Jackson使用`@JsonSubTypes`实现多态解析

Jackson使用@JsonSubTypes实现多态解析

假如有以下格式json
json1.json 文件内容

[
  {
    "id": "1",
    "type": "startEvent",
    "startEvent": "begin",
    "startInfo": "hello world"
  },
  {
    "id": "2",
    "type": "endEvent",
    "endEvent": "end",
    "endInfo": "hello jackson"
  }
]

json2.json 文件内容

{
  "id": "1",
  "type": "startEvent",
  "startEvent": "begin",
  "startInfo": "hello world"
}

特点：

1. 有公共字段，这里是（id&type）
2. type 用来确定是哪个子类
3. 后面两个Event&Info是每个子类特定的字段

Demo-Version-1

定义解析实体

BaseElement

@Data
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property = "type", visible =
  true)
@JsonSubTypes({@JsonSubTypes.Type(value = StartEvent.class, name = "startEvent"),
  @JsonSubTypes.Type(value = EndEvent.class, name = "endEvent")})
public class BaseElement {

  String id;

  String type;
}

StartEvent

@ToString(callSuper = true)
@Data
public class StartEvent extends BaseElement{

  private String startEvent;

  private String startInfo;

}

EndEvent

@ToString(callSuper = true)
@Data
public class EndEvent extends BaseElement{

  private String endEvent;
  private String endInfo;
}

解析

单个 json2.json

 @Test
  public void version1Json2Test() {
    String json = IoUtil.readFileAsString("json2.json");
    ObjectMapper mapper = new ObjectMapper();
    try {
      StartEvent startEvent = (StartEvent) mapper.readValue(json, BaseElement.class);
      System.out.println(startEvent);
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

多个（start和event数组） json1.json

@Test
  public void version1Json1Test() {
    String jsonString = IoUtil.readFileAsString("json1.json");
    System.out.println(jsonString);
    ObjectMapper objectMapper = new ObjectMapper();
    try {
      List<BaseElement> baseElements = new ArrayList<>();
      baseElements = objectMapper.readValue(jsonString,
        new TypeReference<List<BaseElement>>() {
        });
      baseElements.forEach(System.out::println);
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

Demo-Version-2

JsonTypeInfo借助JsonSubTypes注解来感知抽象类的有哪些实现类，并且是如何匹配的。在大型工程中抽象类的子类很多（接口的实现很多），那么JsonSubTypes注解就十分臃肿了。而且这种写法是 违反开闭原则（OCP） 的。借助以下方式可以将JsonSubTypes剔除掉，达到相同的效果。

下面使用一种GitHub上的开源框架 https://github.com/ronmamo/reflections

BaseElement

@Data
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY, property =
  "type", visible =
  true)
public class BaseElement {

  String id;

  String type;
}

EndEvent

@ToString(callSuper = true)
@Data
@JsonTypeName(value = "endEvent")
public class EndEvent extends BaseElement {

  private String endEvent;
  private String endInfo;
}

StartEvent

@ToString(callSuper = true)
@Data
@JsonTypeName(value = "startEvent")
public class StartEvent extends BaseElement {

  private String startEvent;

  private String startInfo;

}

解析

单个 json2.json

 @Test
  public void version2Json2Test() {
    Reflections reflections = new Reflections("org.ctc.googol.jackson.version2");
    Set<Class<? extends BaseElement>> classSet = reflections.getSubTypesOf(BaseElement.class);
    ObjectMapper mapper = new ObjectMapper();
    classSet.stream().forEach(clazz -> mapper.registerSubtypes(clazz));
    String json = IoUtil.readFileAsString("json2.json");
    try {
      StartEvent startEvent = (StartEvent) mapper.readValue(json, BaseElement.class);
      System.out.println(startEvent);
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

多个（start和event数组） json1.json

@Test
  public void version2Json1Test() {
    Reflections reflections = new Reflections("org.ctc.googol.jackson.version2");
    Set<Class<? extends BaseElement>> classSet = reflections.getSubTypesOf(BaseElement.class);
    ObjectMapper mapper = new ObjectMapper();
    classSet.stream().forEach(clazz -> mapper.registerSubtypes(clazz));
    String json = IoUtil.readFileAsString("json1.json");
    try {
      List<BaseElement> result = mapper.readValue(json,
        new TypeReference<List<BaseElement>>() {
        });
      result.forEach(System.out::println);
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

具体Demo源码请见 https://github.com/googalAmbition/googol
Jackson 其他黑科技 https://github.com/FasterXML/jackson