public String getFilterExpression() {
        return getRequest().getFilterExpression();
    }
Any query filters will be ignored if a filter expression has been
specified. When a filter expression is specified, the corresponding
name-map and value-map can also be specified via
{@link #withNameMap(Map)} and {@link #withValueMap(Map)}.

@see ScanRequest#getFilterExpression()
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/document/spec/ScanSpec.java