我不确定是否可以通过这种方式做到这一点。readTree
方法返回扩展的对象JsonNode
,在这种情况下它将是ObjectNode
. ObjectNode
不接受两个同名的属性,最后反序列化后它代表:
{"c":"c","as":"a2"}
之后你想将该节点转换为A
POJO
班级。默认解串器List
期望START_ARRAY
令牌不String
。您可以通过实现扩展的自定义转换器来使其工作StdConverter<String, List>
但列表将被修剪为一个元素。在这种情况下,我认为,你必须使用readValue
方法,因为你需要指示Jackson
that as
元素是展开的数组。
EDIT
在你的评论之后我意识到我们可以欺骗XmlMapper
使用我们想要的任何东西。很明显Jackson
uses JsonNodeDeserializer
反序列化JsonNode
-s。因此,我们需要做的就是找到一个可以注入代码的地方。幸好有一个方法_handleDuplicateField
它处理我们的案例。默认情况下,如果FAIL_ON_READING_DUP_TREE_KEY
标志已启用:
protected void _handleDuplicateField(JsonParser p, DeserializationContext ctxt,
JsonNodeFactory nodeFactory,
String fieldName, ObjectNode objectNode,
JsonNode oldValue, JsonNode newValue)
throws JsonProcessingException
{
// [databind#237]: Report an error if asked to do so:
if (ctxt.isEnabled(DeserializationFeature.FAIL_ON_READING_DUP_TREE_KEY)) {
ctxt.reportInputMismatch(JsonNode.class,
"Duplicate field '%s' for ObjectNode: not allowed when FAIL_ON_READING_DUP_TREE_KEY enabled",
fieldName);
}
}
那么,让我们利用这个事实来扩展这个类:
class MergeDuplicateFieldsJsonNodeDeserializer extends JsonNodeDeserializer {
@Override
protected void _handleDuplicateField(JsonParser p, DeserializationContext ctxt,
JsonNodeFactory nodeFactory, String fieldName, ObjectNode objectNode,
JsonNode oldValue, JsonNode newValue) throws JsonProcessingException {
super._handleDuplicateField(p, ctxt, nodeFactory, fieldName, objectNode, oldValue, newValue);
ArrayNode array;
if (oldValue instanceof ArrayNode) {
// Merge 3-rd, 4-th, ..., n-th element to already existed array
array = (ArrayNode) oldValue;
array.add(newValue);
} else {
// Merge first two elements
array = nodeFactory.arrayNode();
array.add(oldValue);
array.add(newValue);
}
objectNode.set(fieldName, array);
}
}
现在,我们需要注册这个反序列化器。整个测试可能如下所示:
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.deser.std.JsonNodeDeserializer;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.node.ArrayNode;
import com.fasterxml.jackson.databind.node.JsonNodeFactory;
import com.fasterxml.jackson.databind.node.ObjectNode;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlElementWrapper;
import java.util.Arrays;
import java.util.List;
public class XmlMapperApp {
public static void main(String[] args) throws Exception {
A a = new A();
a.c = "String";
a.as = Arrays.asList("1", "2", "tom", "Nick");
SimpleModule mergeDuplicatesModule = new SimpleModule("Merge duplicated fields in array");
mergeDuplicatesModule.addDeserializer(JsonNode.class, new MergeDuplicateFieldsJsonNodeDeserializer());
XmlMapper mapper = new XmlMapper();
mapper.registerModule(mergeDuplicatesModule);
String xml = mapper.writeValueAsString(a);
System.out.println(xml);
System.out.println(mapper.readTree(xml));
}
}
上面的代码打印:
<A><c>String</c><as>1</as><as>2</as><as>tom</as><as>Nick</as></A>
{"c":"String","as":["1","2","tom","Nick"]}