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