ObjectMapper
1. 依赖
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.8</version>
</dependency>
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该依赖同时会将如下库添加到项目路径中:
- jackson-annotations-2.9.8.jar
- jackson-core-2.9.8.jar
- jackson-databind-2.9.8.jar
2. 序列化和反序列化
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Car {
private String color;
private String type;
}
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2.1 Java对象序列化为Json
public class Main {
public static void main(String[] args) throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
Car car = new Car("yellow", "renault");
objectMapper.writeValue(new File("target/car.json"), car);
}
}
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输出结果 car.json:
{"color":"yellow","type":"renault"}
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writeValueAsString: 将生成的JSON转化为字符串
//{"color":"yellow","type":"renault"}
String carAsString = objectMapper.writeValueAsString(car);
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writeValueAsBytes: 将生成的JSON转化为字节数组
2.2 JSON反序列化为Java对象
public class Main {
public static void main(String[] args) throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
String json = "{ \"color\" : \"Black\", \"type\" : \"BMW\"}";
Car car = objectMapper.readValue(json, Car.class);
System.out.println(car);
}
}
//输出结果:Car(color=Black, type=BMW)
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readValue()函数还接受其他形式的输入,例如包含JSON字符串的文件:
ObjectMapper objectMapper = new ObjectMapper();
String json = "{ \"color\" : \"Black\", \"type\" : \"BMW\"}";
Car car = objectMapper.readValue(new File("target/json_car.json"), Car.class);
System.out.println(car);
//输出Car(color=yellow, type=renault)
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或者URL
Car car = objectMapper.readValue(new URL("target/json_car.json"), Car.class);
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2.3 JSON 反序列化为Jackson JsonNode
同样的,JSON可以被解析为JsonNode对象,从某一个具体的节点获取数据。
String json = "{ \"color\" : \"Black\", \"type\" : \"FIAT\" }";
JsonNode jsonNode = objectMapper.readTree(json);
String color = jsonNode.get("color").asText();
// Output: color -> Black
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2.4 JSON数组反序列化为Java List
通过使用TypeReference
可以将数组形式的JSON反序列化为Java 数组
String jsonCarArray =
"[{ \"color\" : \"Black\", \"type\" : \"BMW\" },
{ \"color\" : \"Red\", \"type\" : \"FIAT\" }]";
List<Car> listCar = objectMapper.readValue(jsonCarArray,
new TypeReference<List<Car>>(){});
//[Car(color=Black, type=BMW), Car(color=Red, type=FIAT)]
System.out.println(listCar);
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2.5 JSON字符串反序列化为Java Map
String json = "{ \"color\" : \"Black\", \"type\" : \"BMW\" }";
Map<String, Object> map
= objectMapper.readValue(json, new TypeReference<Map<String,Object>>(){});
//{color=Black, type=BMW}
System.out.println(map);
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3. 高级特性
Jackson
库的一个强大之处在于可以对序列化、反序列化进行定制。
3.1 设置序列化、反序列化特性
默认情况下,当JSON字符串包含Java类没有的属性时,反序列化就会失败。
String jsonString
= "{ \"color\" : \"Black\", \"type\" : \"Fiat\", \"year\" : \"1970\" }";
//UnrecognizedPropertyException
Car car = objectMapper.readValue(jsonString, Car.class);
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通过设置方法我们可以改变默认的行为使其忽略新的字段属性。
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
String jsonString
= "{ \"color\" : \"Black\", \"type\" : \"Fiat\", \"year\" : \"1970\" }";
Car car = objectMapper.readValue(jsonString, Car.class);
//Car(color=Black, type=Fiat)
System.out.println(car);
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类似的 FAIL_ON_NULL_FOR_PRIMITIVES
: 允许基本类型的值为null。
配置方式为
objectMapper.configure(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES, false);
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FAIL_ON_NUMBERS_FOR_ENUM
:控制枚举值是否被允许序列化/反序列化为数字
配置方式为:
objectMapper.configure(DeserializationFeature.FAIL_ON_NUMBERS_FOR_ENUMS, false)
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更多的配置方式详见https://github.com/FasterXML/jackson-databind/wiki/Serialization-Features
3.4 处理集合
DeserializationFeature类提供的另一个小而有用的功能是能够从JSON数组响应生成我们想要的集合类型的功能。
例如,我们可以将结果生成为数组:
String jsonCarArray =
"[{ \"color\" : \"Black\", \"type\" : \"BMW\" }, { \"color\" : \"Red\", \"type\" : \"FIAT\" }]";
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.configure(DeserializationFeature.USE_JAVA_ARRAY_FOR_JSON_ARRAY, true);
Car[] cars = objectMapper.readValue(jsonCarArray, Car[].class);
// Car(color=Black, type=BMW)
//Car(color=Red, type=FIAT)
for (Car car : cars) {
System.out.println(car);
}
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Or as a List:
String jsonCarArray =
"[{ \"color\" : \"Black\", \"type\" : \"BMW\" }, { \"color\" : \"Red\", \"type\" : \"FIAT\" }]";
List<Car> listCar = objectMapper.readValue(jsonCarArray, new TypeReference<List<Car>>(){});
// [Car(color=Black, type=BMW), Car(color=Red, type=FIAT)]
System.out.println(listCar);
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处理集合的更多资料详见https://www.baeldung.com/jackson-collection-array