java jackson在序列化后保留类型信息，如果类型为Object的变量 [英] java jackson keep type information after serialize if variable of type Object

问题描述

ClassA

public class ClassA {
   private String id;
   private Object rawData;
}

B 类

public class ClassB {
   private String name;
}

ClassC

public class ClassC {
   String address;
}

主类

public class MainExample {
   public static void main( String[] args ) throws IOException {

      ObjectMapper mapper = new ObjectMapper(  );

      ClassB classB = new ClassB();
      //ClassC classC = new ClassC();
      ClassA classA = new ClassA();
      classA.setRawData(  classB );
      //classA.setRawData(  classC );

      if (classA.getRawData() instanceof ClassB) {
         System.out.println("true ");
      } else {
         System.out.println("false");
      }

      String classAString = mapper.writeValueAsString( classA );
      ClassA a = mapper.readValue( classAString, ClassA.class );

      if (a.getRawData() instanceof ClassB) {
         System.out.println("true ");
      } else {
         System.out.println("false");
      }
   }
}

为什么第一个 if-else 打印 "TRUE" 和第二个 if-else 打印 "false"??

why first if-else printing "TRUE" and second if-else printing "false"??

如何查看 rawData 的类型?

How can I check the type of rawData?

推荐答案

mapper.writeValueAsString(classA) 会将实例序列化为类似于 {"rawData":{}}.

mapper.writeValueAsString(classA) will serialise the instance into something similar to {"rawData":{}}.

反序列化 {} 时，默认映射器会失败，因为它将 {} 视为不可序列化的类型.如果您之前已将 SerializationFeature.FAIL_ON_EMPTY_BEANS 配置为 false，您将创建一个空的 Object.

While deserialising {} the default mapper would fail, because it treats {} as a non-serializable type. If you've configured SerializationFeature.FAIL_ON_EMPTY_BEANS to false before, you will have an empty Object created.

您可能希望使用 mapper.enableDefaultTyping(); 在 JSON 中包含类型信息，从而反序列化为正确的类型.

You might want to use mapper.enableDefaultTyping(); to include type information in JSON, and thereby deserialize into the correct types.

注意:如果传入的内容来自不受信任的来源，则使用默认类型可能存在潜在的安全风险，建议不要这样做，或者，如果启用，使用 setDefaultTyping 传递一个自定义 TypeResolverBuilder 实现，用于将要使用的合法类型列入白名单.

NOTE: use of Default Typing can be a potential security risk if incoming content comes from untrusted sources, and it is recommended that this is either not done, or, if enabled, use setDefaultTyping passing a custom TypeResolverBuilder implementation that white-lists legal types to use.

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