Jackson 3 (rc6) deserialization of `Locale` (array) fails

[cowtowncoder]

Discussed in #5230

^{Originally posted by tsegall July 22, 2025}
I get the following exception in Jackson 3 (rc6)(the same program works in Jackson 2)

Exception in thread "main" tools.jackson.databind.DatabindException: Cannot cast [Ljava.util.Locale; to java.util.Locale
at [Source: (String)"{"locale":"en-US","locales":["en-US"]}"; line: 1, column: 38] (through reference chain: j3bug.DateTimeParserConfig["locales"])
at tools.jackson.databind.DatabindException.wrapWithPath(DatabindException.java:111)
at tools.jackson.databind.deser.bean.BeanDeserializerBase.wrapAndThrow(BeanDeserializerBase.java:1850)
at tools.jackson.databind.deser.bean.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:534)
at tools.jackson.databind.deser.bean.BeanDeserializer.deserialize(BeanDeserializer.java:200)
at tools.jackson.databind.deser.DeserializationContextExt.readRootValue(DeserializationContextExt.java:265)
at tools.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2688)
at tools.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1600)
at j3bug.J3bug.main(J3bug.java:15)
Caused by: java.lang.ClassCastException: Cannot cast [Ljava.util.Locale; to java.util.Locale
at java.base/java.lang.Class.cast(Class.java:3889)
at tools.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:132)
at tools.jackson.databind.deser.bean.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:532)
... 5 more

Class I am attempting to serialize is here:

package j3bug;

import java.util.Locale;

public class DateTimeParserConfig {
        public Locale[] locales;
        private Locale locale;

        public Locale[] getLocales() {
                return locales;
        }

        public void setLocales(final Locale... locales) {
                this.locales = locales;
                if (locales != null && locales.length == 1)
                        this.locale = this.locales[0];
        }

        protected void setLocale(final Locale locale) {
                this.locale = locale;
        }

        public Locale getLocale() {
                if (locale != null)
                        return locale;

                locales = new Locale[] { Locale.getDefault() };

                return locales.length == 1 ? locales[0] : null;
        }
}

Driver is here:

package j3bug;

import java.util.Locale;

import tools.jackson.core.StreamReadFeature;
import tools.jackson.databind.ObjectMapper;
import tools.jackson.databind.json.JsonMapper;

public abstract class J3bug {
                
        public static void main(String[] args) {
                ObjectMapper mapper = JsonMapper.builder().enable(StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION).build();
                DateTimeParserConfig cfg = new DateTimeParserConfig();
                String serialized = mapper.writeValueAsString(cfg);
                mapper.readValue(serialized, DateTimeParserConfig.class);
        }               
}

Any clues appreciated.