java - 使用 Jackson 反序列化枚举字段

Question

我有一个简单的枚举 Days

public enum Days {
    @JsonProperty("Monday")
    MONDAY("Monday"),
    @JsonProperty("Tuesday")
    TUESDAY("Tuesday");

    private String day;

    Days(String day) {
        this.day = day;
    }

    @JsonValue
    public String getDay() {
        return day;
    }
}

和一个类Event

public class Event {
    private Days day;
    private String name;


    @JsonCreator
    public Event(@JsonProperty("day") Days day,
             @JsonProperty("name") String name) {
    this.day = day;
    this.name = name;
}

    public Days getDay() {
        return day;
    }

    public String getName() {
        return name;
    }
}

我正在使用 Jackson 2.9，this answer 表示使用 @JsonProperty 应该足够了，但是我很难反序列化它:

public static void main(String[] args) throws JsonProcessingException {
    ObjectMapper objectMapper = new ObjectMapper();
    Event event = new Event(Days.MONDAY, "Birthday");

    String serialisedEvent = objectMapper.writeValueAsString(event);
    System.out.println(serialisedEvent);
    // {"day":"Monday","name":"Birthday"}

    Event deserialisedEvent = objectMapper.convertValue(serialisedEvent, Event.class);
    // Exception in thread "main" java.lang.IllegalArgumentException: Cannot construct instance of `xyz.blabla.Event` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('{"day":"Monday","name":"Birthday"}')
    // at [Source: UNKNOWN; line: -1, column: -1]
    // at com.fasterxml.jackson.databind.ObjectMapper._convert(ObjectMapper.java:3750)
    System.out.println(deserialisedEvent.getDay());
}

我在 Spring Boot 2 项目中使用 Java 11 和 Jackson 2.9。我怎样才能使它工作？

Answer 1

Jackson反序列化方法叫做readValue。

convertValue 的目的不同——它首先序列化一个对象(它可能是一个字符串——它会变成一个 JSON-string-literal)，然后将结果反序列化为目标类型的对象。

以下应该有效:

Event deserialisedEvent = objectMapper.readValue(serialisedEvent, Event.class);