Jackson Date

1. Übersicht

In diesem Tutorial werden wir Daten mit Jackson serialisieren. Wir beginnen mit der Serialisierung einer einfachen java.util. Datum , dann Joda-Zeit sowie die Java 8 DateTime .

2. Datum auf Zeitstempel serialisieren

Zuerst wollen wir sehen, wie man ein einfaches java.util.Date mit Jackson serialisiert .

Im folgenden Beispiel - werden wir eine Instanz von „serialisiert Ereignis “ , das eine hat seit Feld „ eventdate „:

@Test public void whenSerializingDateWithJackson_thenSerializedToTimestamp() throws JsonProcessingException, ParseException { SimpleDateFormat df = new SimpleDateFormat("dd-MM-yyyy hh:mm"); df.setTimeZone(TimeZone.getTimeZone("UTC")); Date date = df.parse("01-01-1970 01:00"); Event event = new Event("party", date); ObjectMapper mapper = new ObjectMapper(); mapper.writeValueAsString(event); }

Wichtig ist hierbei, dass Jackson das Datum standardmäßig in ein Zeitstempelformat serialisiert (Anzahl der Millisekunden seit dem 1. Januar 1970, UTC).

Die tatsächliche Ausgabe der " Ereignis " -Serialisierung ist:

{ "name":"party", "eventDate":3600000 }

3. Serialize Datum ISO-8601

Die Serialisierung auf dieses knappe Zeitstempelformat ist nicht optimal. Lassen Sie uns nun das Datum im ISO-8601- Format serialisieren :

@Test public void whenSerializingDateToISO8601_thenSerializedToText() throws JsonProcessingException, ParseException { SimpleDateFormat df = new SimpleDateFormat("dd-MM-yyyy hh:mm"); df.setTimeZone(TimeZone.getTimeZone("UTC")); String toParse = "01-01-1970 02:30"; Date date = df.parse(toParse); Event event = new Event("party", date); ObjectMapper mapper = new ObjectMapper(); mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS); // StdDateFormat is ISO8601 since jackson 2.9 mapper.setDateFormat(new StdDateFormat().withColonInTimeZone(true)); String result = mapper.writeValueAsString(event); assertThat(result, containsString("1970-01-01T02:30:00.000+00:00")); }

Beachten Sie, dass die Darstellung des Datums jetzt viel besser lesbar ist.

4. Konfigurieren ObjectMapper Date

Den vorherigen Lösungen fehlt immer noch die volle Flexibilität bei der Auswahl des genauen Formats für die Darstellung der Instanzen java.util.Date .

Schauen wir uns nun eine Konfiguration an, mit der wir unsere Formate für die Darstellung von Datumsangaben festlegen können :

@Test public void whenSettingObjectMapperDateFormat_thenCorrect() throws JsonProcessingException, ParseException { SimpleDateFormat df = new SimpleDateFormat("dd-MM-yyyy hh:mm"); String toParse = "20-12-2014 02:30"; Date date = df.parse(toParse); Event event = new Event("party", date); ObjectMapper mapper = new ObjectMapper(); mapper.setDateFormat(df); String result = mapper.writeValueAsString(event); assertThat(result, containsString(toParse)); }

Beachten Sie, dass wir, obwohl wir jetzt hinsichtlich des Datumsformats flexibler sind, immer noch eine globale Konfiguration auf der Ebene des gesamten ObjectMapper verwenden .

5. Verwenden Sie @JsonFormat, um das Datum zu formatieren

Schauen wir uns als Nächstes die Annotation @JsonFormat an, um das Datumsformat für einzelne Klassen anstatt global für die gesamte Anwendung zu steuern :

public class Event { public String name; @JsonFormat (shape = JsonFormat.Shape.STRING, pattern = "dd-MM-yyyy hh:mm:ss") public Date eventDate; }

Jetzt - testen wir es:

@Test public void whenUsingJsonFormatAnnotationToFormatDate_thenCorrect() throws JsonProcessingException, ParseException { SimpleDateFormat df = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss"); df.setTimeZone(TimeZone.getTimeZone("UTC")); String toParse = "20-12-2014 02:30:00"; Date date = df.parse(toParse); Event event = new Event("party", date); ObjectMapper mapper = new ObjectMapper(); String result = mapper.writeValueAsString(event); assertThat(result, containsString(toParse)); }

6. Custom Date Serializer

Als Nächstes verwenden wir einen benutzerdefinierten Serializer für Dates, um die vollständige Kontrolle über die Ausgabe zu erhalten:

public class CustomDateSerializer extends StdSerializer { private SimpleDateFormat formatter = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss"); public CustomDateSerializer() { this(null); } public CustomDateSerializer(Class t) { super(t); } @Override public void serialize (Date value, JsonGenerator gen, SerializerProvider arg2) throws IOException, JsonProcessingException { gen.writeString(formatter.format(value)); } }

Weiter - verwenden wir es als Serializer für unser Feld " eventDate ":

public class Event { public String name; @JsonSerialize(using = CustomDateSerializer.class) public Date eventDate; }

Zum Schluss - testen wir es:

@Test public void whenUsingCustomDateSerializer_thenCorrect() throws JsonProcessingException, ParseException { SimpleDateFormat df = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss"); String toParse = "20-12-2014 02:30:00"; Date date = df.parse(toParse); Event event = new Event("party", date); ObjectMapper mapper = new ObjectMapper(); String result = mapper.writeValueAsString(event); assertThat(result, containsString(toParse)); }

7. Serialisieren Sie Joda-Time mit Jackson

Datumsangaben sind nicht immer eine Instanz von java.util.Date . Tatsächlich - sie werden immer mehr von einer anderen Klasse vertreten - und eine übliche ist natürlich die DateTime- Implementierung aus der Joda-Time-Bibliothek.

Mal sehen, wie wir DateTime mit Jackson serialisieren können .

Wir werden das jackson-datatype-joda- Modul für die sofort einsatzbereite Joda-Time-Unterstützung verwenden:

 com.fasterxml.jackson.datatype jackson-datatype-joda 2.9.7 

Und jetzt können wir einfach das JodaModule registrieren und fertig:

@Test public void whenSerializingJodaTime_thenCorrect() throws JsonProcessingException { DateTime date = new DateTime(2014, 12, 20, 2, 30, DateTimeZone.forID("Europe/London")); ObjectMapper mapper = new ObjectMapper(); mapper.registerModule(new JodaModule()); mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS); String result = mapper.writeValueAsString(date); assertThat(result, containsString("2014-12-20T02:30:00.000Z")); }

8. Serialisieren Sie Joda DateTime mit dem benutzerdefinierten Serializer

If we don't want the extra Joda-Time Jackson dependency – we can also make use of a custom serializer (similar to the earlier examples) to get DateTime instances serialized cleanly:

public class CustomDateTimeSerializer extends StdSerializer { private static DateTimeFormatter formatter = DateTimeFormat.forPattern("yyyy-MM-dd HH:mm"); public CustomDateTimeSerializer() { this(null); } public CustomDateTimeSerializer(Class t) { super(t); } @Override public void serialize (DateTime value, JsonGenerator gen, SerializerProvider arg2) throws IOException, JsonProcessingException { gen.writeString(formatter.print(value)); } }

Next – let's use it as our property “eventDate” serializer:

public class Event { public String name; @JsonSerialize(using = CustomDateTimeSerializer.class) public DateTime eventDate; }

Finally – let's put everything together and test it:

@Test public void whenSerializingJodaTimeWithJackson_thenCorrect() throws JsonProcessingException { DateTime date = new DateTime(2014, 12, 20, 2, 30); Event event = new Event("party", date); ObjectMapper mapper = new ObjectMapper(); String result = mapper.writeValueAsString(event); assertThat(result, containsString("2014-12-20 02:30")); }

9. Serialize Java 8 Date With Jackson

Next – let's see how to serialize Java 8 DateTime – in this example, LocalDateTime – using Jackson. We can make use of the jackson-datatype-jsr310 module:

 com.fasterxml.jackson.datatype jackson-datatype-jsr310 2.9.7 

Now, all we need to do is register the JavaTimeModule (JSR310Module is deprecated) and Jackson will take care of the rest:

@Test public void whenSerializingJava8Date_thenCorrect() throws JsonProcessingException { LocalDateTime date = LocalDateTime.of(2014, 12, 20, 2, 30); ObjectMapper mapper = new ObjectMapper(); mapper.registerModule(new JavaTimeModule()); mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS); String result = mapper.writeValueAsString(date); assertThat(result, containsString("2014-12-20T02:30")); }

10. Serialize Java 8 Date Without Any Extra Dependency

If you don't want the extra dependency, you can always use a custom serializer to write out the Java 8 DateTime to JSON:

public class CustomLocalDateTimeSerializer extends StdSerializer { private static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm"); public CustomLocalDateTimeSerializer() { this(null); } public CustomLocalDateTimeSerializer(Class t) { super(t); } @Override public void serialize( LocalDateTime value, JsonGenerator gen, SerializerProvider arg2) throws IOException, JsonProcessingException { gen.writeString(formatter.format(value)); } }

Next – let's use the serializer for our “eventDate” field:

public class Event { public String name; @JsonSerialize(using = CustomLocalDateTimeSerializer.class) public LocalDateTime eventDate; }

Now – let's test it:

@Test public void whenSerializingJava8DateWithCustomSerializer_thenCorrect() throws JsonProcessingException { LocalDateTime date = LocalDateTime.of(2014, 12, 20, 2, 30); Event event = new Event("party", date); ObjectMapper mapper = new ObjectMapper(); String result = mapper.writeValueAsString(event); assertThat(result, containsString("2014-12-20 02:30")); }

11. Deserialize Date

Next – let's see how to deserialize a Date with Jackson. In the following example – we deserialize an “Event” instance containing a date:

@Test public void whenDeserializingDateWithJackson_thenCorrect() throws JsonProcessingException, IOException { String json = "{"name":"party","eventDate":"20-12-2014 02:30:00"}"; SimpleDateFormat df = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss"); ObjectMapper mapper = new ObjectMapper(); mapper.setDateFormat(df); Event event = mapper.readerFor(Event.class).readValue(json); assertEquals("20-12-2014 02:30:00", df.format(event.eventDate)); }

12. Deserialize Joda ZonedDateTime With Time Zone Preserved

In its default configuration, Jackson adjusts the time zone of a Joda ZonedDateTime to the time zone of the local context. As, by default, the time zone of the local context is not set and has to be configured manually, Jackson adjusts the time zone to GMT:

@Test public void whenDeserialisingZonedDateTimeWithDefaults_thenNotCorrect() throws IOException { ObjectMapper objectMapper = new ObjectMapper(); objectMapper.findAndRegisterModules(); objectMapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS); ZonedDateTime now = ZonedDateTime.now(ZoneId.of("Europe/Berlin")); String converted = objectMapper.writeValueAsString(now); ZonedDateTime restored = objectMapper.readValue(converted, ZonedDateTime.class); System.out.println("serialized: " + now); System.out.println("restored: " + restored); assertThat(now, is(restored)); }

This test case will fail with output:

serialized: 2017-08-14T13:52:22.071+02:00[Europe/Berlin] restored: 2017-08-14T11:52:22.071Z[UTC]

Fortunately, there is a quick and simple fix for this odd default-behavior: we just have to tell Jackson, not to adjust the time zone.

This can be done by adding the below line of code to the above test case:

objectMapper.disable(DeserializationFeature.ADJUST_DATES_TO_CONTEXT_TIME_ZONE);

Note that, to preserve time zone we also have to disable the default behavior of serializing the date to the timestamp.

13. Custom Date Deserializer

Let's also see how to use a custom Date deserializer; we'll write a custom deserializer for the property “eventDate“:

public class CustomDateDeserializer extends StdDeserializer { private SimpleDateFormat formatter = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss"); public CustomDateDeserializer() { this(null); } public CustomDateDeserializer(Class vc) { super(vc); } @Override public Date deserialize(JsonParser jsonparser, DeserializationContext context) throws IOException, JsonProcessingException { String date = jsonparser.getText(); try { return formatter.parse(date); } catch (ParseException e) { throw new RuntimeException(e); } } }

Next – let's use it as the “eventDate” deserializer:

public class Event { public String name; @JsonDeserialize(using = CustomDateDeserializer.class) public Date eventDate; }

And finally – let's test it:

@Test public void whenDeserializingDateUsingCustomDeserializer_thenCorrect() throws JsonProcessingException, IOException { String json = "{"name":"party","eventDate":"20-12-2014 02:30:00"}"; SimpleDateFormat df = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss"); ObjectMapper mapper = new ObjectMapper(); Event event = mapper.readerFor(Event.class).readValue(json); assertEquals("20-12-2014 02:30:00", df.format(event.eventDate)); }

14. Fixing InvalidDefinitionException

When creating a LocalDate instance, we may come across an exception:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `java.time.LocalDate`(no Creators, like default construct, exist): no String-argument constructor/factory method to deserialize from String value ('2014-12-20') at [Source: (String)"2014-12-20"; line: 1, column: 1]

This problem occurs because JSON doesn't natively have a date format, so represents dates as String.

The String representation of a date isn't the same as an object of type LocalDate in memory, so we need an external deserializer to read that field from a String, and a serializer to render the date to String format.

These methods also apply to LocalDateTime — the only change is to use an equivalent class for LocalDateTime.

14.1. Jackson Dependency

Jackson allows us to fix this a couple of ways. First, we have to make sure the jsr310 dependency is in our pom.xml:

 com.fasterxml.jackson.datatype jackson-datatype-jsr310 2.11.0 

14.2. Serialization to Single Date Object

In order to be able to handle LocalDate, we need to register the JavaTimeModule with our ObjectMapper.

We also disable the feature WRITE_DATES_AS_TIMESTAMPS in ObjectMapper to prevent Jackson from adding time digits to the JSON output:

@Test public void whenSerializingJava8DateAndReadingValue_thenCorrect() throws IOException { String stringDate = "\"2014-12-20\""; ObjectMapper mapper = new ObjectMapper(); mapper.registerModule(new JavaTimeModule()); mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS); LocalDate result = mapper.readValue(stringDate, LocalDate.class); assertThat(result.toString(), containsString("2014-12-20")); }

Here, we've used Jackson's native support for serializing and deserializing dates.

14.3. Annotation in POJO

Another way to deal with that problem is to use the LocalDateDeserializer and JsonFormat annotations at the entity level:

public class EventWithLocalDate { @JsonDeserialize(using = LocalDateDeserializer.class) @JsonSerialize(using = LocalDateSerializer.class) @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "dd-MM-yyyy") public LocalDate eventDate; }

The @JsonDeserialize annotation is used to specify a custom deserializer to unmarshal the JSON object. Similarly, @JsonSerialize indicates a custom serializer to use when marshaling the entity.

In addition, the annotation @JsonFormat allows us to specify the format to which we will serialize date values. Therefore, this POJO can be used to read and write the JSON:

@Test public void whenSerializingJava8DateAndReadingFromEntity_thenCorrect() throws IOException { String json = "{\"name\":\"party\",\"eventDate\":\"20-12-2014\"}"; ObjectMapper mapper = new ObjectMapper(); EventWithLocalDate result = mapper.readValue(json, EventWithLocalDate.class); assertThat(result.getEventDate().toString(), containsString("2014-12-20")); }

While this approach takes more work than using the JavaTimeModule defaults, it's a lot more customizable.

15. Conclusion

In this extensive Date article, we looked at several ways Jackson can help marshal and unmarshal a date to JSON using a sensible format we have control over.

Der Beispielcode ist wie immer auf GitHub zu finden.