Java中注解@JsonFormat的用法詳解

更新時(shí)間：2023年01月06日 12:04:35 作者：喝汽水的貓^

這篇文章主要給大家介紹了關(guān)于Java中注解@JsonFormat用法的相關(guān)資料,以及分享了@JsonFormat 將枚舉序列化為對(duì)象的方法,文中給出了詳細(xì)的代碼實(shí)例,需要的朋友可以參考下

一、@JsonFormat是什么？

通常日期格式都是以時(shí)間戳的形式存放在數(shù)據(jù)庫里，當(dāng)前端頁面通過接口查詢時(shí)，我們會(huì)將一個(gè)對(duì)象的某些屬性查出來返回給頁面。

類似在實(shí)體類上加上改注解:

二、@JsonFormat參數(shù)講解

1.locale代表中國的意思:中國時(shí)間段

2.pattern寫的就是對(duì)于數(shù)據(jù)庫的時(shí)間段

yyyy代表-年
MM代表-月
dd代表-日
HH代表-時(shí)
mm代表分
ss代表-秒

3.TimeZone

TimeZone 表示時(shí)區(qū)偏移量，也可以計(jì)算夏令時(shí)。

在操作 Date, Calendar等表示日期/時(shí)間的對(duì)象時(shí)，經(jīng)常會(huì)用到TimeZone；因?yàn)椴煌臅r(shí)區(qū)，時(shí)間不同。

那么timezone加上GMT+8什么意思呢?

GMT 就是格林威治標(biāo)準(zhǔn)時(shí)間的英文縮寫(Greenwich Mean Time 格林尼治標(biāo)準(zhǔn)時(shí)間)，是世界標(biāo)準(zhǔn)時(shí)間，gmt+8 是格林威治時(shí)間+8小時(shí)，中國所在時(shí)區(qū)就是gmt+8 。

附：@JsonFormat 將枚舉序列化為對(duì)象

Java Enum

@JsonFormat(shape = JsonFormat.Shape.OBJECT)
public enum EmployeeType {
    FullTime("Full Time"), PartTime("Part Time");

    private String displayName;

    EmployeeType(String displayName) {
        this.displayName = displayName;
    }

    public String getDisplayName() {
        return displayName;
    }
}

public class Employee {
    private String name;
    private EmployeeType employeeType;
    ...
}

Main Class

public class ExampleMain {
    public static void main(String[] args) throws IOException {
        Employee employee = new Employee();
        employee.setName("Amy");
        employee.setEmployeeType(EmployeeType.PartTime);

        System.out.println("-- before serialization --");
        System.out.println(employee);

        System.out.println("-- after serialization --");
        ObjectMapper om = new ObjectMapper();
        String jsonString = om.writeValueAsString(employee);
        System.out.println(jsonString);

        System.out.println("-- after deserialization --");
        System.out.println(om.readValue(jsonString, Employee.class));
    }
}

枚舉被序列化為了 JSON 對(duì)象，但無法反序列化。

-- before serialization --
Employee{name='Amy', employeeType=PartTime}
-- after serialization --
{"name":"Amy","employeeType":{"displayName":"Part Time"}}
-- after deserialization --
Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `org.example.c27.EmployeeType` from Object value (token `JsonToken.START_OBJECT`)
 at [Source: (String)"{"name":"Amy","employeeType":{"displayName":"Part Time"}}"; line: 1, column: 30] (through reference chain: org.example.c27.Employee["employeeType"])
	at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:59)
	at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1601)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1375)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1280)
	at com.fasterxml.jackson.databind.DeserializationContext.extractScalarFromObject(DeserializationContext.java:872)
	at com.fasterxml.jackson.databind.deser.std.EnumDeserializer.deserialize(EnumDeserializer.java:199)
	at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:129)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:324)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:187)
	at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:322)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4593)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3548)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3516)
	at org.example.c27.ExampleMain.main(ExampleMain.java:28)

Process finished with exit code 1

不使用 @JsonFormat

-- before serialization --
Employee{name='Amy', employeeType=PartTime}
-- after serialization --
{"name":"Amy","employeeType":"PartTime"}
-- after deserialization --
Employee{name='Amy', employeeType=PartTime}