[BUG] Codegen incorrectly deserializes Maps because of double jsonReader.readFieldName() call

[artpaym]

Description:
Codegen incorrectly deserializes Maps because of double jsonReader.readFieldName() call.

For example, if your DTO has "elements" Map field, the generated code in ..._FASTJSONReader looks like this (the relevant piece):

...
                                        String elements_key = jsonReader.readFieldName();
                                        if (jsonReader.isReference()) {
                                            String ref = jsonReader.readReference();
                                            jsonReader.addResolveTask(elements, elements_key, JSONPath.of(ref));
                                        } else {
                                            elements.put(jsonReader.readFieldName(), (MyDto)this.itemReader0.readObject(jsonReader, MyDto.class, "elements", features));
                                        }
...

So the flow is:

1. String elements_key = jsonReader.readFieldName();
2. elements.put(jsonReader.readFieldName(), ...
   causing the reader to advance multiple times on the field name hence breaking the reader cursor basically.

Seems like the cause is here:

fastjson2/codegen/src/main/java/com/alibaba/fastjson2/internal/processor/JSONCompiledAnnotationProcessor.java

Line 3042 in 0e7ef92

exec(method(fieldValueVar, "put", mapEntryKeyExpr, mapEntryValueExpr))

Instead of mapEntryKeyExpr it should use ident(mapKey) to fix it, it seems.

It worked fine, when I copy pasted generated code with the fix applied on top, so it's likely the cause.

Environment:

• OS: Linux
• JDK: Openjdk-21.0.5+11
• Fastjson2 and codegen: 2.0.59
• Quarkus: 3.28.4

Reproduction:

1. Use a Map field in your DTO class with codegen, for example:

@JSONCompiled
@JSONType
public class MyDto {
    ...
    public Map<String, MyOtherDto> elements;
    ...

2. Build the project.
3. Send some corresponding valid json.
4. Parse json: MyDto dto = JSON.parseObject(json, MyDto.class);

Actual

Instead of real valid "elements" items, your MyDto will have only a single element with "null" key (though I suppose you might see a different result, because an incorrectly advanced parser might yield different results depending on input data, etc).

The real json had something like this instead:

...
    "elements": {
      ...
      "7": {
        "format": "mmddhhnnss",
        "id": 7,
        "type": "datetime",
        "value": "0302054257"
      },
      ...
    }
...

Expected

Map gets parsed correctly, and then written back to json string also correctly.

[artpaym]

Updated ticket, because found the cause.

[jujn]

Thank you for your feedback. I have attempted to submit the patch.

[wenshao]

https://github.com/alibaba/fastjson2/releases/tag/2.0.60
please use the last version