Generate Java class from JSON?

Question

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!

If you're using Jackson (the most popular library there), try

https://github.com/astav/JsonToJava

Its open source (last updated on Jun 7, 2013 as of year 2021) and anyone should be able to contribute.

Summary

A JsonToJava source class file generator that deduces the schema based on supplied sample json data and generates the necessary java data structures.

It encourages teams to think in Json first, before writing actual code.

Features

• Can generate classes for an arbitrarily complex hierarchy (recursively)
• Can read your existing Java classes and if it can deserialize into those structures, will do so
• Will prompt for user input when ambiguous cases exist

Here's an online tool that will take JSON, including nested objects or nested arrays of objects and generate a Java source with Jackson annotations.

Answering this old question with recent project ;-).

At the moment the best solution is probably JsonSchema2Pojo :

It does the job from the seldom used Json Schema but also with plain Json. It provides Ant and Maven plugin and an online test application can give you an idea of the tool. I put a Json Tweet and generated all the containing class (Tweet, User, Location, etc..).

We'll use it on Agorava project to generate Social Media mapping and follow the contant evolution in their API.

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("}");