Jackson的使用

引入包

<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-core -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.13.1</version>
</dependency>

创建需要序列化的对象

@Data
public class Student {
    private String name;
    private Integer age;
    private Date birthday;
    private LocalDateTime start;
}

序列化

public class jacksonTest {
    private static ObjectMapper objectMapper = new ObjectMapper();
    static {
        // 设置JSON包含
        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
    }
    @Test
    void jacksonTest(){
        Student student = new Student();
        student.setAge(20);
        student.setBirthday(new Date());
        student.setStart(LocalDateTime.now());
        objectMapper.writerWithDefaultPrettyPrinter()

    }
}

处理NULL值

设置JSON是否包含null，有两种方式

1. 全局方式
   objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

2. 针对某一个类，使用注解@JsonInclude(JsonInclude.Include.NON_NULL)

@Data
@JsonInclude(JsonInclude.Include.NON_NULL)
public class Student {
    private String name;
    private Integer age;
    private Date birthday;
    private LocalDateTime start;
}

处理Date格式

针对一个属性

在Bean对象的属性上使用@JsonFormat(pattern = "yyyy-MM-dd",timezone = "GMT+8")注解

@Data
public class Student {
    @JsonInclude(JsonInclude.Include.NON_NULL)
    private String name;
    private Integer age;
    @JsonFormat(pattern = "yyyy-MM-dd",timezone = "GMT+8")
    private Date birthday;
    @JsonFormat(pattern = "yyyyMMdd HH:mm:ss",timezone = "GMT+8")
    private LocalDateTime start;
}

标记完运行发现LocalDateTime类型对应的start是没有被序列化成想要的字符串

需要引入一个jsr310的包，是jackson对LocalDateTime等jar8时间日期处理的Model
在jackson中是作为Model生效的，Model需要注册。

1. 先引入jsr310的包 了解看下格林威治时间、GMT、UTC、CST、ISO、JSR310

<dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-jsr310</artifactId>
    <version>2.13.1</version>
</dependency>

2. 注册

public class JasksonTest {
    private static ObjectMapper objectMapper = new ObjectMapper();
    static {
        // 设置JSON包含
        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        // 自动通过SPI发现jackson的modle并注册
        objectMapper.findAndRegisterModules();
    }
    @Test
    void jasksonTest() throws JsonProcessingException {
        Student student = new Student();
        student.setAge(20);
        student.setBirthday(new Date());
        student.setStart(LocalDateTime.now());
        System.out.println(objectMapper.writeValueAsString(student));;

    }
}

3. 再次查看

LocalDateTime设置全局

public class JasksonTest {
    private static ObjectMapper objectMapper = new ObjectMapper();
    static {
        // 设置JSON包含
        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        JavaTimeModule javaTimeModule = new JavaTimeModule();
        // 反序列化时使用
        javaTimeModule.addDeserializer(LocalDateTime.class,
            new LocalDateTimeDeserializer(DateTimeFormatter.ofPattern("yyyyMMdd HH:mm:ss")));
        // 序列化时使用
        javaTimeModule.addSerializer(LocalDateTime.class,
            new LocalDateTimeSerializer(DateTimeFormatter.ofPattern("yyyyMMdd HH:mm:ss")));
        objectMapper.registerModule(javaTimeModule);
    }
    @Test
    void jasksonTest() throws JsonProcessingException {
        Student student = new Student();
        student.setAge(20);
        student.setBirthday(new Date());
        student.setStart(LocalDateTime.now());
        System.out.println(objectMapper.writeValueAsString(student));;

    }
}

查看

LocalDateTime的时间格式是到yyyyMMdd HH:mm:ss的如果只是yyyyMMdd，反序列化会报错

Cannot deserialize value of type java.time.LocalDateTime from String "20220124"

Date类型也可以但不建议使用
objectMapper.setDateFormat(new SimpleDateFormat("yyyyMMdd HH:mm:ss"));
SimpleDateFormat不是线程安全的，而ObjectMapper是线程安全的。这样设置导致ObjectMapper线程不安全。

美化输出

objectMapper.configure(SerializationFeature.INDENT_OUTPUT,true);

反序列化

忽略不存在的key

objectMapper.configure(DeserializationFeature.FAIL_ON_IGNORED_PROPERTIES,false);

泛型

public class JasksonTest {
    private static ObjectMapper objectMapper = new ObjectMapper();
    static {
        // 设置JSON包含
        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
//        objectMapper.findAndRegisterModules();
        JavaTimeModule javaTimeModule = new JavaTimeModule();
//        // 反序列化时使用
        javaTimeModule.addDeserializer(LocalDateTime.class,
            new LocalDateTimeDeserializer(DateTimeFormatter.ofPattern("yyyyMMdd HH:mm:ss")));
//        // 序列化时使用
        javaTimeModule.addSerializer(LocalDateTime.class,
            new LocalDateTimeSerializer(DateTimeFormatter.ofPattern("yyyyMMdd HH:mm:ss")));
        objectMapper.registerModule(javaTimeModule);
        // 美化输出
        objectMapper.configure(SerializationFeature.INDENT_OUTPUT,true);
        // 反序列化时忽略不存在的key
        objectMapper
            .configure(DeserializationFeature.FAIL_ON_IGNORED_PROPERTIES,false);
    }
    @Test
    void jasksonTest() throws JsonProcessingException {
        Student student = new Student();
        student.setAge(20);
        student.setBirthday(new Date());
        student.setStart(LocalDateTime.now());
        ResponseDataBase<Student> response = new ResponseDataBase<>();
        response.setData(student);
        response.successful();
        String data = objectMapper.writeValueAsString(response);
        System.out.println(data);
        ResponseDataBase<Student> dataResult = 
        objectMapper
            .readValue(data, new TypeReference<ResponseDataBase<Student>>() {});
        System.out.println(dataResult.getData().toString());
    }
}

定制

驼峰转下划线

设置策略 userName 转成 user_name 输出
// 蛇形命名法
objectMapper.setPropertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE);

指定属性名称

使用注解@JsonProperty("address")

指定忽略属性

使用注解@JsonIgnore

使用Jackson做对象更新

对象更新，对象的合并。如果后者属性有值，则用后者，否则的值不变。

@Test
void jasksonTest2() throws JsonProcessingException {
    Student orgUser = new Student();
    orgUser.setName("user1");
    orgUser.setAge(20);
    orgUser.setBirthday(new Date());

    Student newUser = new Student();
    newUser.setName("user1");
    newUser.setAge(22);

    Student student = objectMapper.updateValue(orgUser, newUser);
    System.out.println(student.toString());
}

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