JsonGenerationException when serializing nested object using custom serializer in Jackson

Question

Here is the class that I want to serialize.

public class ItemRow<T> {

    private String id;
    private List<T> items;
}

There are two variations that are allowed. ItemRow<String>, ItemRow<ItemRow>. In the latter case, it will be nested.

eg:

ItemRow item1 = new ItemRow("abc", Arrays.asList("item1", "item2", "item3"));
String result = mapper.writeValueAsString(item1);
System.out.println(result);

should give

{
     "abc":["item1","item2","item3"]
}

Now, the latter case

ItemRow item2 = new ItemRow("cde", Arrays.asList("item4, item5"));
ItemRow item = new ItemRow("combined", Arrays.asList(item1,item2));
result = mapper.writeValueAsString(item);
System.out.println(result);

should give

{
    "combined": {
        "abc": ["item1", "item2", "item3"],
        "cde": ["item4", "item5"]
    }
}

But I get exception while serializing the latter. The first one works as expected. so I believe the recursive serialization is failing, but I am unable to find out why

Here is exception

com.fasterxml.jackson.core.JsonGenerationException: Can not start an object, expecting field name (context: Object)

    at com.fasterxml.jackson.core.JsonGenerator._reportError(JsonGenerator.java:1961)
    at com.fasterxml.jackson.core.json.JsonGeneratorImpl._reportCantWriteValueExpectName(JsonGeneratorImpl.java:244)
    at com.fasterxml.jackson.core.json.WriterBasedJsonGenerator._verifyValueWrite(WriterBasedJsonGenerator.java:866)
    at com.fasterxml.jackson.core.json.WriterBasedJsonGenerator.writeStartObject(WriterBasedJsonGenerator.java:279)
    at hello.ItemRowSerializer.serialize(ItemRow.java:58)
    at hello.ItemRowSerializer.serialize(ItemRow.java:42)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
    at com.fasterxml.jackson.databind.ObjectMapper.writeValue(ObjectMapper.java:2655)
    at com.fasterxml.jackson.core.base.GeneratorBase.writeObject(GeneratorBase.java:381)
    at hello.ItemRowSerializer.serialize(ItemRow.java:67)
    at hello.ItemRowSerializer.serialize(ItemRow.java:42)

Serializer implementation

class ItemRowSerializer extends JsonSerializer<ItemRow> {

    @Override
    public void serialize(ItemRow itemRow, JsonGenerator jgen, SerializerProvider serializerProvider) throws IOException {

        String id = itemRow.getId();
        List<Object> items = itemRow.getItems();

        if (items.isEmpty()) {
            jgen.writeStartObject();
            jgen.writeFieldName(id);
            jgen.writeStartArray();
            jgen.writeEndArray();
            jgen.writeEndObject();
        }
        else {
            jgen.writeStartObject();
            Object item = items.get(0);
            jgen.writeFieldName(id);
            if (item instanceof  ItemRow){
                for (Object i : items) {
                    //ItemRow temp = (ItemRow) i;
                    //jgen.writeObjectField(temp.getId(), temp);
                    //jgen.writeObjectField(id, i);
                    jgen.writeStartObject();
                    jgen.writeObject(i);
                    jgen.writeEndObject();
                }
            }
            else {
                //jgen.writeFieldName(id);
                jgen.writeStartArray();
                for (Object arg : items) {
                    jgen.writeString(arg.toString());
                }
                jgen.writeEndArray();
            }
        }
        jgen.writeEndObject();
    }
}

Answer 1

Your serializer algoritihm is incorrect. The code is down above. You do not need to start object when you are directly deserializing an object. I removed this steps and minimized the code.

Example Test;

@Test
public void serializeTest() throws JsonProcessingException
{
    ObjectMapper mapper = new ObjectMapper();

    SimpleModule module = new SimpleModule();
    module.addSerializer(ItemRow.class, new ItemRowSerializer());
    mapper.registerModule(module);

    ItemRow item1 = new ItemRow("abc", Arrays.asList("item1", "item2", "item3"));
    String result = mapper.writeValueAsString(item1);
    System.out.println(result);


    ItemRow item2 = new ItemRow("cde", Arrays.asList("item4", "item5"));


    ItemRow item6 = new ItemRow("deeper-1", Arrays.asList("item6", "item7"));
    ItemRow item7 = new ItemRow("deeper-2", Arrays.asList("item6", "item7"));
    ItemRow item8 = new ItemRow("deeper", Arrays.asList(item6, item7));

    ItemRow item3 = new ItemRow("inner-1", Arrays.asList("item6", "item7"));
    ItemRow item4 = new ItemRow("inner-2", Arrays.asList("item6", "item7"));
    ItemRow item5 = new ItemRow("inner", Arrays.asList(item3, item4, item8));




    ItemRow item = new ItemRow("combined", Arrays.asList(item1,item2,item5));
    result = mapper.writeValueAsString(item);
    System.out.println(result);

}

Algorithm;

public class ItemRowSerializer extends JsonSerializer<ItemRow>
{

    @Override
    public void serialize(ItemRow itemRow, JsonGenerator jgen, SerializerProvider serializerProvider) throws IOException
    {
        jgen.writeStartObject();
        writeInnerObject(jgen, itemRow);
        jgen.writeEndObject();
    }


    private void writeStringArr(JsonGenerator jgen, List items) throws IOException
    {
        jgen.writeStartArray();
        for (Object arg : items)
        {
            jgen.writeString(arg.toString());
        }
        jgen.writeEndArray();
    }


    private void writeInnerObject(JsonGenerator jgen, ItemRow row) throws IOException
    {
        jgen.writeFieldName(row.getId());

        if (row.getItems().size() > 0 && row.getItems().get(0) instanceof ItemRow)
        {
            jgen.writeStartObject();
            for (int i = 0; i < row.getItems().size(); i++)
            {
                ItemRow innerRow = (ItemRow) row.getItems().get(i);
                if( innerRow.getItems().size() > 0 && innerRow.getItems().get(0) instanceof ItemRow )
                {
                    writeInnerObject(jgen, innerRow);
                }
                else
                {
                    jgen.writeFieldName(innerRow.getId());
                    writeStringArr(jgen, innerRow.getItems());
                }
            }
            jgen.writeEndObject();
        }
        else
        {
            writeStringArr(jgen, row.getItems());
        }
    }
}