使用 Jackson 转换 json 问题 ，请叫高手指导。 实体类里面 有 <M extends TreeEntity> Collection<M> getChildren() {}这种类型的时候 转换报错！

[forbreak]

使用 Jackson转换 json ，实体类里面含有 <M extends TreeEntity> Collection 类型的变量，进行字符串转化对象的时候报错。

实体类如下：

@JsonIgnoreProperties(ignoreUnknown = true)
public class OrgTree {

protected Collection<? extends OrgTree> children = new HashSet<OrgTree>();

@Override
public <M extends OrgTree> Collection<M> getChildren() {
    return super.getChildren();
}

@Override
public <M extends OrgTree> void setChildren(Collection<M> children) {
    super.setChildren(children);
}
}

</code>
</pre>
json 配置处理如下

private static ObjectMapper objectMapper = new ObjectMapper();

static {
    objectMapper = new ObjectMapper();
    Hibernate4Module hibernateModule = new Hibernate4Module();
    hibernateModule.configure(Hibernate4Module.Feature.FORCE_LAZY_LOADING, false);
    objectMapper.registerModule(hibernateModule);


    objectMapper.configure(SerializationFeature.INDENT_OUTPUT, true);
    objectMapper.configure(SerializationFeature.WRITE_NULL_MAP_VALUES, false);
    objectMapper.configure(SerializationFeature.WRITE_EMPTY_JSON_ARRAYS, false);
    objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
    objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false); // 允许序列化空的对象,比如Object

    objectMapper.configure(JsonParser.Feature.ALLOW_SINGLE_QUOTES, true);
    objectMapper.configure(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES, true);
    objectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));

    objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);


}

调用

objectMapper.readValue(json,OrgTree.class);
 //尝试过多种方式 调用 如
 readValue(json,new TypeReference<OrgTree>(){});
 readValue(json,new TypeReference<Collection<OrgTree>>(){});
 readValue(json,new TypeReference<Collection<? extends OrgTree>>(){});

错误信息 如下：

com.fasterxml.jackson.databind.JsonMappingException: Type variable 'M' can not be resolved (with context of class com.biz.framework.domain.OrgTree)
        at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCache2(DeserializerCache.java:269)
        at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCacheValueDeserializer(DeserializerCache.java:244)
        at com.fasterxml.jackson.databind.deser.DeserializerCache.findValueDeserializer(DeserializerCache.java:143)
        at com.fasterxml.jackson.databind.DeserializationContext.findContextualValueDeserializer(DeserializationContext.java:325)
        at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.createContextual(CollectionDeserializer.java:147)
        at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.createContextual(CollectionDeserializer.java:23)
        at com.fasterxml.jackson.databind.DeserializationContext.findRootValueDeserializer(DeserializationContext.java:348)
        at com.fasterxml.jackson.databind.ObjectMapper._findRootDeserializer(ObjectMapper.java:2899)
        at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2793)
        at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1950)
        at com.biz.framework.json.JSON.toObject(JSON.java:112)

[sorra]

把TypeReference换成TypeFactory.constructCollectionType(collectionClass, elementClass)
因为泛型被擦除了，所以必须告诉Jackson你的元素类型M到底是啥具体类型。
必须知道具体类型才能做JSON反序列化。