java - 如何将 DefaultMutableTreeNode (Java) 序列化为 JSON？

Question

如何将树(使用 DefaultMutableTreeNode 类在 Java 中实现)序列化为 JSON(用于通过 RESTful 方法传输到 iOS 客户端)？

我尝试过:

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

它因StackOverflowError而崩溃。

Answer 1

Swing 的 DefaultMutableTreeNode类是一种树状数据结构其中包含相同类型的实例作为 children 和 parent。这就是为什么 Gson 的默认序列化器会陷入无限递归因此引发了 StackOverflowError。

要解决这个问题，您需要使用更智能的 JsonSerializer 自定义您的 Gson专门用于将 DefaultMutableTreeNode 转换为 JSON。作为奖励，您可能还想提供 JsonDeserializer用于将此类 JSON 转换回 DefaultMutableTreeNode。

为此，创建您的 Gson 实例不仅仅是通过 new Gson()，而是通过

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

下面的DefaultMutableTreeNodeSerializer负责用于将 DefaultMutableTreeNode 转换为 JSON。它将其属性 allowsChildren、userObject 和 children 转换为 JSON。请注意，它不会将 parent 属性转换为 JSON，因为这样做会再次产生无限递归。

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;
    }
}

为了进行测试，我们将示例 JTree 的根节点序列化为 JSON，然后再次反序列化它。

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 输出:

{
  "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负责用于将 JSON 转换回 DefaultMutableTreeNode。

它使用与反序列化器相同的想法 How to serialize/deserialize a DefaultMutableTreeNode with Jackson? 。DefaultMutableTreeNode 不太像 POJO，因此不与Gson配合良好。因此，它使用一个行为良好的 POJO 帮助器类(具有属性allowsChildren、userObject 和 children)并让 Gson将 JSON 内容反序列化到此类中。然后，POJO 对象(及其 POJO 子对象)被转换为DefaultMutableTreeNode 对象(带有 DefaultMutableTreeNode 子级)。

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;
        }
    }
}