枚举类使用Jackson进行序列化与反序列化（How To Serialize and Deserialize Enums with Jackson）

逃离我推掉我的手 2024-03-23 00:19 85阅读 0赞

Let’s define the following Enum:

public enum Distance {
        
        KILOMETER("km", 1000), 
        MILE("miles", 1609.34),
        METER("meters", 1), 
        INCH("inches", 0.0254),
        CENTIMETER("cm", 0.01), 
        MILLIMETER("mm", 0.001);
    
        private String unit;
        private final double meters;
    
        private Distance(String unit, double meters) {
        
            this.unit = unit;
            this.meters = meters;
        }
    
        // standard getters and setters
    }

## Serializing Enums to JSON ##

### Default Enum Representation ###

import com.fasterxml.jackson.core.JsonProcessingException;
    import com.fasterxml.jackson.databind.ObjectMapper;
    
    public enum Distance {
         ... }
    
    new ObjectMapper().writeValueAsString(Distance.MILE); 
    
    结果
    "MILE"

### Enum as a JSON Object ###

import com.fasterxml.jackson.annotation.JsonFormat;
    
    @JsonFormat(shape = JsonFormat.Shape.OBJECT)
    public enum Distance {
         ... }
    
    new ObjectMapper().writeValueAsString(Distance.MILE); 
    
    执行结果
    {
        "unit":"miles","meters":1609.34}

### Enums and @JsonValue ###

@JsonFormat(shape = JsonFormat.Shape.OBJECT)
    public enum Distance {
         
        ...
        @JsonValue
        public String getMeters() {
        
            return meters;
        }
    }
    
    new ObjectMapper().writeValueAsString(Distance.MILE); 
    
    执行结果
    1609.34

发现：@JsonValue 优先级高于 @JsonFormat(shape = JsonFormat.Shape.OBJECT)

### Custom Serializer for Enum ###

version of Jackson earlier than 2.1.2, or if even more customization is required for the enum, we can use a custom Jackson serializer.

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 DistanceSerializer extends StdSerializer<Distance> {
        
    
        public DistanceSerializer() {
        
            super(Distance.class);
        }
    
        @Override
        public void serialize(Distance distance, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
        
            jsonGenerator.writeStartObject();
            jsonGenerator.writeFieldName("name");
            jsonGenerator.writeString(distance.name());
            jsonGenerator.writeFieldName("unit");
            jsonGenerator.writeString(distance.getUnit());
            jsonGenerator.writeFieldName("meters");
            jsonGenerator.writeNumber(distance.getMeters());
            jsonGenerator.writeEndObject();
        }
    }

@JsonSerialize(using = DistanceSerializer.class)
    public enum Distance {
         ... }

## Deserializing JSON to Enum ##

### Default Behavior ###

public enum Distance {
         ... }

@Data
    public class City {
        
        private Distance distance;
    }

City city = new City().setDistance(Distance.KILOMETER);
     String s = new ObjectMapper().writeValueAsString(city);
     System.out.println(s); // {"distance":"KILOMETER"}
     
     String json = "{\"distance\":\"KILOMETER\"}";
     City city1 = new ObjectMapper().readValue(json, City.class);
     System.out.println(city1.toString()); // City(distance=KILOMETER)

**do a case-insensitive deserialization**

import com.fasterxml.jackson.databind.MapperFeature;
    import com.fasterxml.jackson.databind.ObjectMapper;
    import com.fasterxml.jackson.databind.json.JsonMapper;
    O
    bjectMapper objectMapper = JsonMapper.builder()
            .enable(MapperFeature.ACCEPT_CASE_INSENSITIVE_ENUMS)
            .build();
            
    String json1 = "{\"distance\":\"KiLoMeTeR\"}";
    
    City city2 = objectMapper.readValue(json1, City.class);
    System.out.println(city2.toString()); // City(distance=KILOMETER)

### Using @JsonValue ###

public enum Distance {
        
        ...
        @JsonValue
        public double getMeters() {
        
            return meters;
        }
    }

@Data
    public class City {
        
        private Distance distance;
    }

import com.fasterxml.jackson.annotation.JsonValue;
    
    String json3 = "{\"distance\":\"0.0254\"}";
    City city3= new ObjectMapper().readValue(json3, City.class);
    System.out.println(city3.toString()); // City(distance=INCH)

### Using @JsonProperty ###

public enum Distance {
        
        @JsonProperty("distance-in-km")
        KILOMETER("km", 1000), 
        @JsonProperty("distance-in-miles")
        MILE("miles", 1609.34);
        ...
    }

@Data
    public class City {
        
        private Distance distance;
    }

import com.fasterxml.jackson.annotation.JsonProperty;
    
    String json = "{\"distance\":\"distance-in-km\"}";
    City city1 = new ObjectMapper().readValue(json, City.class);
    System.out.println(city1.toString()); // City(distance=KILOMETER)

### Using @JsonCreator ###

import com.fasterxml.jackson.annotation.JsonCreator;
    import com.fasterxml.jackson.annotation.JsonProperty;
    
    public enum Distance {
        
    
        @JsonCreator
        public static Distance forValues(@JsonProperty("unit") String unit, @JsonProperty("meters") double meters) {
        
            for (Distance distance : Distance.values()) {
        
                if (distance.unit.equals(unit) && Double.compare(distance.meters, meters) == 0) {
        
                    return distance;
                }
            }
            return null;
        }
    
        ...
    }

String json = "{\n" +
            "    \"distance\": {\n" +
            "        \"unit\":\"miles\", \n" +
            "        \"meters\":1609.34\n" +
            "    }\n" +
            "}";
            
    String json1 = "{" + "\"distance\": {" + "\"unit\":\"miles\", " + "\"meters\":1609.34" + "}" + "}";
    
    City city1 = new ObjectMapper().readValue(json, City.class);
    System.out.println(city1.toString()); // City(distance=MILE)
    
    City city2 = new ObjectMapper().readValue(json1, City.class);
    System.out.println(city2.toString()); // City(distance=MILE)

### Using a Custom Deserializer ###

public class CustomEnumDeserializer extends StdDeserializer<Distance> {
        
    
        @Override
        public Distance deserialize(JsonParser jsonParser, DeserializationContext ctxt)
          throws IOException, JsonProcessingException {
        
            JsonNode node = jsonParser.getCodec().readTree(jsonParser);
    
            String unit = node.get("unit").asText();
            double meters = node.get("meters").asDouble();
    
            for (Distance distance : Distance.values()) {
        
               
                if (distance.getUnit().equals(unit) && Double.compare(
                  distance.getMeters(), meters) == 0) {
        
                    return distance;
                }
            }
    
            return null;
        }
    }

@JsonDeserialize(using = CustomEnumDeserializer.class)
    public enum Distance {
        
       ...
    }

\-----------------------------------------------------------------------------读书笔记摘自 文章：[How To Serialize and Deserialize Enums with Jackson][]

[How To Serialize and Deserialize Enums with Jackson]: https://www.baeldung.com/jackson-serialize-enums