Spring messageSource only works as xml (not as Spring-Java-Config)

I have a working Spring MVC Project and want to migrate the application context configuration from xml to Java-Config. All works fine except messageSource Bean.

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Following works fine:

This config class gets imported by another config class:

package gmm;

import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.ImportResource;

@Configuration
@ImportResource({"classpath:applicationContext.xml"})
public class I18nConfiguration {

}

Referenced applicationContext.xml file:

<beans
    xmlns="http://ift.tt/GArMu6"
    xmlns:xsi="http://ift.tt/ra1lAU"
    xmlns:context="http://ift.tt/GArMu7"
    xsi:schemaLocation="
        http://ift.tt/GArMu6
        http://ift.tt/18sW2ax
        http://ift.tt/GArMu7
        http://ift.tt/1bGeTcI
">

    <bean id="messageSource"
        class="org.springframework.context.support.ResourceBundleMessageSource">
        <property name="basename">
            <value>messages</value>
        </property>
    </bean>

</beans>

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Following does not work:

Moved bean into java config:

package gmm;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.support.ResourceBundleMessageSource;

@Configuration
public class I18nConfiguration {

    public ResourceBundleMessageSource messageSource() {
        ResourceBundleMessageSource source = new ResourceBundleMessageSource();
        source.setBasename("messages");
        return source;
    }
}

When i use this java config, i get just the usual ???key.for.message??? stuff. Debugging output doesn't tell me something unusual.

I don't see what is wrong here. Is there some obvious error in my code? Please tell me even if you don't now the solution cause i feel like Im kind of dumb right now! This is supposed to be super easy isn't it?

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Edit: The message files are in src/main/resources and are named like messages_en.properties.