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JsonMappingException: No suitable constructor found for type [simple type, class ]: can not instantiate from JSON object

Posted by: admin November 2, 2017 Leave a comment

Questions:

I am getting the following error when trying to get a JSON request and process it:

org.codehaus.jackson.map.JsonMappingException: No suitable constructor found for type [simple type, class com.myweb.ApplesDO]: can not instantiate from JSON object (need to add/enable type information?)

Here is the JSON I am trying to send:

{
  "applesDO" : [
    {
      "apple" : "Green Apple"
    },
    {
      "apple" : "Red Apple"
    }
  ]
}

In Controller, I have the following method signature:

@RequestMapping("showApples.do")
public String getApples(@RequestBody final AllApplesDO applesRequest){
    // Method Code
}

AllApplesDO is a wrapper of ApplesDO :

public class AllApplesDO {

    private List<ApplesDO> applesDO;

    public List<ApplesDO> getApplesDO() {
        return applesDO;
    }

    public void setApplesDO(List<ApplesDO> applesDO) {
        this.applesDO = applesDO;
    }
}

ApplesDO:

public class ApplesDO {

    private String apple;

    public String getApple() {
        return apple;
    }

    public void setApple(String appl) {
        this.apple = apple;
    }

    public ApplesDO(CustomType custom){
        //constructor Code
    }
}

I think that Jackson is unable to convert JSON into Java objects for subclasses. Please help with the configuration parameters for Jackson to convert JSON into Java Objects. I am using Spring Framework.

EDIT: Included the major bug that is causing this problem in the above sample class – Please look accepted answer for solution.

Answers:

So, finally I realized what the problem is. It is not a Jackson configuration issue as I doubted.

Actually the problem was in ApplesDO Class:

public class ApplesDO {

    private String apple;

    public String getApple() {
        return apple;
    }

    public void setApple(String apple) {
        this.apple = apple;
    }

    public ApplesDO(CustomType custom) {
        //constructor Code
    }
}

There was a custom constructor defined for the class making it the default constructor. Introducing a dummy constructor has made the error to go away:

public class ApplesDO {

    private String apple;

    public String getApple() {
        return apple;
    }

    public void setApple(String apple) {
        this.apple = apple;
    }

    public ApplesDO(CustomType custom) {
        //constructor Code
    }

    //Introducing the dummy constructor
    public ApplesDO() {
    }

}

Questions:
Answers:

This happens for these reasons:

  1. your inner class should be defined as static

    private static class Conditiont {  //jackson specific 
    
     }
    
  2. It might be that you got no default constructor in your class (UPDATE: This seems not to be the case)

    private static class Condition {
        private Long id;
    
        public Condition() {
        }
    
        // Setters and Getters
    }
    
  3. It could be your Setters are not defined properly or are not visible (e.g. private setter)

Questions:
Answers:

I would like to add another solution to this that does not require a dummy constructor. Since dummy constructors are a bit messy and subsequently confusing. We can provide a safe constructor and by annotating the constructor arguments we allow jackson to determine the mapping between constructor parameter and field.

so the following will also work. Note the string inside the annotation must match the field name.

import com.fasterxml.jackson.annotation.JsonProperty;
public class ApplesDO {

        private String apple;

        public String getApple() {
            return apple;
        }

        public void setApple(String apple) {
            this.apple = apple;
        }

        public ApplesDO(CustomType custom){
            //constructor Code
        }

        public ApplesDO(@JsonProperty("apple")String apple) {
        }

}

Questions:
Answers:

When I ran into this problem, it was a result of trying to use an inner class to serve as the DO. Construction of the inner class (silently) required an instance of the enclosing class — which wasn’t available to Jackson.

In this case, moving the inner class to its own .java file fixed the problem.

Questions:
Answers:

Can you please test this structure. If I remember correct you can use it this way:

{
    "applesRequest": {
        "applesDO": [
            {
                "apple": "Green Apple"
            },
            {
                "apple": "Red Apple"
            }
        ]
    }
}

Second, please add default constructor to each class it also might help.

Questions:
Answers:

Thumb Rule: Add a default constructor for each class you used as a mapping class. You missed this and issue arise!
Simply add default constructor and it should work.

Questions:
Answers:

If you start annotating constructor, you must annotate all fields.

Notice, my Staff.name field is mapped to “ANOTHER_NAME” in JSON string.

     String jsonInString="{\"ANOTHER_NAME\":\"John\",\"age\":\"17\"}";
     ObjectMapper mapper = new ObjectMapper();
     Staff obj = mapper.readValue(jsonInString, Staff.class);
     // print to screen

     public static class Staff {
       public String name;
       public Integer age;
       public Staff() {         
       }        

       //@JsonCreator - don't need this
       public Staff(@JsonProperty("ANOTHER_NAME") String   n,@JsonProperty("age") Integer a) {
        name=n;age=a;
       }        
    }

Questions:
Answers:

You must realize what options Jackson has available for deserialization. In Java, method argument names are not present in compiled code. That’s why Jackson can’t generally use constructors to create a well defined object with everything already set.

So, if there is an empty constructor and there are also setters, it uses the empty constructor and setters. If there are no setters, some dark magic (reflections) is used to do it.

If you want to use constructor with Jackson, you must use the annotations as mentioned by @PiersyP in his answer. You can also use builder pattern. If you encounter some exceptions, good luck. Error handling in Jackson sucks big time, it’s hard to understand that gibberish in error messages.

Questions:
Answers:

You have to create dummy empty constructor in our model class.So while mapping json, it set by setter method.

Questions:
Answers:

Failing custom jackson Serializers/Deserializers could also be the problem. Though it’s not your case, it’s worth mentioning.

I faced the same exception and that was the case.