ObjectMapper

Jackson ObjectMapper类（com.fasterxml.jackson.databind.ObjectMapper）是使用Jackson解析JSON的最简单方法。Jackson ObjectMapper可以从字符串，流或文件中解析JSON，并创建表示已解析的JSON的Java对象或对象图。将JSON解析为Java对象也称为从JSON反序列化Java对象。Jackson ObjectMapper也可以从Java对象创建JSON。从Java对象生成JSON也称为将Java对象序列化为JSON。Jackson Object映射器可以将JSON解析为您开发的类的对象，也可以解析为本教程稍后说明的内置JSON树模型的对象。顺便说一下，之所以称为ObjectMapper是因为它将JSON映射到Java对象（反序列化），或者将Java对象映射到JSON（序列化）。

public class Car {
    private String brand = null;
    private int doors = 0;
}

ObjectMapper objectMapper = new ObjectMapper();

String carJson =
    "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";

try {
    Car car = objectMapper.readValue(carJson, Car.class);

    System.out.println("car brand = " + car.getBrand());
    System.out.println("car doors = " + car.getDoors());
} catch (IOException e) {
    e.printStackTrace();
}

Car.class是我们自定义的类，其作为第二个参数传入到readValue函数中。为了使用Jackson正确地从JSON读取Java对象，重要的是要知道Jackson如何将JSON对象的字段映射到Java对象的字段，因此我将解释Jackson是如何做到的。默认情况下，Jackson通过将JSON字段的名称与Java对象中的getter和setter方法进行匹配，将JSON对象的字段映射到Java对象中的字段。Jackson删除了getter和setter方法名称的“ get”和“ set”部分，并将其余名称的第一个字符转换为小写。

例如，名为brand的JSON字段与名为getBrand()和setBrand()的Java getter和setter方法匹配。名为engineNumber的JSON字段将与名为getEngineNumber()和setEngineNumber()的getter和setter匹配。如果需要以其他方式将JSON对象字段与Java对象字段匹配，则需要使用自定义序列化器和反序列化器，或者使用许多Jackson注释中的一些。

APIs

读取为对象

ObjectMapper objectMapper = new ObjectMapper();
String carJson =
    "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";

// 来自于 JSON 字符串
Car car = objectMapper.readValue(carJson, Car.class);

// 来自于 JSON 数组
String jsonArray = "[{\"brand\":\"ford\"}, {\"brand\":\"Fiat\"}]";

Car[] cars2 = objectMapper.readValue(jsonArray, Car[].class);
List<Car> cars1 = objectMapper.readValue(jsonArray, new TypeReference<List<Car>>(){});

// 从 JSON Byte Array 读取
byte[] bytes = carJson.getBytes("UTF-8");
Car car = objectMapper.readValue(bytes, Car.class);

// 读取为 Map
String jsonObject = "{\"brand\":\"ford\", \"doors\":5}";
Map<String, Object> jsonMap = objectMapper.readValue(jsonObject,
    new TypeReference<Map<String,Object>>(){});

// 来自于 StringReader
Reader reader = new StringReader(carJson);
Car car = objectMapper.readValue(reader, Car.class);

// 来自于 JSON 文件
File file = new File("data/car.json");
Car car = objectMapper.readValue(file, Car.class);

// 从 URL 读取
InputStream input = new FileInputStream("data/car.json");
Car car = objectMapper.readValue(input, Car.class);

我们可以设置在读取到Null的原始类型时抛出异常：

objectMapper.configure(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES, true);
String carJson = "{ \"brand\":\"Toyota\", \"doors\":null }";
Car car = objectMapper.readValue(carJson, Car.class);

Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException:
    Cannot map `null` into type int
    (set DeserializationConfig.DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES to 'false' to allow)
 at [Source: (String)
    "{ "brand":"Toyota", "doors":null }"; line: 1, column: 29] (through reference chain: jackson.Car["doors"])

最后，我们也可以注册自定义的解释器：

public class CarDeserializer extends StdDeserializer<Car> {

    public CarDeserializer(Class<?> vc) {
        super(vc);
    }

    @Override
    public Car deserialize(JsonParser parser, DeserializationContext deserializer) throws IOException {
        Car car = new Car();
        while(!parser.isClosed()){
            JsonToken jsonToken = parser.nextToken();

            if(JsonToken.FIELD_NAME.equals(jsonToken)){
                String fieldName = parser.getCurrentName();
                System.out.println(fieldName);

                jsonToken = parser.nextToken();

                if("brand".equals(fieldName)){
                    car.setBrand(parser.getValueAsString());
                } else if ("doors".equals(fieldName)){
                    car.setDoors(parser.getValueAsInt());
                }
            }
        }
        return car;
    }
}

String json = "{ \"brand\" : \"Ford\", \"doors\" : 6 }";

SimpleModule module =
        new SimpleModule("CarDeserializer", new Version(3, 1, 8, null, null, null));
module.addDeserializer(Car.class, new CarDeserializer(Car.class));

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);

Car car = mapper.readValue(json, Car.class);

序列化为字符串

ObjectMapper objectMapper = new ObjectMapper();

Car car = new Car();
car.brand = "BMW";
car.doors = 4;

objectMapper.writeValue(
    new FileOutputStream("data/output-2.json"), car);

ObjectMapper objectMapper = new ObjectMapper();

Car car = new Car();
car.brand = "BMW";
car.doors = 4;

String json = objectMapper.writeValueAsString(car);
System.out.println(json);

Annotations

Jackson包含一组Java批注，您可以使用这些批注来修改Jackson到Java对象之间读写JSON的方式。Jackson批注@JsonIgnore用于告诉Jackson忽略Java对象的某个属性（字段）。在将JSON读取到Java对象中以及将Java对象写入JSON时，都将忽略该属性。这是使用@JsonIgnore批注的示例类：

import com.fasterxml.jackson.annotation.JsonIgnore;

public class PersonIgnore {

    @JsonIgnore
    public long    personId = 0;

    public String  name = null;
}

最近更新于0001-01-01