【Jackson】注解及其使用

Jackson库提供了多种注解（annotations），可以用来控制JSON序列化和反序列化的行为。这些注解允许你灵活地映射Java对象与JSON数据之间的关系。下面将详细介绍一些常用的Jackson注解及其用法。

1. @JsonProperty

作用: 用于指定JSON属性与Java类字段的映射关系。可以用来重命名字段，或指定序列化/反序列化时的字段名。

示例:

import com.fasterxml.jackson.annotation.JsonProperty;public class User {@JsonProperty("user_name")private String name;@JsonProperty("user_age")private int age;// Getter和Setter方法@Overridepublic String toString() {return "User{" +"name='" + name + '\'' +", age=" + age +'}';}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;public class JsonPropertyExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();// 创建User对象User user = new User();user.setName("John Doe");user.setAge(30);// 序列化为JSONString jsonString = mapper.writeValueAsString(user);System.out.println(jsonString); // 输出: {"user_name":"John Doe","user_age":30}// 反序列化JSONUser deserializedUser = mapper.readValue(jsonString, User.class);System.out.println(deserializedUser); // 输出: User{name='John Doe', age=30}}
}

2. @JsonIgnore

作用: 用于忽略某个字段，避免将其序列化到JSON中或从JSON中反序列化该字段。

示例:

import com.fasterxml.jackson.annotation.JsonIgnore;public class User {private String name;private int age;@JsonIgnoreprivate String password;// Getter和Setter方法@Overridepublic String toString() {return "User{" +"name='" + name + '\'' +", age=" + age +'}';}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;public class JsonIgnoreExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();// 创建User对象User user = new User();user.setName("John Doe");user.setAge(30);user.setPassword("secret");// 序列化为JSONString jsonString = mapper.writeValueAsString(user);System.out.println(jsonString); // 输出: {"name":"John Doe","age":30}}
}

3. @JsonInclude

作用: 用于指定序列化时应包括哪些字段。例如，可以设置为JsonInclude.Include.NON_NULL，仅包括非空字段。

示例:

import com.fasterxml.jackson.annotation.JsonInclude;@JsonInclude(JsonInclude.Include.NON_NULL)
public class User {private String name;private Integer age;  // 使用包装类以允许null值// Getter和Setter方法@Overridepublic String toString() {return "User{" +"name='" + name + '\'' +", age=" + age +'}';}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;public class JsonIncludeExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();// 创建User对象User user = new User();user.setName("John Doe");// age字段没有设置值，因此在序列化时将被忽略// 序列化为JSONString jsonString = mapper.writeValueAsString(user);System.out.println(jsonString); // 输出: {"name":"John Doe"}}
}

4. @JsonFormat

作用: 用于指定日期和时间字段的格式化方式。例如，可以指定日期格式。

示例:

import com.fasterxml.jackson.annotation.JsonFormat;
import java.util.Date;public class Event {private String title;@JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")private Date eventDate;// Getter和Setter方法@Overridepublic String toString() {return "Event{" +"title='" + title + '\'' +", eventDate=" + eventDate +'}';}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;import java.text.SimpleDateFormat;
import java.util.Date;public class JsonFormatExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd"));// 创建Event对象Event event = new Event();event.setTitle("Annual Meeting");event.setEventDate(new Date());// 序列化为JSONString jsonString = mapper.writeValueAsString(event);System.out.println(jsonString); // 输出: {"title":"Annual Meeting","eventDate":"2024-07-25"}}
}

5. @JsonCreator 和 @JsonProperty

作用: @JsonCreator 用于标记构造函数或工厂方法，使Jackson在反序列化时使用这个构造函数或方法。@JsonProperty 用于指定构造函数参数的名称。

示例:

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;public class User {private String name;private int age;@JsonCreatorpublic User(@JsonProperty("name") String name, @JsonProperty("age") int age) {this.name = name;this.age = age;}// Getter和Setter方法@Overridepublic String toString() {return "User{" +"name='" + name + '\'' +", age=" + age +'}';}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;public class JsonCreatorExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();// JSON字符串String jsonString = "{\"name\":\"John Doe\",\"age\":30}";// 反序列化JSONUser user = mapper.readValue(jsonString, User.class);System.out.println(user); // 输出: User{name='John Doe', age=30}}
}

6. @JsonSetter

作用: 指定反序列化时应调用的方法。可以用来调整字段名的映射。

示例:

import com.fasterxml.jackson.annotation.JsonSetter;public class User {private String name;private int age;@JsonSetter("user_name")public void setName(String name) {this.name = name;}@JsonSetter("user_age")public void setAge(int age) {this.age = age;}// Getter方法@Overridepublic String toString() {return "User{" +"name='" + name + '\'' +", age=" + age +'}';}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;public class JsonSetterExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();// JSON字符串String jsonString = "{\"user_name\":\"John Doe\",\"user_age\":30}";// 反序列化JSONUser user = mapper.readValue(jsonString, User.class);System.out.println(user); // 输出: User{name='John Doe', age=30}}
}

7. @JsonDeserialize 和 @JsonSerialize

作用: 用于指定自定义的反序列化和序列化器。可以用来自定义字段的解析和序列化逻辑。

示例:

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonGenerator;
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.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;public class Event {private String title;@JsonSerialize(using = CustomDateSerializer.class)@JsonDeserialize(using = CustomDateDeserializer.class)private Date eventDate;// Getter和Setter方法public static class CustomDateSerializer extends JsonSerializer<Date> {private SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");@Overridepublic void serialize(Date date, JsonGenerator gen, SerializerProvider serializers) throws IOException {gen.writeString(dateFormat.format(date));}}public static class CustomDateDeserializer extends JsonDeserializer<Date> {private SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");@Overridepublic Date deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {try {return dateFormat.parse(p.getText());} catch (ParseException e) {throw new IOException(e);}}}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;import java.text.SimpleDateFormat;
import java.util.Date;public class JsonSerializeDeserializeExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd"));// 创建Event对象Event event = new Event();event.setTitle("Annual Meeting");event.setEventDate(new Date());// 序列化为JSONString jsonString = mapper.writeValueAsString(event);System.out.println(jsonString); // 输出: {"title":"Annual Meeting","eventDate":"2024-07-25"}// 反序列化JSONEvent deserializedEvent = mapper.readValue(jsonString, Event.class);System.out.println(deserializedEvent); // 输出: Event{title='Annual Meeting', eventDate=Tue Jul 25 00:00:00 UTC 2024}}
}

8. @JsonManagedReference 和 @JsonBackReference

作用: 用于处理双向关系中的循环引用问题。@JsonManagedReference 用于标记父对象中的字段，@JsonBackReference 用于标记子对象中的字段。

示例:

import com.fasterxml.jackson.annotation.JsonBackReference;
import com.fasterxml.jackson.annotation.JsonManagedReference;public class User {private String name;@JsonManagedReferenceprivate Address address;// Getter和Setter方法@Overridepublic String toString() {return "User{" +"name='" + name + '\'' +", address=" + address +'}';}
}public class Address {private String street;@JsonBackReferenceprivate User user;// Getter和Setter方法@Overridepublic String toString() {return "Address{" +"street='" + street + '\'' +'}';}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;public class JsonManagedBackReferenceExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();// 创建User和Address对象User user = new User();user.setName("John Doe");Address address = new Address();address.setStreet("123 Main St");address.setUser(user);user.setAddress(address);// 序列化为JSONString jsonString = mapper.writeValueAsString(user);System.out.println(jsonString); // 输出: {"name":"John Doe","address":{"street":"123 Main St"}}// 反序列化JSONUser deserializedUser = mapper.readValue(jsonString, User.class);System.out.println(deserializedUser); // 输出: User{name='John Doe', address=Address{street='123 Main St'}}}
}

9. @JsonAlias

作用: 允许字段有多个别名，以便在反序列化时接受不同名称的JSON属性。

示例:

import com.fasterxml.jackson.annotation.JsonAlias;public class User {@JsonAlias({"username", "user_name"})private String name;@JsonAlias({"userage", "user_age"})private int age;// Getter和Setter方法@Overridepublic String toString() {return "User{" +"name='" + name + '\'' +", age=" + age +'}';}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;public class JsonAliasExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();// JSON字符串String jsonString = "{\"username\":\"John Doe\",\"userage\":30}";// 反序列化JSONUser user = mapper.readValue(jsonString, User.class);System.out.println(user); // 输出: User{name='John Doe', age=30}}
}

10. @JsonUnwrapped

作用: 将嵌套对象的属性“展开”到父对象中。这在处理嵌套结构时特别有用。

示例:

import com.fasterxml.jackson.annotation.JsonUnwrapped;public class User {private String name;private int age;@JsonUnwrappedprivate Address address;// Getter和Setter方法@Overridepublic String toString() {return "User{" +"name='" + name + '\'' +", age=" + age +", address=" + address +'}';}
}public class Address {private String street;private String city;// Getter和Setter方法@Overridepublic String toString() {return "Address{" +"street='" + street + '\'' +", city='" + city + '\'' +'}';}
}

用法:

import com.fasterxml.jackson.databind.ObjectMapper;public class JsonUnwrappedExample {public static void main(String[] args) throws Exception {ObjectMapper mapper = new ObjectMapper();// 创建User和Address对象User user = new User();user.setName("John Doe");user.setAge(30);Address address = new Address();address.setStreet("123 Main St");address.setCity("Springfield");user.setAddress(address);// 序列化为JSONString jsonString = mapper.writeValueAsString(user);System.out.println(jsonString); // 输出: {"name":"John Doe","age":30,"street":"123 Main St","city":"Springfield"}// 反序列化JSONUser deserializedUser = mapper.readValue(jsonString, User.class);System.out.println(deserializedUser); // 输出: User{name='John Doe', age=30, address=Address{street='123 Main St', city='Springfield'}}}
}

总结

Jackson库中的注解可以帮助你灵活地控制JSON数据的序列化和反序列化过程。通过合理使用这些注解，你可以实现字段重命名、忽略字段、设置字段包含策略、处理日期格式、定义自定义序列化和反序列化逻辑、解决循环引用问题、多态处理等功能。这些功能使得Jackson在处理复杂JSON数据时非常强大和灵活。