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

How to Fix JsonMappingException

If you're encountering the dreaded JsonMappingException: No suitable constructor found for type [simple type, class ]: can not instantiate from JSON object error, don't fret! 🤔 I've got your back. In this blog post, I'll walk you through common issues that cause this error and provide easy solutions to fix it. Let's dive in!

The Error and its Context

Here's the error message that's causing all the trouble:

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?)

The error occurs when you're trying to receive and process a JSON request. Here's an example of the JSON you're sending:

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

You have a controller method that is supposed to handle this request:

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

AllApplesDO is a wrapper class for ApplesDO, which looks like this:

public class AllApplesDO {

    private List<ApplesDO> applesDO;

    // Getters and setters

    // Constructor
}

public class ApplesDO {

    private String apple;

    // Getter and setter

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

The Root Cause

The root cause of the JsonMappingException error is most likely the absence of a suitable constructor in your ApplesDO class. Jackson is unable to instantiate the class from the JSON object. This often occurs when you have a subclass that Jackson cannot convert into Java objects.

The Solution

To fix this issue, you need to add or enable type information for Jackson to properly convert JSON into Java objects. Here's what you can do:

1. Add a default constructor to your ApplesDO class:

public class ApplesDO {

    private String apple;

    // Getter, setter, and default constructor

    public ApplesDO(){
        // Default constructor code
    }

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

By adding a default constructor, Jackson will be able to instantiate the ApplesDO class using reflection when converting JSON into Java objects.

1. Enable Jackson's type information handling:

In your Spring configuration, make sure you have the following properties set:

spring.jackson.deserialization.fail-on-unknown-properties=false
spring.jackson.mapper.enable-default-typing=true

The enable-default-typing property enables Jackson's type information handling, which allows the conversion of JSON objects into Java objects, even if the class is a subclass.

Conclusion

Congratulations! You've successfully fixed the JsonMappingException error. 🎉 By adding a default constructor to your class and enabling Jackson's type information handling, you've ensured that JSON objects can be seamlessly converted into Java objects.

If you found this article helpful, why not share it with your fellow developers who might be facing the same issue? 🌟 And if you have any questions or additional tips, feel free to leave a comment below. Let's keep the conversation going!

Happy coding! 👩‍💻👨‍💻