XML Validation with non local schemas

OK, I solved the problem on my own. In short I had to create a class which implements the interface LSResourceResolver and overrides the method resolveResource.

The long answer is:
First I created the class XsdReceiver and let it implement the interface LSResourceResolver. Also the method resolveResource must be overwritten now. This method is finally responsible for querying missing XSD files and returning them as resource LSInput. (Like LSResourceResolver, LSInput is an interface and must therefore be implemented as a separate class. I had simply called it Input). Finally I used the method setResourceResolver and instantiated as parameter the created class XsdReceiver.

Functionality:
The method isValid is supposed to check the XML schema for validity, among other things. The necessary schema file can be found in the root tag. It must also be passed manually to the SchemaFactory class. After the method is called and the main schema file is read, every referenced XML or XSD file is automatically passed to the method resolveResource. The SchemaFactory class does this for us.

Have fun :D

public boolean isValid(byte[] data) {
    if (this.getStamp() == null || this.getStamp().getChecker() == null)
        return false;
    else if (this.getStamp().getCrc().isEmpty())
        return false;

    try {
        SchemaFactory factory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);

        factory.setErrorHandler(new LineNumberErrorHandler());
        factory.setResourceResolver(new XsdReceiver());

        Schema schema = factory.newSchema(XsdReceiver.receive(this.schemaLocation));
        Validator validator = schema.newValidator();

        validator.setResourceResolver(factory.getResourceResolver());
        validator.validate(new StreamSource(new ByteArrayInputStream(data)));
    } catch (Exception e) {
        e.printStackTrace();
        return false;
    }

    return true;
}

public class XsdReceiver implements LSResourceResolver {
    public static Source receive(String url) {
        url = url.contains(" ") ? url.replace(" ", "/") : url;
        HttpURLConnection c = XsdReceiver.followRedirection(url);

        if (c == null)
            return null;

        try {
            return new StreamSource(c.getInputStream());
        } catch (IOException e) {
            return null;
        }
    }

    private static HttpURLConnection followRedirection(String url) {
        HttpURLConnection c = null;

        try {
            URL u = new URL(url);
            c = (HttpURLConnection)u.openConnection();
            int status = c.getResponseCode();

            if (status == HttpURLConnection.HTTP_MOVED_TEMP
                    || status == HttpURLConnection.HTTP_MOVED_PERM
                    || status == HttpURLConnection.HTTP_SEE_OTHER
            )
                c = (HttpURLConnection) new URL(c.getHeaderField("Location")).openConnection();
        } catch (IOException e) {
            return null;
        }

        return c;
    }

    @Override
    public LSInput resolveResource(String type, String namespaceURI, String publicId, String systemId, String baseURI) {
        if (namespaceURI == null || type.equals("http://www.w3.org/TR/REC-xml"))
            return null;

        if (systemId.equals("xml.xsd")) {
            try {
                File f = ResourceUtils.getFile("classpath:data/xml.xsd");

                return new Input(StandardCharsets.UTF_8.name(), new FileInputStream(f), null, systemId, publicId, namespaceURI, true, null);
            } catch (IOException e) {
                return null;
            }
        }

        String file = String.join("/", namespaceURI, systemId);
        HttpURLConnection connection;
        Input i = null;

        connection = XsdReceiver.followRedirection(file);

        if (connection == null)
            return null;

        try {
            i = new Input(StandardCharsets.UTF_8.name(), connection.getInputStream(), null, systemId, publicId, namespaceURI, true, null);
        } catch (IOException e) {
            return null;
        }

        return i;
    }
}

public class Input implements LSInput {
    private String encoding;
    private InputStream byteStream;
    private String stringData;
    private String systemId;
    private String publicId;
    private String baseUri;
    boolean certifiedText;
    private Reader characterStream;

    public Input(String encoding, InputStream byteStream, String stringData, String systemId, String publicId, String baseUri, boolean certifiedText, Reader characterStream) {
        this.encoding = encoding;
        this.byteStream = byteStream;
        this.stringData = stringData;
        this.systemId = systemId;
        this.publicId = publicId;
        this.baseUri = baseUri;
        this.certifiedText = certifiedText;
        this.characterStream = characterStream;
    }

    @Override
    public Reader getCharacterStream() {
        return this.characterStream;
    }

    @Override
    public void setCharacterStream(Reader characterStream) {
        this.characterStream = characterStream;
    }

    @Override
    public InputStream getByteStream() {
        return this.byteStream;
    }

    @Override
    public void setByteStream(InputStream byteStream) {
        this.byteStream = byteStream;
    }

    @Override
    public String getStringData() {
        return this.stringData;
    }

    @Override
    public void setStringData(String stringData) {
        this.stringData = stringData;
    }

    @Override
    public String getSystemId() {
        return this.systemId;
    }

    @Override
    public void setSystemId(String systemId) {
        this.systemId = systemId;
    }

    @Override
    public String getPublicId() {
        return this.publicId;
    }

    @Override
    public void setPublicId(String publicId) {
        this.publicId = publicId;
    }

    @Override
    public String getBaseURI() {
        return this.baseUri;
    }

    @Override
    public void setBaseURI(String baseURI) {
        this.baseUri = baseURI;
    }

    @Override
    public String getEncoding() {
        return this.encoding;
    }

    @Override
    public void setEncoding(String encoding) {
        this.encoding = encoding;
    }

    @Override
    public boolean getCertifiedText() {
        return this.certifiedText;
    }

    @Override
    public void setCertifiedText(boolean certifiedText) {
        this.certifiedText = certifiedText;
    }
}