java - Jackson - 将内部对象列表反序列化为更高级别的列表

Question

使用 Spring Boot 和 Jackson，如何将包装/内部列表反序列化为直接在外部级别的列表？

例如，我有:

{
    "transaction": {
    "items": {
        "item": [
            {
                "itemNumber": "193487654",
                "itemDescription": "Widget",
                "itemPrice": "599.00",
                "itemQuantity": "1",
                "itemBrandName": "ACME",
                "itemCategory": "Electronics",
                "itemTax": "12.95"
            },
            {
                "itemNumber": "193487654",
                "itemDescription": "Widget",
                "itemPrice": "599.00",
                "itemQuantity": "1",
                "itemBrandName": "ACME",
                "itemCategory": "Electronics",
                "itemTax": "12.95"
            }
        ]
    },
    ...
    }
}

在JSON中，item是items下的一个列表；但我想将它解析为一个名为 items 的列表，直接在 transaction 下，而不是定义一个包含名为 的列表的 DTO Items >项目。

这可能吗？如何定义这个 DTO Item？

public class TrasactionDTO {
    private List<Item> items;
    ...
}

public class Item {

}

这个问题类似但是没有解决问题。 Deserialize wrapped list using Jackson

Answer 1

我们需要实现自定义反序列化器。因为我们想跳过一个内部字段，所以我们的实现应该:

1. { - 跳过起始对象
2. "any_field_name" - 跳过任何字段名称。我们假设我们只有一个内场。
3. [{}, ..., {}] - 为 List 使用默认反序列化器。
4. } - 跳过结束对象

使用上面的概念实现应该很容易:

public class InnerListDeserializer extends JsonDeserializer<List> implements ContextualDeserializer {

    private final JavaType propertyType;

    public InnerListDeserializer() {
        this(null);
    }

    public InnerListDeserializer(JavaType propertyType) {
        this.propertyType = propertyType;
    }

    @Override
    public List deserialize(JsonParser p, DeserializationContext context) throws IOException {
        p.nextToken(); // SKIP START_OBJECT
        p.nextToken(); // SKIP any FIELD_NAME

        List list = context.readValue(p, propertyType);

        p.nextToken(); // SKIP END_OBJECT

        return list;
    }

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext context, BeanProperty property) {
        return new InnerListDeserializer(property.getType());
    }
}

假设我们有这样的 JSON 有效载荷:

{
  "transaction": {
    "items": {
      "item": [
        {
          "itemNumber": "193487654",
          "itemDescription": "Widget",
          "itemPrice": "599.00",
          "itemQuantity": "1",
          "itemBrandName": "ACME",
          "itemCategory": "Electronics",
          "itemTax": "12.95"
        },
        {
          "itemNumber": "193487654",
          "itemDescription": "Widget",
          "itemPrice": "599.00",
          "itemQuantity": "1",
          "itemBrandName": "ACME",
          "itemCategory": "Electronics",
          "itemTax": "12.95"
        }
      ]
    },
    "name": "Pickle Rick"
  }
}

在 JSON 之上我们可以映射到下面的 POJO 类:

@JsonRootName("transaction")
public class Transaction {

    private String name;
    private List<Item> items;

    @JsonDeserialize(using = InnerListDeserializer.class)
    public List<Item> getItems() {
        return items;
    }

    // getters, setters, toString
}

public class Item {

    private String itemNumber;

    // getters, setters, toString
}

为了展示它适用于许多不同的模型，让我们再介绍一个 JSON 负载:

{
  "product": {
    "products": {
      "innerArray": [
        {
          "id": "1234"
        }
      ]
    }
  }
}

还有两个 POJO 类:

@JsonRootName("product")
class Product {

    private List<ProductItem> products;

    @JsonDeserialize(using = InnerListDeserializer.class)
    public List<ProductItem> getProducts() {
        return products;
    }

    // getters, setters, toString
}

class ProductItem {

    private String id;

    // getters, setters, toString
}

现在我们可以测试我们的解决方案了:

import com.fasterxml.jackson.annotation.JsonRootName;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.BeanProperty;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.deser.ContextualDeserializer;

import java.io.File;
import java.io.IOException;
import java.util.List;

public class JSoupTest {

    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);
        mapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE);
        mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);

        File jsonFile = new File("Path to 1-st JSON").getAbsoluteFile();
        File jsonFile1 = new File("Path to 2-nd JSON").getAbsoluteFile();

        System.out.println(mapper.readValue(jsonFile, Transaction.class));
        System.out.println(mapper.readValue(jsonFile1, Product.class));
    }
}

上面的例子打印:

Transaction{items=[Item{itemNumber=193487654}, Item{itemNumber=193487654}], name='Pickle Rick'}
Product{products=[ProductItem{id='1234'}]}

更多信息请阅读:

1. Custom Jackson Deserializer Getting Access to Current Field Class
2. Getting Started with Custom Deserialization in Jackson
3. Jackson Exceptions – Problems and Solutions
4. Jackson UNWRAP_ROOT_VALUE
5. Configuring ObjectMapper in Spring