Generate Java class from JSON?

user_238123 0
Viewed 367974

In a Java Maven project, how do you generate java source files from JSON? For example we have

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

When we run mvn generate-sources we want it to generate something like this:

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"));
    }
}

As a Java developer, what lines of XML do I need to write in my pom.xml in order to make this happen?

json java
13 Answers

Try http://www.jsonschema2pojo.org

Or the jsonschema2pojo plug-in for Maven:

<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>

The <sourceType>json</sourceType> covers the case where the sources are json (like the OP). If you have actual json schemas, remove this line.

Updated in 2014: Two things have happened since Dec '09 when this question was asked:

  • The JSON Schema spec has moved on a lot. It's still in draft (not finalised) but it's close to completion and is now a viable tool specifying your structural rules

  • I've recently started a new open source project specifically intended to solve your problem: jsonschema2pojo. The jsonschema2pojo tool takes a json schema document and generates DTO-style Java classes (in the form of .java source files). The project is not yet mature but already provides coverage of the most useful parts of json schema. I'm looking for more feedback from users to help drive the development. Right now you can use the tool from the command line or as a Maven plugin.

Hope this helps!

user_1258250

Thanks all who attempted to help.
For me this script was helpful. It process only flat JSON and don't take care of types, but automate some routine

  String str = 
        "{"
            + "'title': 'Computing and Information systems',"
            + "'id' : 1,"
            + "'children' : 'true',"
            + "'groups' : [{"
                + "'title' : 'Level one CIS',"
                + "'id' : 2,"
                + "'children' : 'true',"
                + "'groups' : [{"
                    + "'title' : 'Intro To Computing and Internet',"
                    + "'id' : 3,"
                    + "'children': 'false',"
                    + "'groups':[]"
                + "}]" 
            + "}]"
        + "}";



    JSONObject json = new JSONObject(str);
    Iterator<String> iterator =  json.keys();

    System.out.println("Fields:");
    while (iterator.hasNext() ){
       System.out.println(String.format("public String %s;", iterator.next()));
    }

    System.out.println("public void Parse (String str){");
    System.out.println("JSONObject json = new JSONObject(str);");

    iterator  = json.keys();
    while (iterator.hasNext() ){
       String key = iterator.next();
       System.out.println(String.format("this.%s = json.getString(\"%s\");",key,key ));

    System.out.println("}");
user_2381230

I know there are many answers but of all these I found this one most useful for me. This link below gives you all the POJO classes in a separate file rather than one huge class that some of the mentioned websites do:

https://json2csharp.com/json-to-pojo

It has other converters too. Also, it works online without a limitation in size. My JSON is huge and it worked nicely.

user_9887380

As far as I know there is no such tool. Yet.

The main reason is, I suspect, that unlike with XML (which has XML Schema, and then tools like 'xjc' to do what you ask, between XML and POJO definitions), there is no fully features schema language. There is JSON Schema, but it has very little support for actual type definitions (focuses on JSON structures), so it would be tricky to generate Java classes. But probably still possible, esp. if some naming conventions were defined and used to support generation.

However: this is something that has been fairly frequently requested (on mailing lists of JSON tool projects I follow), so I think that someone will write such a tool in near future.

So I don't think it is a bad idea per se (also: it is not a good idea for all use cases, depends on what you want to do ).

user_595010
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