@Override
    public void configure(SchemamappingConfig config) {
        if (config == null) {
            return;
        }
        fields.clear();
        for (Fieldmapping m: config.fieldmapping()) {
            SchemaMapKey key = new SchemaMapKey(m.chain(), m.docproc(), ("".equals(m.doctype())?null:m.doctype()), m.indocument());
            fields.put(key, m.inprocessor());
        }
    }
Map key. Doctype can be null, not the others.
docproc/src/main/java/com/yahoo/docproc/proxy/SchemaMap.java