Moshi – java.math.BigDecimal requires explicit JsonAdapter to be registered

Moshi does not have built-in adapters to process types like java.math.BigDecimal; we need to manually provide a custom adapter to handle the BigDecimal type.

Table of contents:

• 1. Download Moshi
• 2. BigDecimal requires explicit JsonAdapter
• 3. Make Moshi supports BigDecimal
  • 3.1 Define BigDecimal Adapter
  • 3.2 Register the Adapter with Moshi
• 4. Download Source Code
• 5. References

P.S Tested with Moshi 1.15.1

1. Download Moshi

Declare moshi in the pom.xml.

    <dependency>
        <groupId>com.squareup.moshi</groupId>
        <artifactId>moshi</artifactId>
        <version>1.15.1</version>
    </dependency>

2. BigDecimal requires explicit JsonAdapter

The following staff object contains the type java.math.BigDecimal.

package com.mkyong.json.model;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Map;

public class Staff {

    private String name;
    private int age;
    private String[] position;              //  Array
    private List<String> skills;            //  List
    private Map<String, BigDecimal> salary; //  Map
    private boolean active;                 // boolean

    // getters, setters and constructors
}

If we use Moshi to serialize the object into JSON, Moshi will throw the error java.math.BigDecimal requires explicit JsonAdapter to be registered.

    Moshi moshi = new Moshi.Builder().build();
    JsonAdapter<Staff> jsonAdapter = moshi.adapter(Staff.class);
        
    Staff staff = createStaff();

    String json = jsonAdapter.toJson(staff); // throws error 

Caused by: java.lang.IllegalArgumentException: 
    Platform class java.math.BigDecimal requires explicit JsonAdapter to be registered
	
    at com.squareup.moshi.ClassJsonAdapter$1.create(ClassJsonAdapter.java:76)
	at com.squareup.moshi.Moshi.adapter(Moshi.java:146)
	... 11 more

3. Make Moshi supports BigDecimal

We need to create and register a custom JsonAdapter to handle the type BigDecimal in Moshi.

3.1 Define `BigDecimal` Adapter

Create logic to handle the BigDecimal.

package com.mkyong.json.moshi.adapter;

import com.squareup.moshi.FromJson;
import com.squareup.moshi.ToJson;

import java.math.BigDecimal;

public class BigDecimalJsonAdapter {

    // Converts BigDecimal to its string representation
    @ToJson
    String toJson(BigDecimal value) {
        return value.toString(); 
    }

    // Constructs a BigDecimal from the string representation
    @FromJson
    BigDecimal fromJson(String value) {
        return new BigDecimal(value); 
    }
}

3.2 Register the Adapter with Moshi

The following example will register a BigDecimalJsonAdapter and convert the staff object that contains the type BigDecimal into JSON.

package com.mkyong.json.moshi;

import com.mkyong.json.model.Staff;
import com.mkyong.json.moshi.adapter.BigDecimalJsonAdapter;
import com.squareup.moshi.JsonAdapter;
import com.squareup.moshi.Moshi;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class ObjectToJsonExample2 {

    public static void main(String[] args) {

        Moshi moshi = new Moshi.Builder()
                .add(new BigDecimalJsonAdapter()) // Register the BigDecimal adapter
                .build();
        JsonAdapter<Staff> jsonAdapter = moshi.adapter(Staff.class).indent("  ");

        Staff staff = createStaff();

        try {

            String json = jsonAdapter.toJson(staff);
            System.out.println(json);

        } catch (Exception e) {
            System.err.println("Error parsing JSON: " + e.getMessage());
        }
    }

    private static Staff createStaff() {

        Staff staff = new Staff();

        staff.setName("mkyong");
        staff.setAge(42);
        staff.setPosition(new String[]{"Founder", "CTO", "Writer", "Minimalists"});

        Map<String, BigDecimal> salary = new HashMap<>();
        salary.put("2010", new BigDecimal(10000));
        salary.put("2012", new BigDecimal(12000));
        salary.put("2018", new BigDecimal(14000));
        staff.setSalary(salary);

        staff.setSkills(Arrays.asList("java", "python", "node", "kotlin"));

        return staff;

    }

}

Output

{
  "active": false,
  "age": 42,
  "name": "mkyong",
  "position": [
    "Founder",
    "CTO",
    "Writer",
    "Minimalists"
  ],
  "salary": {
    "2018": "14000",
    "2012": "12000",
    "2010": "10000"
  },
  "skills": [
    "java",
    "python",
    "node",
    "kotlin"
  ]
}

4. Download Source Code

5. References

• Moshi Github
• Parse JSON using Moshi
• Write JSON to a file using Moshi
• Parse JSON string with Jackson
• Convert Java Objects to JSON with Jackson

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