Java Jackson：反序列化复杂的多态对象模型：JsonMappingException：意外的令牌（START_OBJECT），预期的VALUE_STRING

问题描述

我有这个对象树

A

B延伸A

C延伸B

D延伸B

E延伸C

F延伸A并且有一个参考A

F extends A and has one reference to A

A具有以下注释

@JsonTypeInfo（use = JsonTypeInfo.Id.CLASS，include = JsonTypeInfo.As.PROPERTY，property =@ class）

如果我尝试反序列化扩展A的JSON对象数组，则会抛出以下错误

If i try to deserialize a JSON array of objects that extends A, it throws the following error

json字符串由set的toString（）方法生成，该set参数化为A类，其中A在JSON中使用以下代码序列化：

The json string is generated by toString() method of a set and the set is parametric to type A where A is serialized in JSON with the following code:

ObjectMapper objectMapper=new ObjectMapper();
        objectMapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_CONCRETE_AND_ARRAYS);
        String res="";
        try {
            res = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(t);
        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return res;

反序列化json数组的代码（即上述集合）是：

The code to deserialize the json array (that is the set described above) is:

ObjectMapper mapper = new ObjectMapper();

        mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_CONCRETE_AND_ARRAYS);
        Collection<T> results=null;
        try {
            results =  mapper.readValue(json, TypeFactory.defaultInstance().constructParametricType(Collection.class, clazz ) );
        } catch (JsonParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (JsonMappingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return results;

它解析的json示例如下：

The json sample that it parses is like:

"[{
  "@class" : "pack1.pack2.MyClass",
  "id" : null,
  "f1" : "",
  "f2" : 0.9933817827,
  "f3" : 6.883261E-4,
  "f4" : 0.001375699,
  "f5" : {
    "@class" : "pack1.pack2.MyClass2",
    "id" : null,
    "f1" : "",
    "f2" : 0.0,
    "f3" : 0.0,
    "f4" : 0.0,
    "f5" : [ "java.util.HashSet", [ 0 ] ],
    "f6" : [ "java.util.HashSet", [ 2 ] ],
    "f7" : [ "java.util.ArrayList", [ "scelta", "brani", "buona" ] ],
    "f8" : [ null, "NOM", null ],
    "f9" : false
  },
  "f10" : [ "java.util.HashMap", {
    "2" : "ADJ"
  } ],
  "f11" : [ "java.util.HashSet", [ 0 ] ],
  "f12" : [ "java.util.HashSet", [ 2 ] ],
  "f13" : [ "java.util.ArrayList", [ "scelta", "brani", "buona" ] ],
  "featureIndicator" : false
}]"

这里json字符串只包含我的java集样本的一些对象

Here the json string includes only some objects of my sample of java Set

推荐答案

我认为问题在于默认输入。 JSON的开头并不像杰克逊所期望的默认输入那样生成。 JSON的开头应该是：

I believe the problem is with the default typing. The start of your JSON is not generated as what Jackson expect with default typing. The begining of the JSON should be:

["java.util.HashSet", [{

并且结尾应该有一个额外的结束括号}]] 。

and the end should have an extra closing bracket }]].

这是因为你使用你的集合的 toString（）方法生成JSON。您应该使用 ObjectMapper ，使用默认类型进行配置，如下所示：

That's because you generate the JSON using the toString() method of your set. You should instead use the ObjectMapper, that's configured with default typing, like so:

res = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(yourSet);

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