static public AutoPullMode fromString(String val) {
        String valNorm = val.toLowerCase();
        for (AutoPullMode mode : values()) {
            if (mode.values.contains(valNorm)) {
                return mode;
            }
        }
        throw new IllegalArgumentException("Invalid auto-pull mode " + val + ". Please use 'on', 'off', 'once' or 'always'.");
    }
Enum holding the possible values avalaible for auto-pulling.

@author roland
@since 01/03/15
src/main/java/io/fabric8/maven/docker/util/AutoPullMode.java