如何將DefaultMutableTreeNode（Java）序列化為JSON？

Question

如何將樹（使用DefaultMutableTreeNode類在Java中實現）序列化為JSON（用於通過RESTful方法傳輸到iOS客戶端）？

我試過了：

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

它崩潰與StackOverflowError 。

Answer 1

Swing的DefaultMutableTreeNode類是一個類似樹的數據結構，其中包含與children和parent相同類型的實例。 這就是Gson的默認序列化程序遇到無限遞歸並因此引發StackOverflowError 。

要解決此問題，您需要使用專門設計用於將DefaultMutableTreeNode轉換為JSON的更智能的JsonSerializer來定制Gson 。 另外，您可能還想提供一個JsonDeserializer用於將此類JSON轉換回DefaultMutableTreeNode 。

為此，不僅要通過new Gson()創建Gson實例，還要通過

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

下面的DefaultMutableTreeNodeSerializer負責將DefaultMutableTreeNode轉換為JSON。 它將其屬性allowsChildren ， userObject和children為JSON。 請注意，它不會將parent屬性轉換為JSON，因為這樣做會再次產生無限遞歸。

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;
    }
}

為了進行測試，讓我們將示例JTree的根節點序列化為JSON，然后再次將其反序列化。

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);

它生成以下JSON輸出：

{
  "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"
        }
      ]
    }
  ]
}

下面的DefaultMutableTreeNodeDeserializer負責將JSON轉換回DefaultMutableTreeNode 。

它使用與如何使用Jackson序列化/反序列化DefaultMutableTreeNode中的反序列化器相同的思想。 。 DefaultMutableTreeNode不是非常類似於POJO，因此不能與Gson一起很好地工作。 因此，它使用行為良好的POJO幫助器類（具有屬性allowsChildren ， userObject和children ），並讓Gson將JSON內容反序列化到此類中。 然后，將POJO對象（及其POJO子對象）轉換為DefaultMutableTreeNode對象（具有DefaultMutableTreeNode子對象）。

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

這是對我的較早答案的一種改進的替代方法，該較早的答案對DefaultMutableTreeNode使用JsonSerializer和JsonDeserializer實現。 這兩個接口的API文檔說：

新應用程序應該首選TypeAdapter ，其流API比此接口的樹API更高效。

因此，讓我們使用這種首選方法，並為DefaultMutableTreeNode實現TypeAdapter 。

使用它，您可以像這樣創建Gson實例（而不是僅使用new Gson() ）：

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

下面的DefaultMutableTreeNodeTypeAdapter負責將DefaultMutableTreeNode與JSON相互轉換。 它寫入/讀取其屬性allowsChildren ， userObject和children 。 無需編寫parent屬性，因為父子關系已經在JSON輸出的嵌套結構中進行了編碼。

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;
    }
}