使用JSON编组字符串

Question

I am using JSON Jackson 2.0 to write a data Model to file and then load it back into the application. I can successfully write the object into a string but I encounter the following error message when trying to marshal it back into a object:

No suitable constructor found for type [simple type, class ImportAttributeModel]: can not instantiate from JSON object (need to add/enable type information?) at [Source: user; line: 1, column: 111] (through reference chain: ImportTemplateModel["modelList"])

This is a code snippet of my classes:

public class ImportTemplateModel 
{  
  private Map<Integer, AttributeModel> m_modelList;
  private GraphElementType m_type; 

  public ImportTemplateModel() 
  {

  }
// getters  & setters
}

public class AttributeModel 
{
   private String m_label;
   private String m_key;
   private ElementDefinition m_definition;

   public AttributeModel(String label, String key, ElementDefinition def)
   {
      m_label = label;
      m_key = key;
      m_definition = def;
   }
// getters  & setters
}

Here is the calls I make to write & read the model:

// Write model
ObjectMapper mapper = new ObjectMapper();
ImportTemplateModel itm = new ImportTemplateModel()
// set model atttributes
...
//

try {
  mapper.writeValue(new File(filepath), itm);

} 
catch (IOException e) {

}
.... 

// Read Model
ObjectMapper mapper = new ObjectMapper();
try {
  mapper.readValue(new File(filepath), ImportTemplateModel.class);

}
catch (IOException e) {

}

I am not sure what I am doing wrong at the moment... Is it because AttributeModel is not a POJO? or is it I can't use Maps for this implementation? If so, how do I get around this?

Answer 1

它需要有一个无参数的构造函数，以便Jackson可以实例化它。

Answer 2

Exception:

com.fasterxml.jackson.databind.JsonMappingException: No suitable constructor found for type [simple type, class ImportDataModel$ImportTemplateModel]: can not instantiate from JSON object (need to add/enable type information?)

suggests that ImportTemplateModel is an inner class. Your code snippet shows that this class is not static. If it is not static, it cannot be instantiated outside of its outer class ImportDataModel . Make it static or standalone class and it should work.