java如何使用Jackson序列化对象的嵌套集合

Question

具有以下代码：

public class ToSerialize {
  List<NestedObject> objs;

  public ToSerialize(List<NestedObject> objs) {
    this.objs = objs;
  }
}

public class NestedObject {
  int intValue = 0;
  String strValue = "Hello world";
}

如果可能，我如何设置Jackson，以获得以下CSV作为字符串输出：

obj1.intValue,obj1.strValue,obj2.intValue,obj2.strValue,0,"Hello world",0,"Hello world"

我想写我自己的自定义JsonSerializer，但没办法。不过，这里是我当前无法工作的序列化程序实现

  @Override
  public void serialize(List<NestedObject> values, JsonGenerator gen, SerializerProvider serializers)
    throws IOException
  {
    int i = 1;
    for (Iterator<VsppAssetEntry> iterator = values.iterator(); iterator.hasNext(); ) {
      VsppAssetEntry entry = iterator.next();
      String fieldName = "obj" + i;
      gen.writeObjectField(fieldName, entry);
      i++;
    }

这显然是抛出com.fasterxml.jackson.core.JsonGenerationException: Can not write a field name, expecting a value，因为Jackson在输入此方法之前编写了一个字段名objs

Answer 1

我建议Jackson dataformat CSV

考虑以下CSV模式：

CsvSchema schema = CsvSchema.builder()
        .addColumn("obj")
        .addColumn("intValue")
        .addColumn("strValue")
        .build()
        .withHeader();

ToSerialize toSerialize = new ToSerialize(Arrays.asList(new NestedObject(), new NestedObject()));

String csv = new CsvMapper()
        .writerFor(ToSerialize.class)
        .with(schema)
        .writeValueAsString(toSerialize);

System.out.println(csv);

和以下自定义序列化程序

public class ToSerializeSerializer extends JsonSerializer<ToSerialize> {
    @Override
    public void serialize(ToSerialize toSerialize, JsonGenerator gen, SerializerProvider serializers)
            throws IOException {
        List<NestedObject> objs = toSerialize.getObjs();
        for (int i = 0; i < objs.size(); i++) {
            gen.writeObject("obj" + i);
            gen.writeObject(objs.get(i));
        }
    }
}

…在ToSerialize类上注册它

@JsonSerialize(using = ToSerializeSerializer.class)
public class ToSerialize {
    […]
}

…它会给你

obj,intValue,strValue
obj0,0,"Hello world"
obj1,0,"Hello world"