英文:
com.fasterxml.jackson.databind.exc.MismatchedInputException: Can not deserialize instance of object out of START_ARRAY token - JAVA
问题
得到了 MismatchedInputException 异常。在这里搜索了很多问题，但还没有找到解决方法。
代码：
import /path/to/file/Bars;List<Bars> barResults = null;public boolean validateData() throws IOException {boolean flag = false;try {if (Data.read() != -1) {BufferedReader reader = new BufferedReader(new InputStreamReader(Data));String line;while ((line = reader.readLine()) != null) {line = "[{" + line;System.out.println(line);ObjectMapper om = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);Bars ms = om.readValue(line, Bars.class);System.out.println(ms);break;}reader.close();}} catch (IOException e) {e.printStackTrace();}return flag;}
Json: // 简化示例
[{"createdOn":1601058721310,"lastUpdated":null,"lastUpdatedBy":null,"createdBy":null,"appId":null,"logical":"N","calculationDateTime":1601058721310,"mtaVersionNumber":null,"storageRegionName":"texas","createdOnDate":1601058721310,"lastUpdatedDate":0}]
输出：
[{"createdOn":1601058721310,"lastUpdated":null,"lastUpdatedBy":null,"createdBy":null,"appId":null,"logical":"N","calculationDateTime":1601058721310,"mtaVersionNumber":null,"storageRegionName":"texas","createdOnDate":1601058721310,"lastUpdatedDate":0}]com.fasterxml.jackson.databind.exc.MismatchedInputException: 无法从 START_ARRAY 令牌反序列化 `object` 实例位于 [Source: (StringReader); line: 1, column: 1]
我不确定是什么导致了这个异常。当我启动应用程序时，它可以正常读取 JSON，但会抛出异常。
英文:
Getting the MismatchedInputException. Searched a lot of questions here but havent found a solution yet.
Code:
import /path/to/file/Bars;List<Bars> barResults = null;public boolean validateData() throws IOException {boolean flag = false;try {if (Data.read() != -1) {BufferedReader reader = new BufferedReader(new InputStreamReader(Data));String line;while ((line = reader.readLine()) != null) {line = "[{" + line;System.out.println(line);ObjectMapper om = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);Bars ms = om.readValue(line, Bars.class);System.out.println(ms);break;}reader.close();}} catch (IOException e) {e.printStackTrace();}return flag;}
Json: //shorten for example
[{"createdOn":1601058721310,"lastUpdated":null,"lastUpdatedBy":null,"createdBy":null,"appId":null,"logical":"N","calculationDateTime":1601058721310,"mtaVersionNumber":null,"storageRegionName":"texas","createdOnDate":1601058721310,"lastUpdatedDate":0}]
Output:
[{"createdOn":1601058721310,"lastUpdated":null,"lastUpdatedBy":null,"createdBy":null,"appId":null,"logical":"N","calculationDateTime":1601058721310,"mtaVersionNumber":null,"storageRegionName":"texas","createdOnDate":1601058721310,"lastUpdatedDate":0}]com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `object` out of START_ARRAY tokenat [Source: (StringReader); line: 1, column: 1]
I am not sure what is causing this exception. When I start the application, i runs and reads the JSON fine..but throws exception.
答案1
得分: 1
在你调用 readValue 方法时，你将 Bars.class 作为第二个参数传递，这告诉 Jackson 第一个参数 (line) 是一个 Bars 实例的 JSON 表示，它应该返回这个实例。
JSON 对象以 { 开始，因为你要求 Jackson 反序列化一个对象，所以它期望输入以 { 开头。但是你传递的 JSON，即 line，并不是一个 Bars 实例，它是一个包含 Bars 实例的数组，并且以 [ 开头。
因此，它会抛出一个错误消息，说“我被告知这里应该是一个对象，但我找到了一个数组的开头”。
要修复这个问题，你可以将 readValue 的第二个参数更改为 Bars[].class，这样你可以请求 Jackson 反序列化一个 "Bars" 对象的数组，然后从数组中提取 bar 实例。或者你可以停止在 line 的开头添加 [，并去掉结尾的 ]，这样它就变成了一个单独的对象，而不是包含单个对象的数组。
英文:
In your call to readValue you are passing Bars.class as the second argument, which tells Jackson that the first argument (line) is a JSON representation of a Bars instance and that's what it should return.
JSON objects start with a {, and because you've asked Jackson to deserialize an object, it expects the input to start with a {. But the JSON that you're passing in, line, isn't a Bars instance: it's an array containing a Bars instance, and it starts with a [.
So it throws an error message that says "I was told an object would be here, but instead I found the start of an array".
To fix it, you can either ask Jackson to deserialize an array of "Bar" objects by changing the second argument of readValue to Bars[].class and then extract the bar instance from the array, or you could stop adding a "[" to the start of the line and chop the "]" off the end of it so that it's just a single object and not an array containing that single object.
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