如何忽略Jackson JSON-to-Object映射中的枚举字段？ [英] How to ignore enum fields in Jackson JSON-to-Object mapping?

问题描述

我有一个类似的JSON对象：

I have a JSON Object something like:

{"name":"John", "grade":"A"}

或

{"name":"Mike", "grade":"B"}

或

{"name":"Simon", "grade":"C"}

等

我试图将上面的JSON映射到：

I am trying to map the above JSON to:

@JsonIgnoreProperties(ignoreUnknown = true)
public class Employee{
      @JsonIgnoreProperties(ignoreUnknown = true)
      public enum Grade{ A, B, C }
      Grade grade;
      String name;

  public Grade getGrade() {
    return grade;
  }

  public void setGrade(Grade grade) {
    this.grade = grade;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }
}

上面的映射工作正常，但将来会有更多等级类型让我们说D，E等打破现有的映射并抛出以下异常

the above mapping works fine but in the future there will be more "Grade" types let say D,E etc which breaks the existing mapping and throws the following exception

05-08 09:56:28.130: W/System.err(21309): org.codehaus.jackson.map.JsonMappingException: Can not construct instance of Employee from String value 'D': value not one of declared Enum instance names

有没有办法忽略枚举类型中的未知字段？

Is there a way to ignore unknown fields with in enum types?

谢谢

推荐答案

我认为你应该定义外部反序列化器 等级枚举。

I think you should define external deserializer for Grade enum.

我在enum中添加了额外的字段 - UNKNOWN：

I added additional field to enum - UNKNOWN:

enum Grade {
    A, B, C, UNKNOWN;

    public static Grade fromString(String value) {
        for (Grade grade : values()) {
            if (grade.name().equalsIgnoreCase(value)) {
                return grade;
            }
        }

        return UNKNOWN;
    }
}
class Employee {

    @JsonDeserialize(using = GradeDeserializer.class)
    private Grade grade;
    private String name;

    public Grade getGrade() {
        return grade;
    }

    public void setGrade(Grade grade) {
        this.grade = grade;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Employee [grade=" + grade + ", name=" + name + "]";
    }
}

现在，解析器看起来像这样：

Now, parser could look like that:

class GradeDeserializer extends JsonDeserializer<Grade> {
    @Override
    public Grade deserialize(JsonParser parser, DeserializationContext context)
            throws IOException, JsonProcessingException {
        return Grade.fromString(parser.getValueAsString());
    }
}

用法示例：

public class JacksonProgram {

    public static void main(String[] args) throws Exception {
        ObjectMapper objectMapper = new ObjectMapper();
        JsonFactory jsonFactory = new JsonFactory();
        JsonParser parser = jsonFactory
                .createJsonParser("{\"name\":\"John\", \"grade\":\"D\"}");
        Employee employee = objectMapper.readValue(parser, Employee.class);
        System.out.println(employee);
    }

}

输出：

Employee [grade=UNKNOWN, name=John]

如果你不想添加额外的字段，你会返回 null 。例如。

If you don't want to add additional field, you would return null for example.

这篇关于如何忽略Jackson JSON-to-Object映射中的枚举字段？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！