ObjectMapper常见用法（Intro to the Jackson ObjectMapper）

柔光的暖阳◎ 2024-03-23 17:53 53阅读 0赞

#### 目录 ####

*  maven依赖
 *  Java Object to JSON
 *  JSON to Java Object
 *  JSON to Jackson JsonNode
 *  Creating a Java List From a JSON Array String
 *  Creating Java Map From JSON String
 *  Configuring Serialization or Deserialization Feature
 *  Creating Custom Serializer or Deserializer
 *  Handling Date Formats

--------------------

## maven依赖 ##

<dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-dataformat-xml</artifactId>
        <version>2.14.2</version>
    </dependency>

--------------------

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Car {
        
        private String color;
        private String type;
    }

## Java Object to JSON ##

Car car = new Car("yellow", "renault");
    
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.writeValue(new File("D:/output/car.json"), car);
    
    以下内容输出到文件
    // {"color":"yellow","type":"renault"}

Car car = new Car("yellow", "renault");
    
    ObjectMapper objectMapper = new ObjectMapper();
    String carAsString = objectMapper.writeValueAsString(car);
    
    // {"color":"yellow","type":"renault"}

## JSON to Java Object ##

ObjectMapper objectMapper = new ObjectMapper();
    Car car = objectMapper.readValue(new URL("file:D:/output/car.json"), Car.class);
    
    System.out.println(car); 
    // Car(color=Black, type=BMW)

ObjectMapper objectMapper = new ObjectMapper();
    Car car = objectMapper.readValue(new File("D:/output/car.json"), Car.class);
    
    System.out.println(car); 
    // Car(color=Black, type=BMW)

String json = "{ \"color\" : \"Black\", \"type\" : \"BMW\" }";
    
    ObjectMapper objectMapper = new ObjectMapper();
    Car car = objectMapper.readValue(json, Car.class);
    
    System.out.println(car); // Car(color=Black, type=BMW)

## JSON to Jackson JsonNode ##

String json = "{ \"color\" : \"Black\", \"type\" : \"FIAT\" }";
    
    ObjectMapper objectMapper = new ObjectMapper();
    JsonNode jsonNode = objectMapper.readTree(json);
    
    String color = jsonNode.get("color").asText();
    System.out.println(color); // Black

## Creating a Java List From a JSON Array String ##

String jsonCarArray = "[{ \"color\" : \"Black\", \"type\" : \"BMW\" }, { \"color\" : \"Red\", \"type\" : \"FIAT\" }]";
    
    ObjectMapper objectMapper = new ObjectMapper();
    List<Car> listCar = objectMapper.readValue(jsonCarArray, new TypeReference<List<Car>>() {
        });
    
    System.out.println(listCar); 
    // [Car(color=Black, type=BMW), Car(color=Red, type=FIAT)]

## Creating Java Map From JSON String ##

String json = "{ \"color\" : \"Black\", \"type\" : \"BMW\" }";
    
    ObjectMapper objectMapper = new ObjectMapper();
    Map<String, Object> map = objectMapper.readValue(json, new TypeReference<Map<String, Object>>() {
        });
    
    System.out.println(map); 
    // {color=Black, type=BMW}

## Configuring Serialization or Deserialization Feature ##

String jsonString = "{ \"color\" : \"Black\", \"type\" : \"Fiat\", \"year\" : \"1970\" }";
    
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    Car car = objectMapper.readValue(jsonString, Car.class);
    System.out.println(car); // Car(color=Black, type=Fiat)
    
    JsonNode jsonNodeRoot = objectMapper.readTree(jsonString);
    JsonNode jsonNodeYear = jsonNodeRoot.get("year");
    String year = jsonNodeYear.asText();
    System.out.println(year); // 1970
    
    objectMapper.configure(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES, false);
    objectMapper.configure(DeserializationFeature.FAIL_ON_NUMBERS_FOR_ENUMS, false);

## Creating Custom Serializer or Deserializer ##

public class CustomCarSerializer extends StdSerializer<Car> {
        
    
        public CustomCarSerializer() {
        
            this(null);
        }
    
        public CustomCarSerializer(Class<Car> t) {
        
            super(t);
        }
    
        @Override
        public void serialize(Car car, JsonGenerator jsonGenerator, SerializerProvider serializer) throws IOException {
        
            jsonGenerator.writeStartObject();
            jsonGenerator.writeStringField("car_brand", car.getType());
            jsonGenerator.writeEndObject();
        }
    }

Car car = new Car("yellow", "renault");
    
    ObjectMapper mapper = new ObjectMapper();
    SimpleModule module = new SimpleModule("CustomCarSerializer", new Version(1, 0, 0, null, null, null));
    module.addSerializer(Car.class, new CustomCarSerializer());
    mapper.registerModule(module);
    
    String carJson = mapper.writeValueAsString(car);
    System.out.println(carJson); 
    // {"car_brand":"renault"}

public class CustomCarDeserializer extends StdDeserializer<Car> {
        
    
        public CustomCarDeserializer() {
        
            this(null);
        }
    
        public CustomCarDeserializer(Class<?> vc) {
        
            super(vc);
        }
    
        @Override
        public Car deserialize(JsonParser parser, DeserializationContext deserializer) throws IOException {
        
            Car car = new Car();
            ObjectCodec codec = parser.getCodec();
            JsonNode node = codec.readTree(parser);
            JsonNode colorNode = node.get("color");
            String color = colorNode.asText();
            car.setColor(color);
            return car;
        }
    }

String json = "{ \"color\" : \"Black\", \"type\" : \"BMW\" }";
    
    ObjectMapper mapper = new ObjectMapper();
    SimpleModule module = new SimpleModule("CustomCarDeserializer", new Version(1, 0, 0, null, null, null));
    module.addDeserializer(Car.class, new CustomCarDeserializer());
    mapper.registerModule(module);
    
    Car car = mapper.readValue(json, Car.class);
    System.out.println(car); // Car(color=Black, type=null)

--------------------

## Handling Date Formats ##