在開發中，我們經常需要在前端將數據以樹形結構展現出來。而構造這樣的樹結構是一件很麻煩的事情。在Java中，通過構造JSON樹，我們可以很方便地在前端展現出樹形結構數據。下面就介紹一下如何構造JSON樹。

public static JsonNode buildTree(List<Node> nodeList) {
Map<String, Object> treeMap = nodeList.get(0).getAttributes();
treeMap.put("children", new ArrayList<>());
for (int i = 1; i < nodeList.size(); i++) {
Node node = nodeList.get(i);
recursivelyBuildTree(node, treeMap);
}
ObjectMapper mapper = new ObjectMapper();
String jsonString = mapper.writeValueAsString(treeMap);
return mapper.readTree(jsonString);
}
private static void recursivelyBuildTree(Node node, Map<String, Object> parentMap) {
String parentId = node.getParentId();
String currentId = node.getId();
Map<String, Object> currentMap = new HashMap<>(node.getAttributes());
currentMap.put("children", new ArrayList<>());
if (parentMap.containsKey(currentId)) {
((List) parentMap.get(currentId)).add(currentMap);
}
else {
recursivelyAddNode(parentId, currentMap, parentMap);
}
}
private static void recursivelyAddNode(String parentId, Map<String, Object> currentMap, Map<String, Object> parentMap) {
if (parentMap.containsKey(parentId)) {
List<Map<String, Object>> childrenList = (List<Map<String, Object>>) parentMap.get(parentId);
childrenList.add(currentMap);
}
else {
Node parentNode = getNodeById(parentId, nodeList);
Map<String, Object> parentNodeMap = new HashMap<>(parentNode.getAttributes());
parentNodeMap.put("children", new ArrayList<>());
recursivelyAddNode(parentNode.getParentId(), parentNodeMap, parentMap);
((List) parentNodeMap.get("children")).add(currentMap);
parentMap.put(parentId, parentNodeMap);
}
}
private static Node getNodeById(String nodeId, List<Node> nodeList) {
for (Node node : nodeList) {
if (node.getId().equals(nodeId)) {
return node;
}
}
return null;
}

在這段代碼中，我們構造了一個包含“children”鍵值對的樹形結構。然后使用遞歸的方式依次添加每個節點，并將結果以JSON字符串的形式返回。最終，將JSON字符串解析為JsonNode對象，返回給前端。