Jackson ObjectMapper總結

ObjectMapper

1. 依賴

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.8</version>
</dependency>
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該依賴同時會將以下庫添加到項目路徑中：

• 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對象序列化爲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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輸出結果 car.json:

{"color":"yellow","type":"renault"}
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writeValueAsString: 將生成的JSON轉化爲字符串

//{"color":"yellow","type":"renault"}
String carAsString = objectMapper.writeValueAsString(car);
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writeValueAsBytes: 將生成的JSON轉化爲字節數組

2.2 JSON反序列化爲Java對象

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);
    }
}

//輸出結果:Car(color=Black, type=BMW)
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readValue（）函數還接受其餘形式的輸入，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);

//輸出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 反序列化爲Jackson JsonNode

一樣的，JSON能夠被解析爲JsonNode對象，從某一個具體的節點獲取數據。

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數組反序列化爲Java List

經過使用TypeReference能夠將數組形式的JSON反序列化爲Java 數組

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字符串反序列化爲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. 高級特性

Jackson庫的一個強大之處在於能夠對序列化、反序列化進行定製。

3.1 設置序列化、反序列化特性

默認狀況下，當JSON字符串包含Java類沒有的屬性時，反序列化就會失敗。

String jsonString
            = "{ \"color\" : \"Black\", \"type\" : \"Fiat\", \"year\" : \"1970\" }";

//UnrecognizedPropertyException
Car car = objectMapper.readValue(jsonString, Car.class);
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經過設置方法咱們能夠改變默認的行爲使其忽略新的字段屬性。

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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相似的 FAIL_ON_NULL_FOR_PRIMITIVES: 容許基本類型的值爲null。

配置方式爲

objectMapper.configure(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES, false);
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FAIL_ON_NUMBERS_FOR_ENUM:控制枚舉值是否被容許序列化/反序列化爲數字

配置方式爲:

objectMapper.configure(DeserializationFeature.FAIL_ON_NUMBERS_FOR_ENUMS, false)
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更多的配置方式詳見https://github.com/FasterXML/jackson-databind/wiki/Serialization-Features

3.4 處理集合

DeserializationFeature類提供的另外一個小而有用的功能是可以從JSON數組響應生成咱們想要的集合類型的功能。

例如，咱們能夠將結果生成爲數組：

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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處理集合的更多資料詳見https://www.baeldung.com/jackson-collection-array

4. 參考文獻

• www.baeldung.com/jackson-obj…