Jackson XML Annotations: String element with attribute

You should use JacksonXmlText annotation for value field.

public class Element2 
{
    @JacksonXmlProperty(isAttribute = true)
    private String ns = "yyy";
    @JacksonXmlText
    private String value;

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }
}  

then XML will looks like

<Root>
    <Element1 ns="xxx">
        <Element2 ns="yyy">A String</Element2>
    </Element1>
</Root>

Unfortunately I'm unable to comment but I did want to make a note about flyingAssistant's answer in case anyone else ran into the same issue I was having. You can't add @JacksonXmlText to a constructor property. This feature may be added in build 2.13 based off of this issue reported in the GitHub repo. So for now you'll have to do this

data class Element2(@field:JacksonXmlProperty(isAttribute = true) val ns: String = "yyy") {
    @field:JacksonXmlText
    val value: String? = null
}

For Kotlin, you need to use @field annotation use-site targets:

data class Element2(
        @field:JacksonXmlProperty(isAttribute = true)
        val ns: String = "yyy",
        @field:JacksonXmlText
        val value: String? = null
)

If you don't like defining initial values for ns and value properties by yourself then use Kotlin no-args plugin, which generates a default constructor instead.