使用Java（Jackson）读取JSON中嵌套键的值
我是一名来自Python背景的新Java程序员。 我有收集/返回的天气数据作为带有嵌套键的JSON，我不明白在这种情况下如何拉出值。 我确定之前已经问过这个问题，但是我发誓我已经用Google搜索了很多内容，我似乎无法找到答案。 现在我正在使用json-simple，但我尝试切换到Jackson，但仍然无法弄清楚如何做到这一点。 由于Jackson / Gson似乎是最常用的库，我很乐意看到使用其中一个库的示例。 下面是数据样本，然后是我到目前为止编写的代码。
{ "response": { "features": { "history": 1 } }, "history": { "date": { "pretty": "April 13, 2010", "year": "2010", "mon": "04", "mday": "13", "hour": "12", "min": "00", "tzname": "America/Los_Angeles" }, ... } } 主function
public class Tester { public static void main(String args[]) throws MalformedURLException, IOException, ParseException { WundergroundAPI wu = new WundergroundAPI("*******60fedd095"); JSONObject json = wu.historical("San_Francisco", "CA", "20100413"); System.out.println(json.toString()); System.out.println(); //This only returns 1 level. Further .get() calls throw an exception System.out.println(json.get("history")); } }
函数’historical’调用另一个返回JSONObject的函数
public static JSONObject readJsonFromUrl(URL url) throws MalformedURLException, IOException, ParseException { InputStream inputStream = url.openStream(); try { JSONParser parser = new JSONParser(); BufferedReader buffReader = new BufferedReader(new InputStreamReader(inputStream, Charset.forName("UTF-8"))); String jsonText = readAll(buffReader); JSONObject json = (JSONObject) parser.parse(jsonText); return json; } finally { inputStream.close(); } }
使用Jackson的树模型（ JsonNode ），你有两个“文字”访问器方法（’get’），它为缺失值返回null ，以及“安全”访问器（’path’），它允许你遍历“丢失”节点。 所以，例如：
JsonNode root = mapper.readTree(inputSource); int h = root.path("response").path("history").getValueAsInt();
这将返回给定路径的值，或者，如果缺少路径，则为0（默认值）
但更方便的是，您可以使用JSON指针表达式：
int h = root.at("/response/history").getValueAsInt();
还有其他方法，通常将结构实际建模为Plain Old Java Object（POJO）更方便。 您的内容可能类似于：
public class Wrapper { public Response response; } public class Response { public Map features; // or maybe Map public List history; } public class HistoryItem { public MyDate date; // or just Map // ... and so forth }
如果是这样，您将像任何Java对象一样遍历结果对象。
查看Jackson的ObjectMapper。 您可以创建一个类来为JSON建模，然后使用ObjectMapper的readValue方法将您的JSON String“反序列化”为模型类的实例。 反之亦然。
使用Jsonpath
整数h = JsonPath.parse（json）.read（“$。response.repository.history”，Integer.class）;
试试jpath API。 它是JSON Data的xpath等价物。 您可以通过提供将遍历JSON数据并返回所请求值的jpath来读取数据。
这个Java类是实现，它有关于如何调用API的示例代码。
https://github.com/satyapaul/jpath/blob/master/JSONDataReader.java
自述 –
https://github.com/satyapaul/jpath/blob/master/README.md
例：
JSON数据：
{ "data": [{ "id": "13652355666_10154605514815667", "uid": "442637379090660", "userName": "fanffair", "userFullName": "fanffair", "userAction": "recommends", "pageid": "usatoday", "fanPageName": "USA TODAY", "description": "A missing Indonesian man was found inside a massive python on the island of Sulawesi, according to local authorities and news reports. ", "catid": "NewsAndMedia", "type": "link", "name": "Indonesian man swallowed whole by python", "picture": "https:\/\/external.xx.fbcdn.net\/safe_image.php?d=AQBQf3loH5-XP6hH&w=130&h=130&url=https%3A%2F%2Fwww.gannett-cdn.com%2F-mm-%2F1bb682d12cfc4d1c1423ac6202f4a4e2205298e7%2Fc%3D0-5-1821-1034%26r%3Dx633%26c%3D1200x630%2Flocal%2F-%2Fmedia%2F2017%2F03%2F29%2FUSATODAY%2FUSATODAY%2F636263764866290525-Screen-Shot-2017-03-29-at-9.27.47-AM.jpg&cfs=1&_nc_hash=AQDssV84Gt83dH2A", "full_picture": "https:\/\/external.xx.fbcdn.net\/safe_image.php?d=AQBQf3loH5-XP6hH&w=130&h=130&url=https%3A%2F%2Fwww.gannett-cdn.com%2F-mm-%2F1bb682d12cfc4d1c1423ac6202f4a4e2205298e7%2Fc%3D0-5-1821-1034%26r%3Dx633%26c%3D1200x630%2Flocal%2F-%2Fmedia%2F2017%2F03%2F29%2FUSATODAY%2FUSATODAY%2F636263764866290525-Screen-Shot-2017-03-29-at-9.27.47-AM.jpg&cfs=1&_nc_hash=AQDssV84Gt83dH2A", "message": "Akbar Salubiro was reported missing after he failed to return from harvesting palm oil.", "link": "http:\/\/www.usatoday.com\/story\/news\/nation-now\/2017\/03\/29\/missing-indonesian-man-swallowed-whole-reticulated-python\/99771300\/", "source": "", "likes": { "summary": { "total_count": "500" } }, "comments": { "summary": { "total_count": "61" } }, "shares": { "count": "4" } }] }
代码段：
String jPath = "/data[Array][1]/likes[Object]/summary[Object]/total_count[String]"; String value = JSONDataReader.getStringValue(jPath, jsonData);