Jackson - Convert the Date format to ISO8601

 Problem

在做Rest API的輸出處理時，一定會遇到日期要顯示的格式，而日期也伴隨著timezone的問題。要顯示成甚麼樣子的格式，就看各個應用程式的需求。一開始是想仿效臉書轉為unix time，後來再為這問題爭論不休；最後我在study後，認為ISO8601比較符合我們需求，就決定為我們的顯示格式。

How to?

預設使用Jackson去做Date的轉換，會輸出為unix time。如果要達到我們的目的，可透過@JsonSerialize設定對應序列化的物件；而要接收使用者輸入轉為對應物件的話，可透過@JsonDeserialize設定對應反序列化物件。

Object with JsonSerialize and JsonDeserialize annotation

public class Event {
	@JsonSerialize(using=ISO8601DateSerializer.class)
	@JsonDeserialize(using=ISO8601DateDeserializer.class)
	private Date date;
 
	private String message;
 
	public Date getDate(){
		return date;
	}
 
	public void setDate(Date date){
		this.date = date;
	}
 
        // ... skip
}

ISO8601DateSerializer for Date

public class ISO8601DateSerializer extends StdSerializer<Date> {
	private static final long serialVersionUID = 1L;
 
	public ISO8601DateSerializer() {
		super(Date.class);
	}
 
	@Override
	public void serialize(Date date, JsonGenerator json,
			SerializerProvider provider) throws IOException,
			JsonGenerationException {
		DateFormat df = StdDateFormat.getISO8601Format(
				TimeZone.getTimeZone("UTC"), Locale.getDefault());
		String out = df.format(date);
 
 
		json.writeString(out);
	}
 
}

ISO8601DateDeserializer for Date

public class ISO8601DateDeserializer extends StdDeserializer<Date> {
 
	private static final long serialVersionUID = 1L;
 
	public ISO8601DateDeserializer() {
        super(Date.class);
    }
 
	@Override
	public Date deserialize(JsonParser parser, DeserializationContext context)
			throws IOException, JsonProcessingException {
		try {
 
			DateFormat df = StdDateFormat.getISO8601Format(
					TimeZone.getTimeZone("UTC"), Locale.getDefault());
			return df.parse(parser.getText());
		} catch (Exception e) {
			return null;
		}
	}
}

Convert to Json

簡單寫一個測試案例，去驗證序列化後再反序列化的內容會一致:

	@Test
	public void testDateWithAnnotation() throws Exception {
		ObjectMapper mapper = new ObjectMapper();
 
		Date d = new Date();
		Event e = new Event();
		e.setDate(d);
 
		String ret = mapper.writeValueAsString(e);
		System.out.println(ret);
		Event newEvent = mapper.readValue(ret, Event.class);
		assertEquals(d.getTime(), newEvent.getDate().getTime());
	}

output:

{"date":"2016-03-10T15:40:11.869+0000","message":null}

Apply to all Date fields

上面所敘述的是透過在欄位宣告@JsonSerialize，來達到我們的目的。如果想要讓每一個擁有Date的bean物件都能使用這個格式，可以透過Jackson所提供的Module功能，去設定對應物件的Serializer與Deserializer:

ObjectMapper mapper = new ObjectMapper();
 
SimpleModule m = new SimpleModule();
m.addSerializer(Date.class, new ISO8601DateSerializer());
m.addDeserializer(Date.class, new ISO8601DateDeserializer());
 
mapper.registerModule(m);

後記

• 時間轉換可以透過joda-time去實作你想要的格式。
• Jackson提供了joda模組，讓你轉換更容易。

Reference

• ways-to-convert-unix-linux-time-to-windows-time
• ISO8601
• Designing a REST API: Unix time vs ISO-8601