I'm trying to sign an invoice for the spanish administration using xades4j. They provide an online checker for the signed xml and just one step of the overall check is failing:
Step: Checks if the signature policy is correct
Result: Unknown policy self:policy/general
Other steps like integrity and certificate status are ok.
My guess is self:policy/general is some kind of placeholder in the xades4j library that I should somehow override?
Right now I'm using this code for the policy
SignaturePolicyInfoProvider policyInfoProvider = new SignaturePolicyInfoProvider()
{
    String FACTURAE_URL = "http://www.facturae.es/politica_de_firma_formato_facturae/politica_de_firma_formato_facturae_v3_1.pdf";
    public SignaturePolicyBase getSignaturePolicy()
    {                       
        InputStream is = null;
        try {
            URL url = new URL(FACTURAE_URL);
            URLConnection conn = url.openConnection();
            is = conn.getInputStream();
        } catch (IOException e) {
            e.printStackTrace();
        }
        SignaturePolicyBase base = new SignaturePolicyIdentifierProperty(new ObjectIdentifier(FACTURAE_URL),is);
        return base;
    }
};
The requirements are in this online document (spanish) https://www.facturae.gob.es/politica_de_firma_formato_facturae/politica_de_firma_formato_facturae_v3_1.pdf
My xml output seems to comply:
            <xades:SignaturePolicyIdentifier>
                <xades:SignaturePolicyId>
                    <xades:SigPolicyId>
                        <xades:Identifier>http://www.facturae.es/politica_de_firma_formato_facturae/politica_de_firma_formato_facturae_v3_1.pdf</xades:Identifier>
                    </xades:SigPolicyId>
                    <xades:SigPolicyHash>
                        <ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
                        <ds:DigestValue>2jmj7l5rSw0yVb/vlWAYkK/YBwk=</ds:DigestValue>
                    </xades:SigPolicyHash>
                </xades:SignaturePolicyId>
            </xades:SignaturePolicyIdentifier>