将实体转换为Json时，为什么LocalDate格式会更改？

Question

My objective is to store dates into a database. To do this app I use Springboot, JPA, H2, ...

I use LocalDate and the format wished is yyyy-MM-dd .

Entity

@Entity
public class MyObject {

    @Id
    private String id;
    private LocalDate startdate;
    private LocalDate enddate;

    public MyObject() {}

    public MyObject(LocalDate enddate) {
        this.startdate = LocalDate.now();
        this.enddate = enddate;
    }

    ...
}

Main

private DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
MyObject myObject = new MyObject(LocalDate.parse("2019-03-01", formatter));
myObject.setId(UUID.randomUUID().toString());
myObjectResource.save(myObject);

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON);
System.out.println(myObject.getStartdate()); // 2019-02-23
System.out.println(myObject.getEnddate()); // 2019-03-01
HttpEntity<String> entity = new HttpEntity<>(this.toJsonString(myObject), headers);
System.out.println(entity.toString()); // <{"id":"ba6649e4-6e65-4f54-8f1a-f8fc7143b05a","startdate":{"year":2019,"month":"FEBRUARY","dayOfMonth":23,"dayOfWeek":"SATURDAY","era":"CE","dayOfYear":54,"leapYear":false,"monthValue":2,"chronology":{"id":"ISO","calendarType":"iso8601"}},"enddate":{"year":2019,"month":"MARCH","dayOfMonth":1,"dayOfWeek":"FRIDAY","era":"CE","dayOfYear":60,"leapYear":false,"monthValue":3,"chronology":{"id":"ISO","calendarType":"iso8601"}}},[Content-Type:"application/json"]>

private String toJsonString(Object o) throws Exception {
    ObjectMapper om = new ObjectMapper();
    return om.writeValueAsString(o);
}

Can you help me to understand why dates in entity.toString() are not the same as before with getMethods() ?

Thanks for help!

Answer 1

LocalDate.parse returns a new LocalDate object. The formatting options specified in the DateTimeFormatter get lost aftwerward.

Jackson (the JSON library you're using) doesn't know how you previously "formatted" the LocalDate , so it uses its own formatting.

You can register the JavaTimeModule

final ObjectMapper om = new ObjectMapper();
om.registerModule(new JavaTimeModule());

Or you can provide your custom JsonSerializer<T> .