Java中jackson常用注解

在日常使用中，由于涉及到各种序列化和反序列化的处理，就不能不提 注解，了解注解的常用方式可以极大地方便我们处理序列化，下面介绍一些在使用Jackson中涉及到的注解。

1. @JsonProperty - 字段命名

@JsonProperty 注解用于在序列化时按照给定的字段名命名，在反序列化时，在 json 串中的注解字段给该字段设置属性值。

下面是注解的简单示例：

package org.example;
 
import com.fasterxml.jackson.annotation.JsonProperty;
 
public class PersonProperty {
    @JsonProperty("first_name")
    private String firstName;
 
    public PersonProperty() {
    }
 
    public String getFirstName() {
        return firstName;
    }
 
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
}

public static void jsonPropertyDemo() {
    ObjectMapper objectMapper = new ObjectMapper();
    PersonProperty pp = new PersonProperty();
    pp.setFirstName("Alice");
 
    String jsonString = null;
    try {
        jsonString = objectMapper.writeValueAsString(pp);
        System.out.println("json property: " + jsonString);
    } catch (Exception e) {
        e.printStackTrace();
    }
 
    try {
        PersonProperty pp1 = objectMapper.readValue(jsonString, PersonProperty.class);
        System.out.println(pp1.getFirstName());
    } catch (Exception e) {
        e.printStackTrace();
    }
}

2. @JsonPropertyOrder - 字段序列化顺序

@JsonPropertyOrder加在类上，用以规定数据序列化时字段出现的顺序。

package org.example;
 
import com.fasterxml.jackson.annotation.JsonPropertyOrder;
 
// {"name":"Bob","id":"111","age":25,"phone":"12345678910"}
@JsonPropertyOrder({"name", "id", "age", "phone"}) 
// 没有定义顺序，就按照字典序排列，{"age":25,"id":"111","name":"Bob","phone":"12345678910"}
// @JsonPropertyOrder(alphabetic = true) 
public class PersonPropertyOrder {
    private String id;
 
    private String name;
 
    private int age;
 
    private String phone;
 
    public PersonPropertyOrder() {
    }
 
    public String getId() {
        return id;
    }
 
    public void setId(String id) {
        this.id = id;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public int getAge() {
        return age;
    }
 
    public void setAge(int age) {
        this.age = age;
    }
 
    public String getPhone() {
        return phone;
    }
 
    public void setPhone(String phone) {
        this.phone = phone;
    }
}

public static void jsonPropertyOrder() {
    ObjectMapper objectMapper = new ObjectMapper();
    PersonPropertyOrder ppo = new PersonPropertyOrder();
    ppo.setAge(25);
    ppo.setId("111");
    ppo.setName("Bob");
    ppo.setPhone("12345678910");
 
    String jsonString = null;
    try {
        jsonString = objectMapper.writeValueAsString(ppo);
        System.out.println("json property: " + jsonString);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

3. @JsonAlias - 字段别名，反序列化

在数据反序列化时，通过 @JsonAlias 注解来设置字段的值，只要是 alias中的和字段本身都可以正常反序列化。

package org.example;
 
import com.fasterxml.jackson.annotation.JsonAlias;
 
public class PersonAlias {
    @JsonAlias({"firstName", "personName"})
    private String name;
 
    public PersonAlias() {
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
}

public static void jsonAlias() {
    String jsonString1 = "{"name":"Bob"}";
    String jsonString2 = "{"firstName":"Bob"}";
    String jsonString3 = "{"personName":"Bob"}";
 
    ObjectMapper objectMapper = new ObjectMapper();
    try {
        PersonAlias p1 = objectMapper.readValue(jsonString1, PersonAlias.class);
        PersonAlias p2 = objectMapper.readValue(jsonString2, PersonAlias.class);
        PersonAlias p3 = objectMapper.readValue(jsonString3, PersonAlias.class);
 
        System.out.printf("p1: %s, p2: %s, p3: %s", p1.getName(),p2.getName(), p3.getName());
    } catch (Exception e) {
        e.printStackTrace();
    }
}

4. @JsonIgnore -序列化时忽略字段

@JsonIgnore 加在字段上，用以在序列化时，忽略其，在反序列化时，仅赋值null。

package org.example;
 
import com.fasterxml.jackson.annotation.JsonIgnore;
 
 
public class PersonIgnore {
    private String name;
    @JsonIgnore // 不将其序列化，忽略该字段
    private String[] hobbies;
 
    public PersonIgnore() {
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public String[] getHobbies() {
        return hobbies;
    }
 
    public void setHobbies(String[] hobbies) {
        this.hobbies = hobbies;
    }
}
 

public static void jsonIgnore() {
    ObjectMapper objectMapper = new ObjectMapper();
    String jsonString = null;
 
    try {
        PersonIgnore pi = new PersonIgnore();
        pi.setName("Cathy");
        pi.setHobbies(null);
 
        jsonString = objectMapper.writeValueAsString(pi);
        System.out.println(jsonString);
    } catch (Exception e) {
        e.printStackTrace();
    }
} 

5. @JsonIgnoreProperties - 序列化时忽略某些字段

@JsonIgnoreProperties 加在类上，用于在序列化时，忽略给定的某些字段。

package org.example;
 
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 
@JsonIgnoreProperties({"age"})
public class PersonIgnoreProperties {
    private String name = "Alice";
    private int age;
 
    public PersonIgnoreProperties() {
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public int getAge() {
        return age;
    }
 
    public void setAge(int age) {
        this.age = age;
    }
}

public static void jsonIgnoreProperties() {
    ObjectMapper objectMapper = new ObjectMapper();
    PersonIgnoreProperties pip = new PersonIgnoreProperties();
    pip.setName("Bob");
    pip.setAge(18);
 
    try {
        String jsonString = objectMapper.writeValueAsString(pip);
        System.out.println(jsonString);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

6. @JsonInclude - 序列化时作用于满足条件的

@JsonInclude可以加在类上，也可以加在字段上。该注解表示满足某些条件(NON_NULL,NON_ABSENT,NON_EMPTY,NON_DEFAULT,等)的才能序列化，e.g.如果加在类上，表示只要对象有null就忽略该对象，加在字段上，如果字段是null，则忽略该字段。

package org.example;
 
import com.fasterxml.jackson.annotation.JsonInclude;
 
@JsonInclude(JsonInclude.Include.NON_NULL)
public class PersonInclude {
    private int id;
 
    private String name;
    @JsonInclude(JsonInclude.Include.NON_NULL)
    private String[] hobbies;
 
    public PersonInclude() {
    }
 
    public int getId() {
        return id;
    }
 
    public void setId(int id) {
        this.id = id;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public String[] getHobbies() {
        return hobbies;
    }
 
    public void setHobbies(String[] hobbies) {
        this.hobbies = hobbies;
    }
}

public static void jsonInclude() {
    ObjectMapper objectMapper = new ObjectMapper();
    PersonInclude pi = new PersonInclude();
 
    pi.setName("Cathy");
    pi.setId(1111);
 
    try {
        String jsonString = objectMapper.writeValueAsString(pi);
        System.out.println(jsonString);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

7. @JsonFormat - 设置格式，如日期时间等

用于设置时间格式，或者是数字，或者是日期格式。

package org.example;
 
import com.fasterxml.jackson.annotation.JsonFormat;
 
import java.time.LocalDate;
import java.util.Date;
 
public class PersonFormat {
    @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd hh:mm:ss", timezone = "GMT+8")
    private Date birthDate;
 
    public PersonFormat() {
    }
 
    public Date getBirthDate() {
        return birthDate;
    }
 
    public void setBirthDate(Date birthDate) {
        this.birthDate = birthDate;
    }
}

public static void jsonFormat() {
    ObjectMapper objectMapper = new ObjectMapper();
    PersonFormat pf = new PersonFormat();
    pf.setBirthDate(new Date());
 
    try {
        String jsonString = objectMapper.writeValueAsString(pf);
        System.out.println(jsonString);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

8. @JacksonInject - 反序列化时注入到java对象

该注解用于在数据反序列化时将其他字段注入进Java对象。

package org.example;
 
import com.fasterxml.jackson.annotation.JacksonInject;
 
import java.time.LocalDate;
import java.time.LocalDateTime;
 
public class PersonInject {
 
    private String name;
    
    private int age;
    
    @JacksonInject("responseTime")
    private LocalDateTime responseTime;
 
    public PersonInject() {
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public int getAge() {
        return age;
    }
 
    public void setAge(int age) {
        this.age = age;
    }
 
    public LocalDateTime getResponseTime() {
        return responseTime;
    }
 
    public void setResponseTime(LocalDateTime responseTime) {
        this.responseTime = responseTime;
    }
}

public static void jsonInject() {
    InjectableValues.Std iv = new InjectableValues.Std();
    ObjectMapper objectMapper = new ObjectMapper();
    iv.addValue("responseTime", LocalDateTime.now());
 
    //将JSON字符串反序列化为java对象
    String jsonString = "{"name":"Alice","age":23}";
    objectMapper.setInjectableValues(iv);
 
    try {
        PersonInject pi = objectMapper.readValue(jsonString, PersonInject.class);
        System.out.println(pi.getResponseTime());
    } catch (Exception e) {
        e.printStackTrace();
    }
}

9. @JsonCreator && @ConstructorProperties - 反序列化时采用的构造方法

@JsonCreator 用于在json数据反序列化到实例对象时采用哪个构造方法，同时搭配 @JsonProperty 注解用于相关属性的。

package org.example;
 
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.ObjectMapper;
 
public class PersonCreator {
    private String name;
 
    private int age;
 
    // 构造方法1
    public PersonCreator(String name) {
        this.name = name;
    }
 
    // 构造方法2
    @JsonCreator // 用于反序列化时的处理
    public PersonCreator(@JsonProperty("username") String name,
                         @JsonProperty("age") int age) {
        this.name = name;
        this.age = age;
    }
 
    @Override
    public String toString() {
        return "Test{" +
                "name='" + name + ''' +
                ", age='" + age + ''' +
                '}';
    }
 
    public static void main(String[] args) throws Exception {
        String jsonString = "{"username": "Alice", "age": 18}"; // username -> name
        ObjectMapper objectMapper = new ObjectMapper();
 
        try {
            PersonCreator pc = objectMapper.readValue(jsonString, PersonCreator.class);
            System.out.println(pc);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
 

@ConstructorProperties 也用于构造方法，但相比 @JsonCreator 的使用要简单，可以认为 @ConstructorProperties = @JsonCreator + @JsonProperty。

package org.example;
 
import com.fasterxml.jackson.databind.ObjectMapper;
 
import java.beans.ConstructorProperties;
 
public class PersonConstructorProperties {
    private String username;
 
    private int age;
 
    public PersonConstructorProperties(String username) {
        this.username = username;
    }
 
    @ConstructorProperties({"name", "age"})
    public PersonConstructorProperties(String username, int age) {
        System.out.println("全参构造函数...");
        this.username = username;
        this.age = age;
    }
 
    @Override
    public String toString() {
        return "Test{" +
                "username='" + username + ''' +
                ", age='" + age + ''' +
                '}';
    }
 
    public static void main(String[] args) {
        String jsonString = "{"name": "Bob", "age": 29}";
        ObjectMapper objectMapper = new ObjectMapper();
 
        try {
            PersonConstructorProperties pcp = objectMapper.readValue(jsonString, PersonConstructorProperties.class);
            System.out.println(pcp);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}
 

10. @JsonSerialize && @JsonDeserialize - 自定义序列化方法

这两个注解用于实现自定义的序列化和反序列化的处理，比如我们有个需求，需要将小数的某个字段规定精确位数，为空时输出空字符串。

@JsonSerialize
package org.example;
 
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 
import java.io.IOException;
import java.math.RoundingMode;
import java.text.DecimalFormat;
 
public class PersonSerialize {
    @JsonSerialize(using = CustomDoubleSerialize.class, nullsUsing = NullNumberSerialize.class)
    private Double model;
 
    @JsonSerialize(nullsUsing = NullNumberSerialize.class)
    private Double business;
 
    private String name;
 
    public PersonSerialize() {
    }
 
    public Double getModel() {
        return model;
    }
 
    public void setModel(Double model) {
        this.model = model;
    }
 
    public Double getBusiness() {
        return business;
    }
 
    public void setBusiness(Double business) {
        this.business = business;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
}
 
/**
 * Double保留4位小数，输出string
 */
class CustomDoubleSerialize extends JsonSerializer<Double> {
    private static final DecimalFormat df = new DecimalFormat("#.####");
 
    @Override
    public void serialize(Double value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        df.setRoundingMode(RoundingMode.HALF_UP); // 4
        gen.writeString(df.format(value));
    }
}
 
/**
 * 任意类型null值，改为空字符串输出
 */
class NullNumberSerialize extends JsonSerializer<Object> {
 
    @Override
    public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        gen.writeString("");
    }
}

public static void jsonSerialize() {
    ObjectMapper objectMapper = new ObjectMapper();
    PersonSerialize ps = new PersonSerialize();
    ps.setName("Alice");
    ps.setModel(1.2345678);
 
    try {
        String jsonString = objectMapper.writeValueAsString(ps);
        System.out.println(jsonString); // {"model":"1.2346","business":"","name":"Alice"}
    } catch (Exception e) {
        e.printStackTrace();
    }
}

@JsonDeserialize
package org.example;
 
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;
 
import java.io.IOException;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.time.format.FormatStyle;
 
public class PersonDeserialize {
    @JsonSerialize(using = LocalDatetimeSerialize.class)
    @JsonDeserialize(using = LocalDatetimeDeserialize.class)
    private LocalDateTime birthDate;
 
    private String name;
 
    public PersonDeserialize() {
    }
 
    public LocalDateTime getBirthDate() {
        return birthDate;
    }
 
    public void setBirthDate(LocalDateTime birthDate) {
        this.birthDate = birthDate;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
}
 
class LocalDatetimeSerialize extends JsonSerializer<LocalDateTime> {
    static final DateTimeFormatter DATE_FORMATTER = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM);
 
    @Override
    public void serialize(LocalDateTime value, JsonGenerator gen, SerializerProvider provider) throws IOException {
        String str = value.format(DATE_FORMATTER);
        gen.writeString(str);
    }
}
 
class LocalDatetimeDeserialize extends JsonDeserializer<LocalDateTime> {
    @Override
    public LocalDateTime deserialize(JsonParser p, DeserializationContext ctx) throws IOException {
        String str = p.getText();
        return LocalDateTime.parse(str, LocalDatetimeSerialize.DATE_FORMATTER);
    }
}

public static void jsonDeserialize() {
    ObjectMapper objectMapper = new ObjectMapper();
    PersonDeserialize pd = new PersonDeserialize();
    pd.setName("Dav");
    pd.setBirthDate(LocalDateTime.of(2000, 12, 5, 0, 0));
 
    String jsonString = null;
 
    // serialize
    try {
        jsonString = objectMapper.writeValueAsString(pd);
        System.out.println(jsonString); // {"birthDate":"2000年12月5日 00:00:00","name":"Dav"}
    } catch (Exception e) {
        e.printStackTrace();
    }
 
    // deserialize
    try {
        PersonDeserialize pd1 = objectMapper.readValue(jsonString, PersonDeserialize.class);
        // person -> name: Dav, birthdate: 2000-12-05T00:00
        System.out.printf("person -> name: %s, birthdate: %s\n", pd1.getName(), pd1.getBirthDate());
    } catch (Exception e) {
        e.printStackTrace();
    }
}

11. @JsonAnyGetter && @JsonANySetter - 序列化对map字段的处理

这两个注解用于在序列化和反序列化时map结构的处理，具体说来：

• @JsonAnyGetter，加在getField()上，用于序列化时将此map字段的键值对移至json中的键值对
• @JsonAnySetter，加在字段上或者setField()都可以，加一个即可，用于反序列化时构造类实例，设置类实例属性，将json中的非明确定义的键值对都设置到map结构的字段中

package org.example;
 
import com.fasterxml.jackson.annotation.JsonAnyGetter;
import com.fasterxml.jackson.annotation.JsonAnySetter;
 
import java.util.HashMap;
import java.util.Map;
 
public class PersonGetAndSet {
    private String username;
 
    private String pwd;
 
    private int age;
 
    // @JsonAnySetter // 加方法或者属性都可以，但1个即可
    private Map<String, String> map;
 
    public PersonGetAndSet() {
        this.map = new HashMap<>();
    }
 
    public String getUsername() {
        return username;
    }
 
    public void setUsername(String username) {
        this.username = username;
    }
 
    public String getPwd() {
        return pwd;
    }
 
    public void setPwd(String pwd) {
        this.pwd = pwd;
    }
 
    public int getAge() {
        return age;
    }
 
    public void setAge(int age) {
        this.age = age;
    }
 
    @JsonAnyGetter // serialize, {"username":"Ada","pwd":"123456","age":26,"key1":"val1","key2":"val2"}
    public Map<String, String> getMap() {
        return map;
    }
 
    @JsonAnySetter // deserialize, pwd: 123456, age: 26, map: {key1=val1, key2=val2}
    public void setMap(String key, String value) {
        this.map.put(key, value);
    }
}

public static void jsonGetterAndSetter() {
    PersonGetAndSet pgs = new PersonGetAndSet();
    pgs.setUsername("Ada");
    pgs.setAge(26);
    pgs.setPwd("123456");
    pgs.setMap("key1", "val1");
    pgs.setMap("key2", "val2");
 
    ObjectMapper objectMapper = new ObjectMapper();
 
    String jsonString = null;
 
    try {
        jsonString = objectMapper.writeValueAsString(pgs);
        System.out.println(jsonString);
    } catch (Exception e) {
        e.printStackTrace();
    }
 
    try {
        PersonGetAndSet pgs1 = objectMapper.readValue(jsonString, PersonGetAndSet.class);
        System.out.printf("person -> username: %s, pwd: %s, age: %d, map: %s\n", pgs1.getUsername(), pgs1.getPwd(), pgs1.getAge(), pgs1.getMap());
    } catch (Exception e) {
        e.printStackTrace();
    }
}

12. @JsonNaming - 序列化时输出格式

@JsonNaming 加在类上，用以规范序列化时输出的字段键值的形式，主要有以下格式：

• SnakeCaseStrategy，蛇形体， 如first_name
• UpperCamelCaseStrategy，大写驼峰体，如FirstName
• LowerCaseStratey，小写连体，如firstname
• LowerDotCaseStratey，小写点分，如first.name

package org.example;
 
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.PropertyNamingStrategy;
import com.fasterxml.jackson.databind.annotation.JsonNaming;
 
//@JsonNaming(value = PropertyNamingStrategy.SnakeCaseStrategy.class) // 蛇形体：{"first_name":"Matt","second_name":"Damon"}
//@JsonNaming(value = PropertyNamingStrategy.UpperCamelCaseStrategy.class)  // {"FirstName":"Matt","SecondName":"Damon"}
//@JsonNaming(value = PropertyNamingStrategy.LowerCaseStrategy.class)  // {"firstname":"Matt","secondname":"Damon"}
@JsonNaming(value = PropertyNamingStrategy.LowerDotCaseStrategy.class)  // {"first.name":"Matt","second.name":"Damon"}
public class PersonNaming {
    private String firstName;
 
    private String secondName;
 
    public PersonNaming() {
    }
 
    public PersonNaming(String firstName, String secondName) {
        this.firstName = firstName;
        this.secondName = secondName;
    }
 
    public String getFirstName() {
        return firstName;
    }
 
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
 
    public String getSecondName() {
        return secondName;
    }
 
    public void setSecondName(String secondName) {
        this.secondName = secondName;
    }
 
    public static void main(String[] args) {
        PersonNaming pn = new PersonNaming();
        pn.setFirstName("Matt");
        pn.setSecondName("Damon");
 
        ObjectMapper objectMapper = new ObjectMapper();
 
        try {
            String jsonString = objectMapper.writeValueAsString(pn);
            System.out.println(jsonString);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

13. staic和transient字段

如果字段属性中有这两个修饰符，则在序列化处理时忽略相关字段。