我如何使用 Jackson 展开 JSON 中的特定字段？

Question

我有一个 JSON 有效负载，如下所示：

{
    "id": 32,
    "name": "[Sample] Tomorrow is today, Red printed scarf",
    "primary_image": {
      "id": 247,
      "zoom_url": "www.site.com/in_123__14581.1393831046.1280.1280.jpg",
      "thumbnail_url": "www.site.com/in_123__14581.1393831046.220.290.jpg",
      "standard_url": "www.site.com/in_123__14581.1393831046.386.513.jpg",
      "tiny_url": "www.site.com/in_123__14581.1393831046.44.58.jpg"
    }
  }

我可以打开特定字段并丢弃所有其他字段吗？换句话说，我可以像这样将它直接绑定到 POJO 吗：

public class Product {

    private Integer id;
    private String name;
    private String standardUrl;
}

Answer 1

Jackson 提供了@JsonUnwrapped 注解。

见下文link:

http://jackson.codehaus.org/1.9.9/javadoc/org/codehaus/jackson/annotate/JsonUnwrapped.html

Answer 2

有很多方法。您需要反序列化、序列化还是两者都需要？

反序列化的一种方法是使用将图像作为树节点的创建器方法：

public static class Product {
    private Integer id;
    private String name;
    private String standardUrl;

    public Product(@JsonProperty("id") Integer id,
                   @JsonProperty("name") String name,
                   @JsonProperty("primary_image") JsonNode primaryImage) {
        this.id = id;
        this.name = name;
        this.standardUrl = primaryImage.path("standard_url").asText();
    }
}

创建者不必是构造函数，您可以有一个仅用于 Jackson 反序列化的静态方法。

不过，您必须定义一个自定义序列化程序来重新序列化它（例如，一个 StdDelegatingSerializer 和一个将字符串包装回 ObjectNode 的转换器）

Answer 3

有多种方法可以给这只猫剥皮，我希望你可以为此使用 Jackson 2，因为它提供了反序列化 Json 数据的好方法，我最喜欢的反序列化功能之一就是我将在这里向您展示（使用 Builder Pattern），因为允许您在构造实例时验证它们（或使它们不可变！）。对你来说，这看起来像这样：

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;

import java.util.Map;

@JsonDeserialize(builder = Product.Builder.class)
public class Product {

private Integer id;

private String name;

private String standardUrl;

private Product(Builder builder) {
    //Here you can make validations for your new instance.

    this.id = builder.id;
    this.name = builder.name;

    //Here you have access to the primaryImage map in case you want to add new properties later.
    this.standardUrl = builder.primaryImage.get("standard_url");
}

@Override
public String toString() {
    return String.format("id [%d], name [%s], standardUrl [%s].", id, name, standardUrl);
}

@JsonIgnoreProperties(ignoreUnknown = true)
public static class Builder {

    private Integer id;

    private String name;

    private Map<String, String> primaryImage;

    public Builder withId(Integer id) {
        this.id = id;
        return this;
    }

    public Builder withName(String name) {
        this.name = name;
        return this;
    }

    @JsonProperty("primary_image")
    public Builder withPrimaryImage(Map<String, String> primaryImage) {
        this.primaryImage = primaryImage;
        return this;
    }

    public Product build() {
        return new Product(this);
    }
}
}

为了测试它，我创建了这个 class:

import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.IOException;

public class Test {
public static void main(String[] args) {


    String serialized = "{" +
                        "    \"id\": 32," +
                        "    \"name\": \"[Sample] Tomorrow is today, Red printed scarf\"," +
                        "    \"primary_image\": {" +
                        "      \"id\": 247," +
                        "      \"zoom_url\": \"www.site.com/in_123__14581.1393831046.1280.1280.jpg\"," +
                        "      \"thumbnail_url\": \"www.site.com/in_123__14581.1393831046.220.290.jpg\"," +
                        "      \"standard_url\": \"www.site.com/in_123__14581.1393831046.386.513.jpg\"," +
                        "      \"tiny_url\": \"www.site.com/in_123__14581.1393831046.44.58.jpg\"" +
                        "    }" +
                        "  }";


    ObjectMapper objectMapper = new ObjectMapper();

    try {

        Product deserialized = objectMapper.readValue(serialized, Product.class);

        System.out.print(deserialized.toString());
    } catch (IOException e) {
        e.printStackTrace();
    }
}

输出是（使用 Product 中的覆盖 toString() 方法：

id [32], name [[Sample] Tomorrow is today, Red printed scarf], standardUrl [www.site.com/in_123__14581.1393831046.386.513.jpg].

Answer 4

有两种方法可以获得您需要的回复。对于这两种方法，我们都将使用 JsonView。

创建两种类型的 JsonView：

public interface JViews {
    public static class Public { }
    public static class Product extends Public { }
}

第一种方法

@JsonView(JViews.Public.class)
public class Product {
    private Integer id;
    private String name;
    @JsonIgnore
    private Image primaryImage;
    @JsonView(JViews.Product.class)
    public String getStandardUrl{
       return this.primaryImage.getStandardUrl();
    }
}

第二种方式

同时使用杰克逊的@JsonView 和@JsonUnwrapped。

@JsonView(JViews.Public.class)
public class Product {
    private Integer id;
    private String name;

    @JsonUnwrapped
    private Image primaryImage;
}

public class Image {
   private String zoomUrl;
   @JsonView(JViews.Product.class)
   private String standardUrl;
}

@JsonUnwrapped 注释将您的嵌套对象展平为 Product 对象。而 JsonView 用于过滤可访问的字段。在这种情况下，产品视图只能访问 standardUrl 字段，结果预计为：

{
    "id": 32,
    "name": "[Sample] Tomorrow is today, Red printed scarf",
    "standard_url": "url"
}

如果您在不使用视图的情况下展平嵌套对象，结果将如下所示：

{
    "id": 32,
    "name": "[Sample] Tomorrow is today, Red printed scarf",
    "id":1,
    "standard_url": "url", 
    "zoom_url":"",
    ...
}

我如何使用 Jackson 展开 JSON 中的特定字段？

How can I unwrap a specific field in a JSON using Jackson?

java

json

jackson