使用 Jackson 进行不可变/多态 POJO <-> JSON 序列化

我正在尝试使用 Jackson 2.1.4 将不可变的 POJO 与 JSON 进行序列化，而无需编写自定义序列化程序并使用尽可能少的注释。我还希望避免仅仅为了满足 Jackson 库而添加不必要的 getter 或默认构造函数。

我现在陷入了例外：

JsonMappingException：找不到类型 [简单类型，类 Circle] 的合适构造函数：无法从 JSON 对象实例化（需要添加/启用类型信息？）

代码：

public abstract class Shape {}


public class Circle extends Shape {
  public final int radius; // Immutable - no getter needed

  public Circle(int radius) {
    this.radius = radius;
  }
}


public class Rectangle extends Shape {
  public final int w; // Immutable - no getter needed
  public final int h; // Immutable - no getter needed

  public Rectangle(int w, int h) {
    this.w = w;
    this.h = h;
  }
}

测试代码：

ObjectMapper mapper = new ObjectMapper();
mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.PROPERTY); // Adds type info

Shape circle = new Circle(10);
Shape rectangle = new Rectangle(20, 30);

String jsonCircle = mapper.writeValueAsString(circle);
String jsonRectangle = mapper.writeValueAsString(rectangle);

System.out.println(jsonCircle); // {"@class":"Circle","radius":123}
System.out.println(jsonRectangle); // {"@class":"Rectangle","w":20,"h":30}

// Throws:
//  JsonMappingException: No suitable constructor found.
//  Can not instantiate from JSON object (need to add/enable type information?)
Shape newCircle = mapper.readValue(jsonCircle, Shape.class);
Shape newRectangle = mapper.readValue(jsonRectangle, Shape.class);

System.out.println("newCircle = " + newCircle);
System.out.println("newRectangle = " + newRectangle);

非常感谢任何帮助，谢谢！

您可以（根据 API）用以下方式注释构造函数@JsonCreator http://fasterxml.github.com/jackson-annotations/javadoc/2.1.0/com/fasterxml/jackson/annotation/JsonCreator.html和参数@JsonProperty http://fasterxml.github.com/jackson-annotations/javadoc/2.1.0/com/fasterxml/jackson/annotation/JsonProperty.html.

public class Circle extends Shape {
    public final int radius; // Immutable - no getter needed

    @JsonCreator
    public Circle(@JsonProperty("radius") int radius) {
        this.radius = radius;
    }
}

public class Rectangle extends Shape {
    public final int w; // Immutable - no getter needed
    public final int h; // Immutable - no getter needed

    @JsonCreator        
    public Rectangle(@JsonProperty("w") int w, @JsonProperty("h") int h) {
        this.w = w;
        this.h = h;
    }
}

编辑：也许你必须用注释 Shape 类@JsonSubTypes http://fasterxml.github.com/jackson-annotations/javadoc/2.1.0/com/fasterxml/jackson/annotation/JsonSubTypes.html从而可以确定Shape的具体子类。

@JsonSubTypes({@JsonSubTypes.Type(Circle.class), @JsonSubTypes.Type(Rectangle.class)})
public abstract class Shape {}