JSON解析-Jackson

1.Jackson簡介

參考連結：

簡介：Jackson是一個簡潔的方式去解析JSON開源包。Jackson可以解析JSON從String，Stream，或者file的方式去建立Java對象。Jackson不僅僅可以解析JSON到Java對象，也可以将Java對象解析為JSON字元串。
原理：Java反射機制實作JSON和Java對象間資料自動轉換。

2.Jackson下載下傳安裝：maven依賴方式添加

<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對象映射

從JSON字元串轉換為Java對象

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

從JSON Reader轉換為Java對象

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

從File 檔案中轉換為Java對象

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

從URL（網絡或本地資源）資源轉換為Java對象

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

從InputStream 輸入流中轉換為Java對象

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

從Byte Array 中轉換為Java對象

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

從JSON字元數組中轉換為Java對象數組（Array）

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

從JSON字元數組中轉換為Java對象清單（List）

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

從JSON字元串中轉換為Java的Map存儲

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字元串中Java對象中沒有的字段設定

objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

原始類型的空JSON值失敗

int 值為null問題。

自定義反序列化器

1.應用
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.定義
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;
    }
}

從Java對象輸出JSON

可以将Java對象輸出到，JSON字元串，Byte數組，File外部檔案中，OutPutStream輸出流中
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);

自定義序列化器

1.應用
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.定義
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.預設Jackson 日期是以1970為開始的int類型的時間值。
2.可以通過
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
objectMapper2.setDateFormat(dateFormat);設定具體Date日期的格式。

Jackson JSON的樹模型

1.讀取JSON作為Jackson JSON樹形式
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.完整執行個體
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 還可以讀取COBR，MessagePack，YAML類型的格式化資料

具體操作流程請查閱資料，另需要對用jar包支援。

4.Jackson 的JsonNode對象（樹模型）

Read JsonNode From JSON從JSON中讀取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寫JsonNode到JSON中

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

Get JsonNode Field取得JsonNode的成員和成員值

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

//取得成員值
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生産器

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讀和寫注解

@JsonIgnore：忽略屬性
import com.fasterxml.jackson.annotation.JsonIgnore;
public class PersonIgnore {
   @JsonIgnore
    public long    personId = 0;
    public String  name = null;}
@JsonIgnoreProperties：忽略屬性組
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：忽略屬性的類型
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自動檢查
import com.fasterxml.jackson.annotation.JsonAutoDetect;
@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY )
public class PersonAutoDetect {
    private long  personId = 123;
    public String name     = null;
}

Read Annotations讀注解

@JsonSetter：設定自定義名稱
@JsonAnySetter
@JsonCreator
@JacksonInject
@JsonDeserialize

Write Annotations寫注解

@JsonGetter：自定義寫值
@JsonInclude
@JsonAnyGetter
@JsonPropertyOrder
@JsonRawValue
@JsonValue
@JsonSerialize

JSON解析-Jackson

1.Jackson簡介

2.Jackson下載下傳安裝：maven依賴方式添加

3.Jackson ObjectMapper Jackson對象映射

4.Jackson 的JsonNode對象（樹模型）

5.Jackson JsonParser json解析器

6.Jackson JsonParser json生産器

6.Jackson Annotations Jackson 注解

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