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方法向iOS客户端传输)？

我试过：

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

它和StackOverflowError坠毁了。

Answer 1

Swing的DefaultMutableTreeNode类是一个类似树的数据结构，它包含与children和parent相同类型的实例。这就是为什么Gson的默认序列化程序会遇到无限递归，从而抛出一个StackOverflowError。

要解决这个问题，您需要使用专门为将Gson转换为JSON而精心编制的更智能的JsonSerializer来自定义DefaultMutableTreeNode。另外，您还可能希望提供一个JsonDeserializer，用于将此类JSON转换回DefaultMutableTreeNode。

为此，您的Gson实例不仅通过new Gson()创建，而且还通过

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

下面的DefaultMutableTreeNodeSerializer负责将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不确定的递归。

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，让我们将示例JTree的根节点序列化为JSON，然后再反序列化它。

​

​

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负责将JSON转换回DefaultMutableTreeNode。

它使用与来自How to serialize/deserialize a DefaultMutableTreeNode with Jackson?的反序列化器相同的思想。DefaultMutableTreeNode不是很像POJO，因此不能很好地与Gson一起工作。因此，它使用了一个行为良好的POJO助手类(带有属性allowsChildren、userObject和children)，并允许Gson将JSON内容反序列化到该类中。然后将POJO对象(及其POJO子对象)转换为DefaultMutableTreeNode对象(带有DefaultMutableTreeNode子对象)。

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

这是一个改进的替代方案，我以前的答案使用了JsonSerializer和JsonDeserializer的DefaultMutableTreeNode实现。这两个接口的API文档说：

新的应用程序应该更喜欢TypeAdapter，它的流API比这个接口的树API更有效。

因此，让我们使用这个首选方法，并为TypeAdapter实现一个DefaultMutableTreeNode。

为了使用它，您可以像这样创建Gson实例(而不是仅仅使用new Gson())：

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

下面的DefaultMutableTreeNodeTypeAdapter负责将DefaultMutableTreeNode与JSON之间的转换。它写入/读取其属性allowsChildren、userObject和children。不需要编写parent属性，因为父-子关系已经编码在JSON-输出的嵌套结构中。

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;
    }
}