Jackson之LocalDateTime转换，无需改实体类

【问题】　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

Demo：

  LocalDateTime dt = LocalDateTime.now();
        ObjectMapper mapper = new ObjectMapper();
        try {
            String json = mapper.writeValueAsString(dt);
            System.out.println(json);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }

Jackson默认序列化会将LocalDateTime序列化成：

{"dayOfMonth":15,"dayOfWeek":"SUNDAY","dayOfYear":258,"hour":14,"minute":6,"month":"SEPTEMBER","monthValue":9,"nano":359000000,"second":55,"year":2019,"chronology":{"id":"ISO","calendarType":"iso8601"}}

并且该字符串反序列化成LocalDateTime会报错：

Cannot construct instance of `java.time.LocalDateTime`

Demo：

    String json = "{\"dayOfMonth\":15,\"dayOfWeek\":\"SUNDAY\",\"dayOfYear\":258,\"hour\":14,\"minute\":2,\"month\":\"SEPTEMBER\",\"monthValue\":9,\"nano\":71000000,\"second\":46,\"year\":2019,\"chronology\":{\"id\":\"ISO\",\"calendarType\":\"iso8601\"}}";
        ObjectMapper mapper = new ObjectMapper();
        try {
            LocalDateTime dt = mapper.readValue(json, LocalDateTime.class);
            System.out.println(dt);
        } catch (IOException e) {
            e.printStackTrace();
        }

就算加了：

mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
mapper.registerModule(new JavaTimeModule());

也会报如下错误：

com.fasterxml.jackson.databind.exc.MismatchedInputException: Expected array or string.

Demo：

 String json = "{\"dayOfMonth\":15,\"dayOfWeek\":\"SUNDAY\",\"dayOfYear\":258,\"hour\":14,\"minute\":2,\"month\":\"SEPTEMBER\",\"monthValue\":9,\"nano\":71000000,\"second\":46,\"year\":2019,\"chronology\":{\"id\":\"ISO\",\"calendarType\":\"iso8601\"}}";
        ObjectMapper mapper = new ObjectMapper();
        try {
            mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
            mapper.registerModule(new JavaTimeModule());
            LocalDateTime dt = mapper.readValue(json, LocalDateTime.class);
            System.out.println(dt);
        } catch (IOException e) {
            e.printStackTrace();
        }

【解决方案】　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

思路：

　　改变jackson内部序列化和反序列化LocalDateTime的方式，序列化时转换为时间戳，反序列化时将时间戳转换为LocalDateTime即可。

代码：

创建两个类，分别继承JsonSerializer和JsonDeserializer

//时间序列化时变为时间戳
public class LocalDateTimeSerializer extends JsonSerializer<LocalDateTime> {
    @Override
    public void serialize(LocalDateTime localDateTime, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
        jsonGenerator.writeNumber(localDateTime.toInstant(ZoneOffset.ofHours(8)).toEpochMilli());
    }
}

//时间戳反序列化时间
public class LocalDateTimeDeserializer extends JsonDeserializer<LocalDateTime> {
    @Override
    public LocalDateTime deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
        Long timestamp = jsonParser.getLongValue();
        return LocalDateTime.ofEpochSecond(timestamp / 1000, 0, ZoneOffset.ofHours(8));
    }
}

使用方法（序列化添加如下3行红色代码）

      LocalDateTime dt = LocalDateTime.now();
        ObjectMapper mapper = new ObjectMapper();
        try {
            JavaTimeModule timeModule = new JavaTimeModule();
            timeModule.addSerializer(LocalDateTime.class, new LocalDateTimeSerializer());
            mapper.registerModule(timeModule);
            String json = mapper.writeValueAsString(dt);
            System.out.println(json);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }

* 经过此方式序列化得到的结果为："1568528927273"

使用方法（反序列化添加如下3行红色代码）

String json = "1568528927273";
        ObjectMapper mapper = new ObjectMapper();
        try {
            JavaTimeModule timeModule = new JavaTimeModule();
            timeModule.addDeserializer(LocalDateTime.class, new LocalDateTimeDeserializer());
            mapper.registerModule(timeModule);
            LocalDateTime dt = mapper.readValue(json, LocalDateTime.class);
            System.out.println(dt);
        } catch (IOException e) {
            e.printStackTrace();
        }

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