杰克逊：忽略空@XmlWrapperElement集合中的空格 [英] Jackson: Ignore whitespace in empty @XmlWrapperElement collection

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注释的集合可能没有元素，但XML包含空格（在我的例子中是一个换行符） 。 Jackson在此内容上抛出JsonMappingException，并显示消息无法从VALUE_STRING标记中反序列化java.util.ArrayList的实例。我无法改变XML的生成方式。

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>\n</inners></outer>";
Outer outer = mapper.readValue(xml, Outer.class);

以下变通办法不起作用：

The following workarounds do not work:

1. 启用 DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY ：在这种情况下，Jackson想要实例化 Inner 使用空格作为内容。


2. 为字符串和集合类型创建此字段的setter。在这种情况下，我得到一个JsonMappingException（属性冲突的setter定义inners）。


3. 在类似Stackoverflow问题建议将Jackson降级到2.2.3。这对我来说无法解决问题。

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.

有任何建议吗？

编辑：我可以通过包装CollectionDeserializer并检查空白标记来解决此问题。然而，这看起来非常脆弱，例如我不得不重写另一个方法来重新包装对象。我可以发布变通方法，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 返回包含空格的标记的空集合并注册新的反序列化程序。我将代码放入模块中，因此可以轻松注册：

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);
                }
            }
        });
    }

}

之后你可以将它添加到你的 ObjectMapper 是这样的：

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

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

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