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184.5.2. 什么是多态对象映射？

184.5.2. 什么是多态对象映射？

在某些情况下，序列化对象的接收器无法预先知道对象类型。例如，这适用于一个多形对象数组。考虑抽象类型、Shape 及其子类型、三角、Square （等等）：

package com.example;
...
public abstract class Shape {
}

public class Triangle extends Shape {
  ...
}

public class Square extends Shape {
  ...
}

public class ListOfShape {
  public List<Shape> shapes;
  ...
}

package com.example;
...
public abstract class Shape {
}

public class Triangle extends Shape {
  ...
}

public class Square extends Shape {
  ...
}

public class ListOfShape {
  public List<Shape> shapes;
  ...
}

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您可以实例化并序列化一个格式数组列表(ListOfShape)，如下所示：

ObjectMapper objectMapper = new ObjectMapper();

ListOfShape shapeList = new ListOfShape();
shapeList.shapes = new ArrayList<Shape>();
shapeList.shapes.add(new Triangle());
shapeList.shapes.add(new Square());

String serialized = objectMapper.writeValueAsString(shapeList);

ObjectMapper objectMapper = new ObjectMapper();

ListOfShape shapeList = new ListOfShape();
shapeList.shapes = new ArrayList<Shape>();
shapeList.shapes.add(new Triangle());
shapeList.shapes.add(new Square());

String serialized = objectMapper.writeValueAsString(shapeList);

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但是，接收器中现在存在一个问题。您可以通过将 type 指定为 readValue （） 的第二个参数来告知接收器预期 ListOfShape 对象：

MyClass myobject = objectMapper.readValue(serialized, ListOfShape.class);
ObjectMapper objectMapper = new ObjectMapper();

MyClass myobject = objectMapper.readValue(serialized, ListOfShape.class);
ObjectMapper objectMapper = new ObjectMapper();

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但是，没有接收器可以知道列表的第一个元素是 三角，第二个元素是 Square。要解决此问题，您需要启用多 形对象映射，如下一节中所述。

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184.5.2. 什么是多态对象映射？

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