多层次的类如何使用Jackson序列化/反序列化（Inheritance with Jackson）

Dear 丶 2024-03-23 18:27 59阅读 0赞

Have a look at working with class hierarchies in Jackson.  
Jackson如何处理类中类

--------------------

## Inclusion of Subtype Information ##

There are two ways to add type information when serializing and deserializing data objects, namely `global default typing` and `per-class annotations`.

### Global Default Typing ###

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public abstract class Vehicle {
        
        private String make;
        private String model;
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Car extends Vehicle {
        
        private int seatingCapacity;
        private double topSpeed;
    
        public Car(String make, String model, int seatingCapacity, double topSpeed) {
        
            super(make, model);
            this.seatingCapacity = seatingCapacity;
            this.topSpeed = topSpeed;
        }
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Truck extends Vehicle {
        
        private double payloadCapacity;
        public Truck(String make, String model, double payloadCapacity) {
        
            super(make, model);
            this.payloadCapacity = payloadCapacity;
        }
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Fleet {
        
        private List<Vehicle> vehicles;
    }

Car car = new Car("Mercedes-Benz", "S500", 5, 250.0);
    Truck truck = new Truck("Isuzu", "NQR", 7500.0);
    List<Vehicle> vehicles = new ArrayList<>();
    vehicles.add(car);
    vehicles.add(truck);
    Fleet serializedFleet = new Fleet();
    serializedFleet.setVehicles(vehicles);
    
    安全白名单
    PolymorphicTypeValidator ptv = BasicPolymorphicTypeValidator.builder()
            .allowIfSubType("javabasic.enumprac")
            .allowIfSubType("java.util.ArrayList")
            .build();        
            
    ObjectMapper mapper = new ObjectMapper();
    mapper.activateDefaultTyping(ptv, ObjectMapper.DefaultTyping.NON_FINAL);
    String jsonDataString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(serializedFleet);
    
    System.out.println(jsonDataString);

[ "javabasic.enumprac.Fleet", {
        
      "vehicles" : [ "java.util.ArrayList", [ [ "javabasic.enumprac.Car", {
        
        "make" : "Mercedes-Benz",
        "model" : "S500",
        "seatingCapacity" : 5,
        "topSpeed" : 250.0
      } ], [ "javabasic.enumprac.Truck", {
        
        "make" : "Isuzu",
        "model" : "NQR",
        "payloadCapacity" : 7500.0
      } ] ] ]
    } ]

String JSON= "[ \"javabasic.enumprac.Fleet\", {\n" +
            "  \"vehicles\" : [ \"java.util.ArrayList\", [ [ \"javabasic.enumprac.Car\", {\n" +
            "    \"make\" : \"Mercedes-Benz\",\n" +
            "    \"model\" : \"S500\",\n" +
            "    \"seatingCapacity\" : 5,\n" +
            "    \"topSpeed\" : 250.0\n" +
            "  } ], [ \"javabasic.enumprac.Truck\", {\n" +
            "    \"make\" : \"Isuzu\",\n" +
            "    \"model\" : \"NQR\",\n" +
            "    \"payloadCapacity\" : 7500.0\n" +
            "  } ] ] ]\n" +
            "} ]";
            
    Fleet deserializedFleet = mapper.readValue(JSON, Fleet.class);
    System.out.println(deserializedFleet);
    // Fleet(vehicles=[Car(seatingCapacity=5, topSpeed=250.0), Truck(payloadCapacity=7500.0)])

--------------------

### Per-Class Annotations ###

@JsonTypeInfo(
            use = JsonTypeInfo.Id.NAME,
            include = JsonTypeInfo.As.PROPERTY,
            property = "type")
    @JsonSubTypes({
        
            @JsonSubTypes.Type(value = Car.class, name = "car"),
            @JsonSubTypes.Type(value = Truck.class, name = "truck")
    })
    @Data
    @AllArgsConstructor
    @NoArgsConstructor
    public abstract class Vehicle {
        
        private String make;
        private String model;
    }

{
        
      "vehicles" : [ {
        
        "type" : "car",
        "make" : "Mercedes-Benz",
        "model" : "S500",
        "seatingCapacity" : 5,
        "topSpeed" : 250.0
      }, {
        
        "type" : "truck",
        "make" : "Isuzu",
        "model" : "NQR",
        "payloadCapacity" : 7500.0
      } ]
    }

Fleet deserializedFleet = mapper.readValue(jsonDataString, Fleet.class);
    System.out.println(deserializedFleet);
    // Fleet(vehicles=[Car(seatingCapacity=5, topSpeed=250.0), Truck(payloadCapacity=7500.0)])

--------------------

## Ignoring Properties from a Supertype ##

Sometimes, some properties inherited from superclasses need to be ignored during serialization or deserialization. This can be achieved by one of three methods: `annotations`, `mix-ins` and `annotation introspection`.

### Annotations ###

有两种常用的Jackson注释来忽略属性，它们是`@JsonIgnore`和`@JsonIgnoreProperties`。

`@JsonIgnore` directly applied to type members.  
`@JsonIgnoreProperties` applied to type and type member.

`@JsonIgnoreProperties` is more powerful  
1、it can ignore properties inherited from supertypes that we do not have control of.  
2、it can ignore many properties at once.

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public abstract class Vehicle {
        
        private String make;
        private String model;
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    @JsonIgnoreProperties({
         "model", "seatingCapacity" })
    public class Car extends Vehicle {
        
        private int seatingCapacity;
        @JsonIgnore
        private double topSpeed;
    
        public Car(String make, String model, int seatingCapacity, double topSpeed) {
        
            super(make, model);
            this.seatingCapacity = seatingCapacity;
            this.topSpeed = topSpeed;
        }
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Crossover extends Car {
        
        private double towingCapacity;
    
        public Crossover(String make, String model, int seatingCapacity, double topSpeed, double towingCapacity) {
        
            super(make, model, seatingCapacity, topSpeed);
            this.towingCapacity = towingCapacity;
        }
    }

@Data
    @AllArgsConstructor
    public class Sedan extends Car {
        
        public Sedan(String make, String model, int seatingCapacity, double topSpeed) {
        
            super(make, model, seatingCapacity, topSpeed);
        }
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Fleet {
        
        private List<Vehicle> vehicles;
    }

Sedan sedan = new Sedan("Mercedes-Benz", "S500", 5, 250.0);
    Crossover crossover = new Crossover("BMW", "X6", 5, 250.0, 6000.0);
    List<Vehicle> vehicles = new ArrayList<>();
    vehicles.add(sedan);
    vehicles.add(crossover);
    
    ObjectMapper mapper = new ObjectMapper();
    String jsonDataString = mapper.writeValueAsString(vehicles);
    
    System.out.println(jsonDataString);

[{
        "make":"Mercedes-Benz"},{
        "make":"BMW","towingCapacity":6000.0}]

--------------------

### Mix-ins ###

Mix-ins allow us to ignoring properties when serializing and deserializing without the need to directly apply annotations to a class.  
This is especially useful when **dealing with third-party classes**

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public abstract class Vehicle {
        
        private String make;
        private String model;
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Fleet {
        
        private List<Vehicle> vehicles;
    }

public static void main(String[] args) throws JsonProcessingException {
        
         Sedan sedan = new Sedan("Mercedes-Benz", "S500", 5, 250.0);
         Crossover crossover = new Crossover("BMW", "X6", 5, 250.0, 6000.0);
         List<Vehicle> vehicles = new ArrayList<>();
         vehicles.add(sedan);
         vehicles.add(crossover);
         
         ObjectMapper mapper = new ObjectMapper();
         mapper.addMixIn(Car.class, CarMixIn.class);
         String jsonDataString = mapper.writeValueAsString(vehicles);
         System.out.println(jsonDataString);
     }
     
     private abstract class CarMixIn {
        
         @JsonIgnore
         public String make;
         @JsonIgnore
         public String topSpeed;
     }

原始值:
    [{
        "make":"Mercedes-Benz","model":"S500","seatingCapacity":5,"topSpeed":250.0},{
        "make":"BMW","model":"X6","seatingCapacity":5,"topSpeed":250.0,"towingCapacity":6000.0}]
    
    ignore以后得值
    [{
        "model":"S500","seatingCapacity":5},{
        "model":"X6","seatingCapacity":5,"towingCapacity":6000.0}]

--------------------

### Annotation Introspection ###

the most powerful method to ignore supertype properties since it allows for detailed customization using the AnnotationIntrospector.

class IgnoranceIntrospector extends JacksonAnnotationIntrospector {
        
        public boolean hasIgnoreMarker(AnnotatedMember m) {
        
            return m.getDeclaringClass() == Vehicle.class && m.getName() == "model"
                    || m.getDeclaringClass() == Car.class
                    || m.getName() == "towingCapacity"
                    || super.hasIgnoreMarker(m);
        }
    }

Sedan sedan = new Sedan("Mercedes-Benz", "S500", 5, 250.0);
    Crossover crossover = new Crossover("BMW", "X6", 5, 250.0, 6000.0);
    List<Vehicle> vehicles = new ArrayList<>();
    vehicles.add(sedan);
    vehicles.add(crossover);
    
    ObjectMapper mapper = new ObjectMapper();
    mapper.setAnnotationIntrospector(new IgnoranceIntrospector());
    String jsonDataString = mapper.writeValueAsString(vehicles);
    
    System.out.println(jsonDataString);
    // [{"make":"Mercedes-Benz"},{"make":"BMW"}]

--------------------

## Subtype Handling Scenarios ##

### Conversion Between Subtypes ###

Jackson allows an object to be converted to aother type object.  
it is most helpful when used between two subtypes of the same interface or class to secure values and functionality.

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public abstract class Vehicle {
        
        private String make;
        private String model;
    }

在Car和Truck的属性上添加@JsonIgnore注释以避免不兼容

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Car extends Vehicle {
        
        @JsonIgnore
        private int seatingCapacity;
        @JsonIgnore
        private double topSpeed;
    
        public Car(String make, String model, int seatingCapacity, double topSpeed) {
        
            super(make, model);
            this.seatingCapacity = seatingCapacity;
            this.topSpeed = topSpeed;
        }
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Truck extends Vehicle {
        
        @JsonIgnore
        private double payloadCapacity;
        public Truck(String make, String model, double payloadCapacity) {
        
            super(make, model);
            this.payloadCapacity = payloadCapacity;
        }
    }

Car car = new Car("Benz", "S500", 5, 250.0);
    Truck truck1 = new Truck("Isuzu", "NQR", 7500.0);
    
    ObjectMapper mapper = new ObjectMapper();
    String s = mapper.writeValueAsString(car);
    System.out.println(s);
    
    String s1 = mapper.writeValueAsString(truck1);
    System.out.println(s1);
    
    Truck truck = mapper.convertValue(car, Truck.class);
    System.out.println(truck);

{
        "make":"Benz","model":"S500"}
    {
        "make":"Isuzu","model":"NQR"}
    Truck(payloadCapacity=0.0)

### Deserialization Without No-arg Constructors ###

默认情况下，Jackson通过使用无参数构造函数来重新创建数据对象。  
By default, Jackson recreates data objects by using no-arg constructors.

In a class hierarchy where a no-arg constructor must be added to a class and all those higher in the inheritance chain. In these cases, **creator methods** come to the rescue.  
Specifically, all no-arg constructors are dropped, and constructors of concrete subtypes are annotated with `@JsonCreator` and `@JsonProperty` to make them creator methods.

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public abstract class Vehicle {
        
        private String make;
        private String model;
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Car extends Vehicle {
        
        private int seatingCapacity;
        private double topSpeed;
    
        @JsonCreator
        public Car(
                @JsonProperty("make") String make,
                @JsonProperty("model") String model,
                @JsonProperty("seating") int seatingCapacity,
                @JsonProperty("topSpeed") double topSpeed) {
        
            super(make, model);
            this.seatingCapacity = seatingCapacity;
            this.topSpeed = topSpeed;
        }
    }

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    public class Truck extends Vehicle {
        
        private double payloadCapacity;
        @JsonCreator
        public Truck(
                @JsonProperty("make") String make,
                @JsonProperty("model") String model,
                @JsonProperty("payload") double payloadCapacity) {
        
            super(make, model);
            this.payloadCapacity = payloadCapacity;
        }
    }

Car car = new Car("Mercedes-Benz", "S500", 5, 250.0);
    Truck truck = new Truck("Isuzu", "NQR", 7500.0);
    List<Vehicle> vehicles = new ArrayList<>();
    vehicles.add(car);
    vehicles.add(truck);
    Fleet serializedFleet = new Fleet();
    serializedFleet.setVehicles(vehicles);
    
    ObjectMapper mapper = new ObjectMapper();
    mapper.enableDefaultTyping();
    
    String jsonDataString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(serializedFleet);
    System.out.println(jsonDataString);
    
    Fleet fleet = mapper.readValue(jsonDataString, Fleet.class);
    System.out.println(fleet);

{
        
      "vehicles" : [ "java.util.ArrayList", [ [ "javabasic.enumprac.prac.Car", {
        
        "make" : "Mercedes-Benz",
        "model" : "S500",
        "topSpeed" : 250.0,
        "seatingCapacity" : 5
      } ], [ "javabasic.enumprac.prac.Truck", {
        
        "make" : "Isuzu",
        "model" : "NQR",
        "payloadCapacity" : 7500.0
      } ] ] ]
    }
    
    Fleet(vehicles=[Car(seatingCapacity=5, topSpeed=250.0), Truck(payloadCapacity=7500.0)])

\-----------------------------------------------------------------------------读书笔记摘自 文章：**[Inheritance with Jackson][]**

[Inheritance with Jackson]: https://www.baeldung.com/jackson-inheritance