How to serialize DefaultMutableTreeNode (Java) to JSON?

Question

How can I serialize a tree (implemented in Java using the DefaultMutableTreeNode class) to JSON (for transferring via RESTful method to an iOS client)?

I tried:

String jsonString = (new Gson()).toJson(topNode);
// topNode is DefaultMutableTreeNode at the root

It crashed with StackOverflowError .

Answer 1

Swing's DefaultMutableTreeNode class is a tree-like data structure which contains instances of this same type both as children and as parent . That's why Gson's default serializer ran into infinite recursion and hence threw a StackOverflowError .

To solve this problem you need to customize your Gson with a smarter JsonSerializer specially crafted for converting a DefaultMutableTreeNode to JSON. As a bonus you might also want to provide a JsonDeserializer for converting such JSON back to a DefaultMutableTreeNode .

For that create your Gson instance not just by new Gson() , but by

Gson gson = new GsonBuilder()
        .registerTypeAdapter(DefaultMutableTreeNode.class, new DefaultMutableTreeNodeSerializer())
        .registerTypeAdapter(DefaultMutableTreeNode.class, new DefaultMutableTreeNodeDeserializer())
        .setPrettyPrinting()
        .create();

The DefaultMutableTreeNodeSerializer below is responsible for converting a DefaultMutableTreeNode to JSON. It converts its properties allowsChildren , userObject and children to JSON. Note that it does not convert the parent property to JSON, because doing that would produce an inifinite recursion again.

public class DefaultMutableTreeNodeSerializer implements JsonSerializer<DefaultMutableTreeNode> {

    @Override
    public JsonElement serialize(DefaultMutableTreeNode src, Type typeOfSrc, JsonSerializationContext context) {
        JsonObject jsonObject = new JsonObject();
        jsonObject.addProperty("allowsChildren", src.getAllowsChildren());
        jsonObject.add("userObject", context.serialize(src.getUserObject()));
        if (src.getChildCount() > 0) {
            jsonObject.add("children", context.serialize(Collections.list(src.children())));
        }
        return jsonObject;
    }
}

For testing let us serialize the root node of a sample JTree to JSON, and then deserialize it again.

JTree tree = new JTree();  // create a sample tree
Object topNode = tree.getModel().getRoot();  // a DefaultMutableTreeNode
String jsonString = gson.toJson(topNode);
System.out.println(jsonString);
DefaultMutableTreeNode topNode2 = gson.fromJson(jsonString, DefaultMutableTreeNode.class);

It generates the following JSON output:

{
  "allowsChildren": true,
  "userObject": "JTree",
  "children": [
    {
      "allowsChildren": true,
      "userObject": "colors",
      "children": [
        {
          "allowsChildren": true,
          "userObject": "blue"
        },
        {
          "allowsChildren": true,
          "userObject": "violet"
        },
        {
          "allowsChildren": true,
          "userObject": "red"
        },
        {
          "allowsChildren": true,
          "userObject": "yellow"
        }
      ]
    },
    {
      "allowsChildren": true,
      "userObject": "sports",
      "children": [
        {
          "allowsChildren": true,
          "userObject": "basketball"
        },
        {
          "allowsChildren": true,
          "userObject": "soccer"
        },
        {
          "allowsChildren": true,
          "userObject": "football"
        },
        {
          "allowsChildren": true,
          "userObject": "hockey"
        }
      ]
    },
    {
      "allowsChildren": true,
      "userObject": "food",
      "children": [
        {
          "allowsChildren": true,
          "userObject": "hot dogs"
        },
        {
          "allowsChildren": true,
          "userObject": "pizza"
        },
        {
          "allowsChildren": true,
          "userObject": "ravioli"
        },
        {
          "allowsChildren": true,
          "userObject": "bananas"
        }
      ]
    }
  ]
}

The DefaultMutableTreeNodeDeserializer below is responsible for converting JSON back to a DefaultMutableTreeNode .

It uses the same idea as the deserializer from How to serialize/deserialize a DefaultMutableTreeNode with Jackson? . The DefaultMutableTreeNode is not very POJO-like and thus doesn't work well together with Gson. Therefore it uses a well-behaving POJO helper class (with properties allowsChildren , userObject and children ) and lets Gson deserialize the JSON content into this class. Then the POJO object (and its POJO children) is converted to a DefaultMutableTreeNode object (with DefaultMutableTreeNode children).

public class DefaultMutableTreeNodeDeserializer implements JsonDeserializer<DefaultMutableTreeNode> {

    @Override
    public DefaultMutableTreeNode deserialize(JsonElement json, Type type, JsonDeserializationContext context) {
        return context.<POJO>deserialize(json, POJO.class).toDefaultMutableTreeNode();
    }

    private static class POJO {

        private boolean allowsChildren;
        private Object userObject;
        private List<POJO> children;
        // no need for: POJO parent

        public DefaultMutableTreeNode toDefaultMutableTreeNode() {
            DefaultMutableTreeNode node = new DefaultMutableTreeNode();
            node.setAllowsChildren(allowsChildren);
            node.setUserObject(userObject);
            if (children != null) {
                for (POJO child : children) {
                    node.add(child.toDefaultMutableTreeNode()); // recursion!
                    // this did also set the parent of the child-node
                }
            }
            return node;
        }

        // Following setters needed by Gson's deserialization:

        public void setAllowsChildren(boolean allowsChildren) {
            this.allowsChildren = allowsChildren;
        }

        public void setUserObject(Object userObject) {
            this.userObject = userObject;
        }

        public void setChildren(List<POJO> children) {
            this.children = children;
        }
    }
}

Answer 2

This is an improved alternative to my older answer which used implementations of JsonSerializer and JsonDeserializer for DefaultMutableTreeNode . The API doc of these 2 interfaces says:

New applications should prefer TypeAdapter , whose streaming API is more efficient than this interface's tree API.

Let's therefore use this preferred approach and implement a TypeAdapter for DefaultMutableTreeNode .

For using it you create your Gson instance like this (instead of just using new Gson() ):

Gson gson = new GsonBuilder()
        .registerTypeAdapterFactory(DefaultMutableTreeNodeTypeAdapter.FACTORY)
        .setPrettyPrinting()
        .create();

The DefaultMutableTreeNodeTypeAdapter below is responsible for converting a DefaultMutableTreeNode to and from JSON. It writes/reads its properties allowsChildren , userObject and children . There is no need to write the parent property, because the parent-child relations are already encoded in the nested structure of the JSON-output.

public class DefaultMutableTreeNodeTypeAdapter extends TypeAdapter<DefaultMutableTreeNode> {

    public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() {

        @Override
        @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
            if (type.getRawType() == DefaultMutableTreeNode.class) {
                return (TypeAdapter<T>) new DefaultMutableTreeNodeTypeAdapter(gson);
            }
            return null;
        }
    };

    private final Gson gson;

    private DefaultMutableTreeNodeTypeAdapter(Gson gson) {
        this.gson = gson;
    }

    @Override
    public void write(JsonWriter out, DefaultMutableTreeNode node) throws IOException {
        out.beginObject();
        out.name("allowsChildren");
        out.value(node.getAllowsChildren());
        out.name("userObject");
        gson.toJson(node.getUserObject(), Object.class, out);
        if (node.getChildCount() > 0) {
            out.name("children");
            gson.toJson(Collections.list(node.children()), List.class, out); // recursion!
        }
        // No need to write node.getParent(), it would lead to infinite recursion.
        out.endObject();
    }

    @Override
    public DefaultMutableTreeNode read(JsonReader in) throws IOException {
        in.beginObject();
        DefaultMutableTreeNode node = new DefaultMutableTreeNode();
        while (in.hasNext()) {
            switch (in.nextName()) {
            case "allowsChildren":
                node.setAllowsChildren(in.nextBoolean());
                break;
            case "userObject":
                node.setUserObject(gson.fromJson(in, Object.class));
                break;
            case "children":
                in.beginArray();
                while (in.hasNext()) {
                    node.add(read(in)); // recursion!
                    // this did also set the parent of the child-node
                }
                in.endArray();
                break;
            default:
                in.skipValue();
                break;
            }
        }
        in.endObject();
        return node;
    }
}