如何从 Jackson 中的数组开始反序列化 JSON 文件？
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How to deserialize JSON file starting with an array in Hymanson?
提问by pixel
I have a Json file that looks like this:
我有一个看起来像这样的 Json 文件：
[
{ "field":"val" },
....
]
I have Java object representing single object and collection of them:
我有代表单个对象和它们的集合的 Java 对象：
public class Objects{
public Collection<List> myObject;
}
I would like to deserialize JSON using ObjectMapper.
我想使用ObjectMapper.
ObjectMapper mapper = new ObjectMapper();
mapper.readValue(in, Objects.class);
But I get:
但我得到：
11-24 23:19:19.828: W/UpdateService(6084): org.codehaus.Hymanson.map.JsonMappingException:
Can not deserialize instance of com.project.my.Objects out of START_ARRAY token
回答by dmon
Try
尝试
mapper.readValue(in, ObjectClass[].class);
Where ObjectClass is something like:
其中 ObjectClass 是这样的：
public class ObjectClass {
String field;
public ObjectClass() { }
public void setField(String value) {
this.field = value;
}
}
Note: in your posted version of the Objectsclass, you're declaring a Collection of Lists (i.e. a list of lists), which is not what you want. You probably wanted a List<ObjectClass>. However, it's much simpler to just do YourObject[].classwhen deserializing with Hymanson, and then converting into a list afterwards.
注意：在您发布的Objects课程版本中，您声明了一个列表集合（即列表列表），这不是您想要的。你可能想要一个List<ObjectClass>. 但是，YourObject[].class在使用 Hymanson 反序列化然后转换为列表时要简单得多。
回答by Han He
You can directly get a list through the following way:
您可以通过以下方式直接获取列表：
List<ObjectClass> objs =
mapper.readValue(in, new TypeReference<List<ObjectClass>>() {});
回答by StaxMan
Passing an array or List type works, as @dmon answered.
正如@dmon 回答的那样，传递数组或列表类型有效。
For sake of completeness, there is also an incremental approach if you wanted to read contents one-by-one:
为了完整起见，如果您想逐个阅读内容，还有一种增量方法：
Iterator<Objects> it = mapper.reader(Objects.class).readValues(in);
while (it.hasNext()) {
Objects next = it.next();
// ... process it
}
This is useful if you have huge lists or sequences of objects; either with enclosing JSON Array, or just root-level values separated by spaces or linefeeds.
如果您有大量的对象列表或序列，这很有用；要么带有封闭的 JSON 数组，要么只是由空格或换行符分隔的根级值。
