I looked at Google's Gson as a potential JSON plugin. Can anyone offer some form of guidance as to how I can generate Java from this JSON string?

Google Gson supports generics and nested beans. The [] in JSON represents an array and should map to a Java collection such as List or just a plain Java array. The {} in JSON represents an object and should map to a Java Map or just some JavaBean class.

You have a JSON object with several properties of which the groups property represents an array of nested objects of the very same type. This can be parsed with Gson the following way:

package com.stackoverflow.q1688099;

import java.util.List;
import com.google.gson.Gson;

public class Test {

    public static void main(String... args) throws Exception {
        String json = 
            "{"
                + "'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':[]"
                    + "}]" 
                + "}]"
            + "}";

        // Now do the magic.
        Data data = new Gson().fromJson(json, Data.class);

        // Show it.
        System.out.println(data);
    }

}

class Data {
    private String title;
    private Long id;
    private Boolean children;
    private List<Data> groups;

    public String getTitle() { return title; }
    public Long getId() { return id; }
    public Boolean getChildren() { return children; }
    public List<Data> getGroups() { return groups; }

    public void setTitle(String title) { this.title = title; }
    public void setId(Long id) { this.id = id; }
    public void setChildren(Boolean children) { this.children = children; }
    public void setGroups(List<Data> groups) { this.groups = groups; }
    
    public String toString() {
        return String.format("title:%s,id:%d,children:%s,groups:%s", title, id, children, groups);
    }
}

Fairly simple, isn't it? Just have a suitable JavaBean and call Gson#fromJson().

See also:

• Json.org - Introduction to JSON
• Gson User Guide - Introduction to Gson

JSON.parse(jsonString) is a pure JavaScript approach so long as you can guarantee a reasonably modern browser.

If you can't fix the JSON document you can create a custom JSON type converter and apply it to the style property. And whoever created that document needs to fix their bug.

If you use System.Text.Json, a possible converter could be :

public class StyleStringJsonConverter : JsonConverter<Style>
{
    public override Style Read(
        ref Utf8JsonReader reader,
        Type typeToConvert,
        JsonSerializerOptions options) =>
            JsonSerializer.Deserialize<Style>(reader.GetString()!);

    public override void Write(
        Utf8JsonWriter writer,
        Style style,
        JsonSerializerOptions options) =>
            writer.WriteStringValue(JsonSerializer.Serialize(style));
}

This can be applied through an attribute :

public class Root
{
    public string lng_x { get; set; }
    [JsonConverter(typeof(StyleStringJsonConverter))]
    public Style style { get; set; }
}

JSON.NET also has custom converters:

public class StyleStringConverter : JsonConverter<Style>
{
    public override void WriteJson(JsonWriter writer, Style value, JsonSerializer serializer)
    {
        writer.WriteValue(JsonConvert.SerializeObject(value));
    }

    public override Style ReadJson(JsonReader reader, Type objectType, Style existingValue, bool hasExistingValue, JsonSerializer serializer)
    {
        string s = (string)reader.Value;

        return JsonConvert.DeserializeObject<Style>(s);
    }

}

This can be applied using an attribute too:

public class Root
{
    public string lng_x { get; set; }
    [JsonConverter(typeof(StyleStringJsonConverter))]
    public Style style { get; set; }
}