JSON解析-Jackson

1.Jackson简介

参考链接：link

简介：Jackson是一个简洁的方式去解析JSON开源包。Jackson可以解析JSON从String，Stream，或者file的方式去创建Java对象。Jackson不仅仅可以解析JSON到Java对象，也可以将Java对象解析为JSON字符串。
原理：Java反射机制实现JSON和Java对象间数据自动转换。

2.Jackson下载安装：maven依赖方式添加

<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-core</artifactId>
  <version>2.9.6</version>
</dependency>

<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-annotations</artifactId>
  <version>2.9.6</version>
</dependency>

<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>2.9.6</version>
</dependency>

3.Jackson ObjectMapper Jackson对象映射

从JSON字符串转换为Java对象

ObjectMapper objectMapper = new ObjectMapper();
String carJson ="{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";
Car car = objectMapper.readValue(carJson, Car.class);

从JSON Reader转换为Java对象

ObjectMapper objectMapper = new ObjectMapper();
String carJson = "{ \"brand\" : \"Mercedes\", \"doors\" : 4 }";
Reader reader = new StringReader(carJson);
Car car = objectMapper.readValue(reader, Car.class);

从File 文件中转换为Java对象

ObjectMapper objectMapper = new ObjectMapper();
File file = new File("data/car.json");
Car car = objectMapper.readValue(file, Car.class);

从URL（网络或本地资源） 资源转换为Java对象

ObjectMapper objectMapper = new ObjectMapper();
URL url = new URL("file:data/car.json");
Car car = objectMapper.readValue(url, Car.class);

从InputStream 输入流中转换为Java对象

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

从Byte Array 中转换为Java对象

ObjectMapper objectMapper = new ObjectMapper();
String carJson = "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";
byte[] bytes = carJson.getBytes("UTF-8");
Car car = objectMapper.readValue(bytes, Car.class);

从JSON字符数组中转换为Java对象数组（Array）

String jsonArray = "[{\"brand\":\"ford\"}, {\"brand\":\"Fiat\"}]";
ObjectMapper objectMapper = new ObjectMapper();
Car[] cars2 = objectMapper.readValue(jsonArray, Car[].class);

从JSON字符数组中转换为Java对象列表（List）

String jsonArray = "[{\"brand\":\"ford\"}, {\"brand\":\"Fiat\"}]";
ObjectMapper objectMapper = new ObjectMapper();
List<Car> cars1 = objectMapper.readValue(jsonArray, new TypeReference<List<Car>>(){});

从JSON字符串中转换为Java的Map存储

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

忽略解析JSON字符串中Java对象中没有的字段设置

objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

原始类型的空JSON值失败

int 值为null问题。

自定义反序列化器

1.应用
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);
2.定义
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;

import java.io.IOException;

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;
    }
}

从Java对象输出JSON

可以将Java对象输出到，JSON字符串，Byte数组，File外部文件中，OutPutStream输出流中
ObjectMapper objectMapper = new ObjectMapper();
Car car = new Car();
car.brand = "BMW";
car.doors = 4;

objectMapper.writeValue(
    new FileOutputStream("data/output-2.json"), car);
String json = objectMapper.writeValueAsString(car);
System.out.println(json);
String json = objectMapper.writeValueAsBytes(car);

自定义序列化器

1.应用
CarSerializer carSerializer = new CarSerializer(Car.class);
ObjectMapper objectMapper = new ObjectMapper();

SimpleModule module =
        new SimpleModule("CarSerializer", new Version(2, 1, 3, null, null, null));
module.addSerializer(Car.class, carSerializer);

objectMapper.registerModule(module);

Car car = new Car();
car.setBrand("Mercedes");
car.setDoors(5);

String carJson = objectMapper.writeValueAsString(car);
2.定义
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;

import java.io.IOException;

public class CarSerializer extends StdSerializer<Car> {

    protected CarSerializer(Class<Car> t) {
        super(t);
    }

    public void serialize(Car car, JsonGenerator jsonGenerator,
                          SerializerProvider serializerProvider)
            throws IOException {

        jsonGenerator.writeStartObject();
        jsonGenerator.writeStringField("producer", car.getBrand());
        jsonGenerator.writeNumberField("doorCount", car.getDoors());
        jsonGenerator.writeEndObject();
    }
}

Jackson Date 日期格式化

1.默认Jackson 日期是以1970为开始的int类型的时间值。
2.可以通过
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
objectMapper2.setDateFormat(dateFormat);设置具体Date日期的格式。

Jackson JSON的树模型

1.读取JSON作为Jackson JSON树形式
String carJson =
        "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";
ObjectMapper objectMapper = new ObjectMapper();
try {
    JsonNode jsonNode = objectMapper.readValue(carJson, JsonNode.class);
} catch (IOException e) {    e.printStackTrace();
}
String carJson =
        "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";
ObjectMapper objectMapper = new ObjectMapper();
try {
    JsonNode jsonNode = objectMapper.readTree(carJson);
} catch (IOException e) {
    e.printStackTrace();
}
2.完整实例
String carJson =
        "{ \"brand\" : \"Mercedes\", \"doors\" : 5," +
        "  \"owners\" : [\"John\", \"Jack\", \"Jill\"]," +
        "  \"nestedObject\" : { \"field\" : \"value\" } }";

ObjectMapper objectMapper = new ObjectMapper();


try {

    JsonNode jsonNode = objectMapper.readValue(carJson, JsonNode.class);

    JsonNode brandNode = jsonNode.get("brand");
    String brand = brandNode.asText();
    System.out.println("brand = " + brand);

    JsonNode doorsNode = jsonNode.get("doors");
    int doors = doorsNode.asInt();
    System.out.println("doors = " + doors);

    JsonNode array = jsonNode.get("owners");
    JsonNode jsonNode = array.get(0);
    String john = jsonNode.asText();
    System.out.println("john  = " + john);

    JsonNode child = jsonNode.get("nestedObject");
    JsonNode childField = child.get("field");
    String field = childField.asText();
    System.out.println("field = " + field);

} catch (IOException e) {
    e.printStackTrace();
}

Jackson 还可以读取COBR，MessagePack，YAML类型的格式化数据

具体操作流程请查阅资料，另需要对用jar包支持。

4.Jackson 的JsonNode对象（树模型）

Read JsonNode From JSON从JSON中读取JsonNode

String json = "{ \"f1\" : \"v1\" } ";
ObjectMapper objectMapper = new ObjectMapper();
JsonNode jsonNode = objectMapper.readTree(json);
System.out.println(jsonNode.get("f1").asText());

Write JsonNode to JSON写JsonNode到JSON中

ObjectMapper objectMapper = new ObjectMapper();
JsonNode jsonNode = readJsonIntoJsonNode();
String json = objectMapper.writeValueAsString(jsonNode);

Get JsonNode Field取得JsonNode的成员和成员值

//取得成员
JsonNode field1 = jsonNode.get("field1");

//取得成员值
String f2Str = jsonNode.get("f2").asText();
double f2Dbl = jsonNode.get("f2").asDouble();
int    f2Int = jsonNode.get("f2").asInt();
long   f2Lng = jsonNode.get("f2").asLong();

5.Jackson JsonParser json解析器

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

JsonFactory factory = new JsonFactory();
JsonParser  parser  = factory.createParser(carJson);

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

System.out.println("car.brand = " + car.brand);
System.out.println("car.doors = " + car.doors);

6.Jackson JsonParser json生产器

JsonFactory factory = new JsonFactory();

JsonGenerator generator = factory.createGenerator(
    new File("data/output.json"), JsonEncoding.UTF8);

generator.writeStartObject();
generator.writeStringField("brand", "Mercedes");
generator.writeNumberField("doors", 5);
generator.writeEndObject();

generator.close();

6.Jackson Annotations Jackson 注解

Read + Write Annotations读和写注解

@JsonIgnore：忽略属性
import com.fasterxml.jackson.annotation.JsonIgnore;
public class PersonIgnore {
   @JsonIgnore
    public long    personId = 0;
    public String  name = null;}
@JsonIgnoreProperties：忽略属性组
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
@JsonIgnoreProperties({"firstName", "lastName"})
public class PersonIgnoreProperties {
    public long    personId = 0;
    public String  firstName = null;
    public String  lastName  = null;}
@JsonIgnoreType：忽略属性的类型
import com.fasterxml.jackson.annotation.JsonIgnoreType;
public class PersonIgnoreType {
    @JsonIgnoreType
    public static class Address {
        public String streetName  = null;
        public String houseNumber = null;
        public String zipCode     = null;
        public String city        = null;
        public String country     = null;
    }
    public long    personId = 0;
    public String  name = null;
    public Address address = null;
}
@JsonAutoDetect：json自动检查
import com.fasterxml.jackson.annotation.JsonAutoDetect;
@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY )
public class PersonAutoDetect {
    private long  personId = 123;
    public String name     = null;
}

Read Annotations读注解

@JsonSetter：设置自定义名称
@JsonAnySetter
@JsonCreator
@JacksonInject
@JsonDeserialize

Write Annotations写注解

@JsonGetter：自定义写值
@JsonInclude
@JsonAnyGetter
@JsonPropertyOrder
@JsonRawValue
@JsonValue
@JsonSerialize