adam bien's blog

import javax.json.Json;
import javax.json.JsonObject;
import static javax.json.Json.createObjectBuilder;
...

public class JsonMerge {

    @Test
    public void merge() {

        JsonObject runtime = createObjectBuilder().
            add("runtime", "Jakarta EE").
            add("speed", "lightspeed").
        build();

        JsonObject project = createObjectBuilder().
            add("project", "thinwars").
            add("dev", createObjectBuilder().
            add("name", "duke").
            add("age", 18)).
        build();

        JsonObject projectWithRuntime = Json.
            createMergePatch(runtime).
            apply(project).
        asJsonObject();

        assertThat(projectWithRuntime.getString("runtime"), equalTo("Jakarta EE"));
        assertThat(projectWithRuntime.getString("project"), equalTo("thinwars"));
        System.out.println(projectWithRuntime);
    }
}

Yields:

{
    "project": "thinwars",
    "dev": {
        "name": "duke",
        "age": 18
    },
    "runtime": "Jakarta EE",
    "speed": "lightspeed"
}    

Unit Test setup requires the following dependency to JSON-P (reference) implementation:

<dependency>
    <groupId>org.glassfish</groupId>
    <artifactId>javax.json</artifactId>
    <version>1.1.4</version>
</dependency>