java - jackson 2.2.x : recursive deserialization fails, 为什么？

Question

准确地说，这是 Jackson 2.2.3。

反序列化过程非常复杂，因为根据我反序列化的 JSON 值的类型，类会发生变化:

• 基本抽象类是JsonMergePatch；
• 当 JSON 值不是 JSON 对象时，我会反序列化为 NonObjectMergePatch；
• 否则，我会递归反序列化为 ObjectMergePatch。

这是代码(为了“简洁”，省略了导入)；首先是 JsonMergePatch:

@JsonDeserialize(using = JsonMergePatchDeserializer.class)
public abstract class JsonMergePatch
    implements JsonSerializable
{
    public abstract JsonNode apply(final JsonNode input)
        throws JsonPatchException;
}

NonObjectMergePatch:

final class NonObjectMergePatch
    extends JsonMergePatch
{
    private final JsonNode node;

    NonObjectMergePatch(@Nonnull final JsonNode node)
    {
        this.node = Preconditions.checkNotNull(node);
    }

    @Override
    public JsonNode apply(final JsonNode input)
        throws JsonPatchException
    {
        return null; // TODO!
    }

    @Override
    public void serialize(final JsonGenerator jgen,
        final SerializerProvider provider)
        throws IOException, JsonProcessingException
    {
        jgen.writeTree(node);
    }

    @Override
    public void serializeWithType(final JsonGenerator jgen,
        final SerializerProvider provider, final TypeSerializer typeSer)
        throws IOException, JsonProcessingException
    {
        serialize(jgen, provider);
    }
}

ObjectMergePatch:

final class ObjectMergePatch
    extends JsonMergePatch
{
    private final Set<String> removedMembers;
    private final Map<String, JsonMergePatch> modifiedMembers;

    ObjectMergePatch(final Set<String> removedMembers,
        final Map<String, JsonMergePatch> modifiedMembers)
    {
        this.removedMembers = ImmutableSet.copyOf(removedMembers);
        this.modifiedMembers = ImmutableMap.copyOf(modifiedMembers);
    }

    @Override
    public JsonNode apply(final JsonNode input)
        throws JsonPatchException
    {
        return null;
    }

    @Override
    public void serialize(final JsonGenerator jgen,
        final SerializerProvider provider)
        throws IOException, JsonProcessingException
    {
        jgen.writeStartObject();

        /*
         * Write removed members as JSON nulls
         */
        for (final String member: removedMembers)
            jgen.writeNullField(member);

        /*
         * Write modified members; delegate to serialization for writing values
         */
        for (final Map.Entry<String, JsonMergePatch> entry:
            modifiedMembers.entrySet()) {
            jgen.writeFieldName(entry.getKey());
            entry.getValue().serialize(jgen, provider);
        }

        jgen.writeEndObject();
    }

    @Override
    public void serializeWithType(final JsonGenerator jgen,
        final SerializerProvider provider, final TypeSerializer typeSer)
        throws IOException, JsonProcessingException
    {
        serialize(jgen, provider);
    }
}

最后，自定义反序列化器:

final class JsonMergePatchDeserializer
    extends JsonDeserializer<JsonMergePatch>
{
    @Override
    public JsonMergePatch deserialize(final JsonParser jp,
        final DeserializationContext ctxt)
        throws IOException, JsonProcessingException
    {
        /* FAILS HERE */
        final JsonNode node = jp.readValueAsTree();
        /*
         * Not an object: the simple case
         */
        if (!node.isObject())
            return new NonObjectMergePatch(node);

        /*
         * The complicated case...
         *
         * We have to build a set of removed members, plus a map of modified
         * members.
         */

        final Set<String> removedMembers = Sets.newHashSet();
        final Map<String, JsonMergePatch> modifiedMembers = Maps.newHashMap();
        final Iterator<Map.Entry<String, JsonNode>> iterator = node.fields();

        Map.Entry<String, JsonNode> entry;

        while (iterator.hasNext()) {
            entry = iterator.next();
            if (entry.getValue().isNull())
                removedMembers.add(entry.getKey());
            else {
                final JsonMergePatch value 
                    = deserialize(entry.getValue().traverse(), ctxt);
                modifiedMembers.put(entry.getKey(), value);
            }            
        }

        return new ObjectMergePatch(removedMembers, modifiedMembers);
    }

    /*
     * This method MUST be overriden... The default is to return null, which is
     * not what we want.
     */
    @Override
    public JsonMergePatch getNullValue()
    {
        return new NonObjectMergePatch(NullNode.getInstance());
    }
}

我已经编写了测试代码，只要要反序列化的 JSON 不是对象，该测试就可以正常工作；当它是一个对象时，我递归地调用 deserialize() 方法来反序列化对象成员值...但是从解析器读取值失败(标记为 /* FAILS HERE */上面)，但有一个异常(exception):

java.lang.IllegalStateException: No ObjectCodec defined for the parser, can not deserialize JSON into JsonNode tree
    at com.fasterxml.jackson.core.JsonParser.readValueAsTree(JsonParser.java:1352)
    at com.github.fge.jsonpatch.rfc7386.JsonMergePatchDeserializer.deserialize(JsonMergePatchDeserializer.java:67)
    at com.github.fge.jsonpatch.rfc7386.JsonMergePatchDeserializer.deserialize(JsonMergePatchDeserializer.java:92)
    at com.github.fge.jsonpatch.rfc7386.JsonMergePatchDeserializer.deserialize(JsonMergePatchDeserializer.java:36)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2888)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2034)
    at com.github.fge.jsonpatch.rfc7386.SerializationTest.objectSerDeserWorksCorrectly(SerializationTest.java:102)

为什么？我该如何解决这个问题？

Answer 1

嗯，在谷歌搜索时，我偶然发现了 a previous question of mine并给出了答案；但我说它很丑......

正如异常提到的，问题是 JsonParser 实例没有关联的 ObjectCodec。所以，我们必须喂它一个；这里的问题是为什么这不会自动完成......

因此，在自定义反序列化器中，我声明了一个新的 ObjectCodec，如下所示:

// JacksonUtils is a class of mine; the method returns an ObjectMapper,
// which extends ObjectCodec
private static final ObjectCodec CODEC = JacksonUtils.newMapper();

在 .deserialize() 方法中，我这样做:

jp.setCodec(CODEC);

并且它有效。

但是很丑。太糟糕了。它是可怕的。我想要一个更好的解决方案:/