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.
Example:
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);
The following workarounds do not work:
DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY
: In this case Jackson wants to instantiate a bogus instance of Inner
using the whitespace as content.Any suggestions?
Edit: 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.
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);
}
}
});
}
}
After that you can add it to your ObjectMapper
like this:
ObjectMapper mapper = new XmlMapper().registerModule(new XmlWhitespaceModule());
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