使用没有注释的 Jackson 将列表序列化为 xml？

Question

I'm looking for a way to (de-)serialize a List of items without using Annotations in Jackson. Is this possible? What I'm doing up to now is trying to replace the <item> -tag with a tag telling about the item's class, but no avail. And even if this worked, I'm not sure whether Jackson would offer a way to process this tag information.

To give a better of what I'm aiming at, here's a sample:

public class JacksonTest {

    private static class ListElement {
        private boolean value;
        // getters, setters, constructors omitted
    }

    @Test
    public void testDeSerialization() throws Exception {
        final List<ListElement> existing = Arrays.asList(new ListElement(true));
        final ObjectMapper mapper = new XmlMapper();
        final JavaType listJavaType = mapper.getTypeFactory().constructCollectionType(List.class, ListElement.class);
        final String listString = mapper.writerFor(listJavaType).writeValueAsString(existing);
        System.out.println(listString);
        // "<List><item><value>true</value></item></List>"
    }

}

So, the result is <List><item><value>true</value></item></List> , while I want the <item> -tag to be replaced with the (qualified) class name or offering a type -attribute. Of course, even this would not help if there's no way in Jackson to process this class name.

Do I have reached a dead end here or is there a way to go?

Answer 1

You can define your own JsonSerializer (also used for XML) and add it to a JacksonXmlModule.

ToXmlGenerator has a setNextName function that allows you to override the default item name

private class MyListSerializer extends JsonSerializer<List> {
    @Override
    public void serialize(List list, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
            throws IOException {
        for (Object obj : list) {
            if (jsonGenerator instanceof ToXmlGenerator) {
                ToXmlGenerator xmlGenerator = (ToXmlGenerator) jsonGenerator;
                String className = obj.getClass().getSimpleName();
                xmlGenerator.setNextName(new QName(className));
            }
            jsonGenerator.writeObject(obj);
            // this is overridden at the next iteration
            // and ignored at the last
            jsonGenerator.writeFieldName("dummy");
        }
    }

    @Override
    public Class<List> handledType() {
        return List.class;
    }
}

@Test
public void testDeSerialization() throws Exception {
    final List<ListElement> existing = Arrays.asList(new ListElement(true));
    JacksonXmlModule module = new JacksonXmlModule();
    module.addSerializer(new MyListSerializer());
    final ObjectMapper mapper = new XmlMapper(module);
    final JavaType listJavaType = mapper.getTypeFactory().constructCollectionType(List.class, ListElement.class);
    final ObjectWriter writer = mapper.writerFor(listJavaType);
    final String listString = writer.writeValueAsString(existing);
    System.out.println(listString);
    // "<List><ListElement><value>true</value></ListElement></List>"
}

Answer 2

Okay, after some tinkering and debugging with Evertude's proposal I've figured out a solution. I'm not really happy with the serialization part and honestly I don't know why I was supposed to do it this way. When debugging I've noticed that XmlGenerator::setNextName is required to be called once but does not have any effect on the next call, so I had to implement a switch there and set the field name for the next item in the loop directly.

I'ld be glad if somebody has an idea what I'm doing wrong, but at least my attempt is working for now:

@Test
public void testDeSerialization() throws Exception {
    final List<ListElement> existing = Arrays.asList(new ListElement(true), new ListElement(false));
    JacksonXmlModule module = new JacksonXmlModule();
    module.addSerializer(new MyListSerializer());
    final ObjectMapper mapper = new XmlMapper(module);
    final JavaType listJavaType = mapper.getTypeFactory().constructCollectionType(List.class, ListElement.class);
    final ObjectWriter writer = mapper.writerFor(listJavaType);
    final String listString = writer.writeValueAsString(existing);
    module.addDeserializer(List.class, new MyListDeserializer());
    List<ListElement> deserialized = mapper.readValue(listString, List.class);
    assertEquals(existing, deserialized); // provided there're proper hash() and equals() methods
}

private class MyListSerializer extends JsonSerializer<List> {

    @Override
    public void serialize(List list, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
            throws IOException {
        boolean done = false;
        for (Object obj : list) {
            if (jsonGenerator instanceof ToXmlGenerator) {
                ToXmlGenerator xmlGenerator = (ToXmlGenerator) jsonGenerator;
                String className = obj.getClass().getSimpleName();
                // weird switch
                if (!done) xmlGenerator.setNextName(new QName(className));
                else jsonGenerator.writeFieldName(className);
                done = true;
            }
            jsonGenerator.writeObject(obj);
        }
    }

    @Override
    public Class<List> handledType() {
        return List.class;
    }
}

private class MyListDeserializer extends JsonDeserializer<List> {

    @Override
    public List deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        List<Object> items = new ArrayList<>();
        JsonToken nextToken;
        while ((nextToken = p.nextToken()) != JsonToken.END_OBJECT) {
            String currentName = p.currentName();
            try {
                String className = "my.test.project.JacksonCustomSerializer$" + currentName;
                Class<?> loadClass = getClass().getClassLoader().loadClass(className);
                p.nextToken();
                items.add(p.readValueAs(loadClass));
            } catch (ClassNotFoundException e) {
                // some handling
            }
        }
        return items;
    }

    @Override
    public Class<List> handledType() {
        return List.class;
    }
}