The annotation XmlSchemaType defines a mapping between an arbitrary Java type and a simple schema built-in type. Most of the time the default mapping is satisfactory, but every now and then an alternative may be more convenient. Let's assume that the processing of chunks of text requires their extension, either after unmarshalling or before the emitting marshalling. For this, a StringBuffer is better than String, which is the default mapping for xs:string. Below are the essential Java classes, one defining TextType as a container for a string, and the type adapter class for the simple conversion between String and StringBuffer. Notice that the latter class is specified in a separate annotation, i.e., XmlJavaTypeAdapter.

public class TextType {
    @XmlElement
    @XmlSchemaType(name="string")
    @XmlJavaTypeAdapter( String2StrBuf.class )
    public StringBuffer strbuf;
}

public class String2StrBuf
    extends XmlAdapter<String,StringBuffer> {
    @Override
    public String marshal( StringBuffer strbuf ){
        return strbuf.toString();
    }
    @Override
    public StringBuffer unmarshal( String string ){
        return new StringBuffer( string );
    }
}

TextType text = new TextType();
text.strbuf = new StringBuffer( "This is the house" );
// ...
text.strbuf.append( " that Jack built." );

Such a type mapping can be defined either for an individual element or for all occurrences within a package. If you need multiple mappings at package level, you'll have to bundle the XmlSchemaType annotations in an XmlSchemaTypes (note the plural) annotation, and the XmlJavaTypeAdapter annotiations are packed into a single XmlJavaTypeAdapters annotation.