如何将 Map<Key, Value> 转换为 Jackson 的 {"keyprop" : key, "valueprop" : value} 数组？

Question

我正在尝试执行以下操作

给定

Map<String, String> labels = {"en_GB" : "English", "de" : "German", "it" : "Italian"}....

我想用Jackson把它序列化成

[{"language" : "en_GB", "label" : "English"}, {"language" : "de", "label" : "German"}, {"language" : "it", "label" : "Italian"}]

本质上是将地图拆分为对象数组，键和值作为单独的属性

代替

 {"en_GB" : "English", "de" : "German", "it" : "Italian"}

我已经搜索了所有 Jackson 文档，但找不到答案。我会很感激一些帮助。提前致谢！

Answer 1

方法一：定义一个中间POJO，将map转换为中间对象列表

• 首先定义一个中间POJO：

  public class LanguageInfo {
      private String language;
      private String label;
  }
  

• 将 map 转换为中间 POJO 列表并将列表序列化为字符串：

  Map<String, String> labels = new HashMap<>();
  labels.put("en_GB", "English");
  labels.put("de", "German");
  labels.put("it", "Italian");
  
  List<LanguageInfo> languageInfoList = labels.entrySet().stream()
    .map(entry -> {
        LanguageInfo info = new LanguageInfo();
        info.setLabel(entry.getValue());
        info.setLanguage(entry.getKey());
        return info;
    }).collect(Collectors.toList());
  ObjectMapper mapper = new ObjectMapper();
  System.out.println(mapper.writeValueAsString(languageInfoList));
  

输出是：

[
    {
        "language":"de",
        "label":"German"
    },
    {
        "language":"en_GB",
        "label":"English"
    },
    {
        "language":"it",
        "label":"Italian"
    }
]

方法二：定义一个JsonSerializer并用它来做序列化工作

假设你想直接序列化下面的类：

@Data
public class LanguageInfos {
    private Map<String, String> labels;
}

• 首先，定义一个JsonSerializer来序列化LanguageInfos：

  需要上面定义的中间POJOLanguageInfo：

  public class LanguageInfosJsonSerializer extends JsonSerializer<LanguageInfos> {
  
      @Override
      public void serialize(LanguageInfos languageInfos, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
          // LanguageInfo defined in below code
          List<LanguageInfo> languageInfoList = languageInfos.getLabels().entrySet().stream()
              .map(entry -> {
                  LanguageInfo info = new LanguageInfo();
                  info.setLabel(entry.getValue());
                  info.setLanguage(entry.getKey());
                  return info;
              }).collect(Collectors.toList());
          jsonGenerator.writeObject(languageInfoList);
      }
  }
  

  不需要中间POJO：

  public class LanguageInfosJsonSerializer extends JsonSerializer<LanguageInfos> {
  
      @Override
      public void serialize(LanguageInfos languageInfos, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
          jsonGenerator.writeStartArray(languageInfos.getLabels().size());
          languageInfos.getLabels().entrySet().forEach(new Consumer<Map.Entry<String, String>>() {
              @SneakyThrows
              @Override
              public void accept(Map.Entry<String, String> entry) {
                      ObjectNode node = new ObjectMapper().createObjectNode();
                      node.put("language",entry.getKey());
                      node.put("label",entry.getValue());
                      jsonGenerator.writeObject(node);
              }
          });
          jsonGenerator.writeEndArray();
      }
  }
  

• 其次，将此序列化器注册到 ObjectMapper 对象中：

  SimpleModule simpleModule = new SimpleModule();
  simpleModule.addSerializer(LanguageInfos.class, new LanguageInfosJsonSerializer());
  ObjectMapper mapper = new ObjectMapper();
  mapper.registerModule(simpleModule);
  

• 三、直接使用ObjectMapper序列化：

  Map<String, String> labels = new HashMap<>();
  labels.put("en_GB", "English");
  labels.put("de", "German");
  labels.put("it", "Italian");
  
  LanguageInfos languageInfos = new LanguageInfos();
  languageInfos.setLabels(labels);
  
  ObjectMapper mapper = new ObjectMapper();
  SimpleModule simpleModule = new SimpleModule();
  simpleModule.addSerializer(LanguageInfos.class, new LanguageInfosJsonSerializer());
  mapper.registerModule(simpleModule);
  System.out.println(mapper.writeValueAsString(languageInfos));
  

输出是：

[
    {
        "language":"de",
        "label":"German"
    },
    {
        "language":"en_GB",
        "label":"English"
    },
    {
        "language":"it",
        "label":"Italian"
    }
]