How can I generate Java classes from a JSON schema in a Maven project?

Generating Java Classes from JSON using Maven

Problem:

In a Java Maven project, how can you generate Java source files from a JSON schema? For instance, given the following JSON:

{
  "firstName": "John",  
  "lastName": "Smith",  
  "address": {  
    "streetAddress": "21 2nd Street",  
     "city": "New York"
  }
}

You want to generate Java classes like:

class Address  {
    JSONObject mInternalJSONObject;
     
    Address (JSONObject json){
        mInternalJSONObject = json;
    }
     
    String  getStreetAddress () {
        return mInternalJSONObject.getString("streetAddress");
    }
    
    String  getCity (){
        return mInternalJSONObject.getString("city");
    }
}

class Person {        
    JSONObject mInternalJSONObject;
    
    Person (JSONObject json){
        mInternalJSONObject = json;
    }
    
    String  getFirstName () {
        return mInternalJSONObject.getString("firstName");
    }
    
    String  getLastName (){
        return mInternalJSONObject.getString("lastName");
    }
    
    Address getAddress (){
        return Address(mInternalJSONObject.getString("address"));
    }
}

Solution:

1. Use JSON Schema: For a more robust solution, consider using JSON Schema, which provides a formal definition of the JSON's structure.

2. jsonschema2pojo Maven Plugin:

   <plugin>
       <groupid>org.jsonschema2pojo</groupid>
       <artifactid>jsonschema2pojo-maven-plugin</artifactid>
       <version>1.0.2</version>
       <configuration>
           <sourcedirectory>${basedir}/src/main/resources/schemas</sourcedirectory>
           <targetpackage>com.myproject.jsonschemas</targetpackage>
           <sourcetype>json</sourcetype>
       </configuration>
       <executions>
           <execution>
               <goals>
                   <goal>generate</goal>
               </goals>
           </execution>
       </executions>
   </plugin>

   Set sourceType to json if your source is JSON directly. For JSON schemas, remove this line.

Additional Tips:

• JSON Schema: If you already have JSON schemas, you can generate Java classes by converting them to org.jsonschema2pojo.model.Schema objects using the com.sun.codemodel.jsonschema.JsonUnboxer class.
• Custom Code Generation: Use the jsonschema2pojo tool directly to generate Java code, giving you more flexibility but requiring manual changes if the JSON schema changes.

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