Jackson:忽略空 @XmlWrapperElement 集合中的空格

问题描述

使用 Jackson 和 jackson-dataformat-xml 2.4.4，我试图反序列化一个 XML 文档，其中用 @XmlWrapperElement 注释的集合可能有零个元素，但 XML 包含空格(在我的情况下是换行符).Jackson 对此内容抛出 JsonMappingException 并显示消息无法从 VALUE_STRING 令牌中反序列化 java.util.ArrayList 的实例".我无法更改生成 XML 的方式.

Using Jackson and jackson-dataformat-xml 2.4.4, I'm trying to deserialize a XML document where a collection annotated with @XmlWrapperElement may have zero elements, but where the XML contains whitespace (in my case a line break). Jackson throws a JsonMappingException on this content with the message "Can not deserialize instance of java.util.ArrayList out of VALUE_STRING token". I cannot change the way the XML is produced.

示例:

static class Outer {
    @XmlElementWrapper
    List<Inner> inners;
}

static class Inner {
    @XmlValue
    String foo;
}
ObjectMapper mapper = new XmlMapper().registerModules(new JaxbAnnotationModule());
String xml = "<outer><inners>
</inners></outer>";
Outer outer = mapper.readValue(xml, Outer.class);

以下解决方法不起作用:

The following workarounds do not work:

1. 启用 DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY:在这种情况下，Jackson 想要使用空格作为内容实例化 Inner 的虚假实例.
2. 为该字段为字符串和集合类型创建设置器.在这种情况下，我得到一个 JsonMappingException(冲突的属性内部"的 setter 定义").
3. 在类似的 Stackoverflow 问题中，建议将 Jackson 降级到 2.2.3.这不能解决我的问题.

1. Enabling DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY: In this case Jackson wants to instantiate a bogus instance of Inner using the whitespace as content.
2. Creating setters for this field for both String and the collection type. In this case I get a JsonMappingException ("Conflicting setter definitions for property "inners"").
3. In a similar Stackoverflow question it is suggested to downgrade Jackson to 2.2.3. This does not fix the problem for me.

有什么建议吗?

我可以通过包装 CollectionDeserializer 并检查空白标记来解决这个问题.然而，这对我来说看起来非常脆弱，例如我不得不重写另一个方法来重新包装对象.我可以发布解决方法，但更简洁的方法会更好.

I can work around this issue by wrapping the CollectionDeserializer and checking for a whitespace token. This looks however very fragile to me, e.g. I had to override another method to rewrap the object. I can post the workaround, but a cleaner approach would be better.

推荐答案

此问题的解决方法是包装标准 CollectionDeserializer 以返回包含空格的标记的空集合并注册新的 Deserializer.我把代码放到一个Module中，这样就可以很容易地注册了:

A workaround for this problem is to wrap the standard CollectionDeserializer to return an empty collection for tokens containing whitespace and register the new Deserializer. I put the code into a Module so it can be registered easily:

import java.io.IOException;
import java.util.Collection;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.BeanProperty;
import com.fasterxml.jackson.databind.DeserializationConfig;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.deser.BeanDeserializerModifier;
import com.fasterxml.jackson.databind.deser.std.CollectionDeserializer;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.type.CollectionType;


public class XmlWhitespaceModule extends SimpleModule {
    private static class CustomizedCollectionDeserialiser extends CollectionDeserializer {

        public CustomizedCollectionDeserialiser(CollectionDeserializer src) {
            super(src);
        }

        private static final long serialVersionUID = 1L;

        @SuppressWarnings("unchecked")
        @Override
        public Collection<Object> deserialize(JsonParser jp, DeserializationContext ctxt)
                throws IOException, JsonProcessingException {
            if (jp.getCurrentToken() == JsonToken.VALUE_STRING
                    && jp.getText().matches("^[\r\n\t ]+$")) {
                return (Collection<Object>) _valueInstantiator.createUsingDefault(ctxt);
            }
            return super.deserialize(jp, ctxt);
        }

        @Override
        public CollectionDeserializer createContextual(DeserializationContext ctxt,
                BeanProperty property) throws JsonMappingException {
            return new CustomizedCollectionDeserialiser(super.createContextual(ctxt, property));
        }
    }

    private static final long serialVersionUID = 1L;

    @Override
    public void setupModule(SetupContext context) {
        super.setupModule(context);
        context.addBeanDeserializerModifier(new BeanDeserializerModifier() {
            @Override
            public JsonDeserializer<?> modifyCollectionDeserializer(
                    DeserializationConfig config, CollectionType type,
                    BeanDescription beanDesc, JsonDeserializer<?> deserializer) {
                if (deserializer instanceof CollectionDeserializer) {
                    return new CustomizedCollectionDeserialiser(
                        (CollectionDeserializer) deserializer);
                } else {
                    return super.modifyCollectionDeserializer(config, type, beanDesc,
                        deserializer);
                }
            }
        });
    }

}

之后你可以像这样将它添加到你的ObjectMapper:

After that you can add it to your ObjectMapper like this:

ObjectMapper mapper = new XmlMapper().registerModule(new XmlWhitespaceModule());

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