojoCodecImpl.decodeProperties(PojoCodecImpl.java:168)
at org.bson.codecs.pojo.PojoCodecImpl.decode(PojoCodecImpl.java:122)
at org.bson.codecs.pojo.PojoCodecImpl.decode(PojoCodecImpl.java:126)
at com.mongodb.operation.CommandResultArrayCodec.decode(CommandResultArrayCodec.java:52)

The enumeration:

public enum SimplePhaseEnumType {

PROPOSED("Proposed"),
INTERIM("Interim"),
MODIFIED("Modified"),
ASSIGNED("Assigned");
private final String value;

SimplePhaseEnumType(String v) {
    value = v;
}

public String value() {
    return value;
}

public static SimplePhaseEnumType fromValue(String v) {
    for (SimplePhaseEnumType c: SimplePhaseEnumType.values()) {
        if (c.value.equals(v)) {
            return c;
        }
    }
    throw new IllegalArgumentException(v);
}}

And the class the uses the enumeration (only showing the relevant fields):

public class SpecificPhaseType {

protected SimplePhaseEnumType value;
protected String date;

public SimplePhaseEnumType getValue() {
    return value;
}

public void setValue(SimplePhaseEnumType value) {
    this.value = value;
}}

I was looking for a way to maybe annotate the class to tell the driver to use a different method to serialize / deserialize those fields when they are encountered. I know how to skip them during the serialization / deserialization but that doesn't fix the problem:

public class SpecificPhaseType {

@BsonIgnore
protected SimplePhaseEnumType value;

Any help on where I could look (code, documentation)?. I already checked PojoQuickTour.java , MongoDB Driver Quick Start - POJOs and POJOs - Plain Old Java Objects

Thanks!

--Jose

Answer 1

I figured out what to do, you first need to write a custom Codec to read and write the enum as a String (an ordinal is another option if you want to save space, but string was more than OK with me):

package com.kodegeek.cvebrowser.persistence.serializers;

import com.kodegeek.cvebrowser.entity.SimplePhaseEnumType;
import org.bson.BsonReader;
import org.bson.BsonWriter;
import org.bson.codecs.Codec;
import org.bson.codecs.DecoderContext;
import org.bson.codecs.EncoderContext;

public class SimplePhaseEnumTypeCodec implements Codec<SimplePhaseEnumType>{
    @Override
    public SimplePhaseEnumType decode(BsonReader reader, DecoderContext decoderContext) {
        return SimplePhaseEnumType.fromValue(reader.readString());
    }

    @Override
    public void encode(BsonWriter writer, SimplePhaseEnumType value, EncoderContext encoderContext) {
        writer.writeString(value.value());
    }

    @Override
    public Class<SimplePhaseEnumType> getEncoderClass() {
        return SimplePhaseEnumType.class;
    }
}

Then you need to register the new codec so MongoDB can handle the enum using your class:

/**
 * MongoDB could not make this any simpler ;-)
 * @return a Codec registry
 */
public static CodecRegistry getCodecRegistry() {
    final CodecRegistry defaultCodecRegistry = MongoClient.getDefaultCodecRegistry();
    final CodecProvider pojoCodecProvider = PojoCodecProvider.builder().register(packages).build();
    final CodecRegistry cvePojoCodecRegistry = CodecRegistries.fromProviders(pojoCodecProvider);
    final CodecRegistry customEnumCodecs = CodecRegistries.fromCodecs(
            new SimplePhaseEnumTypeCodec(),
            new StatusEnumTypeCodec(),
            new TypeEnumTypeCodec()
    );
    return CodecRegistries.fromRegistries(defaultCodecRegistry, customEnumCodecs, cvePojoCodecRegistry);
}

Jackson makes it easier to register custom serializer/ deserializer with annotations like @JsonSerializer / @JsonDeserializer and while Mongo forces you to deal with the registry. Not a big deal :-)

You can peek at the full source code here . Hope this saves some time to anyone who has to deal with a similar issue.