杰克逊:使用列表反序列化来映射XML

Question

有没有办法使用Jackson将以下xml反序列化为Map holding项目列表？

<order>
    <number>12345678</number>
    <amount>100.10</amount>
    <items>
        <item>
            <itemId>123</itemId>
            <amount>100.0</amount>
            <itemName>Item Name1</itemName>
        </item>
        <item>
            <itemId>234</itemId>
            <amount>200.00</amount>
            <itemName>Item Name1</itemName>
        </item>
    </items>
</order>

我试过了

XmlMapper mapper = new XmlMapper();
LinkedHashMap map = (LinkedHashMap)mapper.readValue(xml, Object.class);

并获得以下地图.列表中的第一项缺失.

{
    order={
        number=12345678,
        amount=100.1,
        items={
            item={
                amount=200.0,
                itemName=ItemName2,
                itemId=234
            }
        }
    }
}

Answer 1

这是jackson-dataformat-xml在issue 205下提交的已知错误。简而言之，XML 中的重复元素被当前的UntypedObjectDeserializer实现所吞噬。幸运的是，该报告的作者 ( João Paulo Varandas ) 还以自定义UntypedObjectDeserializer实现的形式提供了临时修复。下面我分享我对修复的解释：

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.deser.std.UntypedObjectDeserializer;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;

import javax.annotation.Nullable;
import java.io.IOException;
import java.util.*;

public enum JacksonDataformatXmlIssue205Fix {;

    public static void main(String[] args) throws IOException {
        String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
                "<items>\n" +
                "    <item><id>1</id></item>\n" +
                "    <item><id>2</id></item>\n" +
                "    <item><id>3</id></item>\n" +
                "</items>";
        SimpleModule module = new SimpleModule().addDeserializer(Object.class, Issue205FixedUntypedObjectDeserializer.getInstance());
        XmlMapper xmlMapper = (XmlMapper) new XmlMapper().registerModule(module);
        Object object = xmlMapper.readValue(xml, Object.class);
        System.out.println(object);     // {item=[{id=1}, {id=2}, {id=3}]}
    }

    @SuppressWarnings({ "deprecation", "serial" })
    public static class Issue205FixedUntypedObjectDeserializer extends UntypedObjectDeserializer {

        private static final Issue205FixedUntypedObjectDeserializer INSTANCE = new Issue205FixedUntypedObjectDeserializer();

        private Issue205FixedUntypedObjectDeserializer() {}

        public static Issue205FixedUntypedObjectDeserializer getInstance() {
            return INSTANCE;
        }

        @Override
        @SuppressWarnings({ "unchecked", "rawtypes" })
        protected Object mapObject(JsonParser parser, DeserializationContext context) throws IOException {

            // Read the first key.
            @Nullable String firstKey;
            JsonToken token = parser.getCurrentToken();
            if (token == JsonToken.START_OBJECT) {
                firstKey = parser.nextFieldName();
            } else if (token == JsonToken.FIELD_NAME) {
                firstKey = parser.getCurrentName();
            } else {
                if (token != JsonToken.END_OBJECT) {
                    throw context.mappingException(handledType(), parser.getCurrentToken());
                }
                return Collections.emptyMap();
            }

            // Populate entries.
            Map<String, Object> valueByKey = new LinkedHashMap<>();
            String nextKey = firstKey;
            do {

                // Read the next value.
                parser.nextToken();
                Object nextValue = deserialize(parser, context);

                // Key conflict? Combine existing and current entries into a list.
                if (valueByKey.containsKey(nextKey)) {
                    Object existingValue = valueByKey.get(nextKey);
                    if (existingValue instanceof List) {
                        List<Object> values = (List<Object>) existingValue;
                        values.add(nextValue);
                    } else {
                        List<Object> values = new ArrayList<>();
                        values.add(existingValue);
                        values.add(nextValue);
                        valueByKey.put(nextKey, values);
                    }
                }

                // New key? Put into the map.
                else {
                    valueByKey.put(nextKey, nextValue);
                }

            } while ((nextKey = parser.nextFieldName()) != null);

            // Ship back the collected entries.
            return valueByKey;

        }

    }

}

Answer 2

通过扩展 UntypedObjectDeserializer 创建自定义反序列化器来完成此工作。