How to use opensaml v3? There is little to no documentation and v2 is EOL

Is there an end to end example of using Open SAML library v3? I can't find any documentation and as per https://wiki.shibboleth.net/confluence/display/OpenSAML/Home the v2 is EOL.

I'm using following code to get SAML assertion-

  private UnmarshallerFactory unmarshallerFactory;
  private DocumentBuilder docBuilder;

  @PostConstruct
  public void init() {

    try {
      InitializationService.initialize();
      DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();
      documentBuilderFactory.setNamespaceAware(true);
      docBuilder = documentBuilderFactory.newDocumentBuilder();
      unmarshallerFactory = XMLObjectProviderRegistrySupport.getUnmarshallerFactory();
    } catch (Exception e){
      logger.error("Error: ",e);
    }
  }

  public Assertion getSamlAssertion(String samlResponse)
      throws IOException, XMLParserException, UnmarshallingException, SAXException {

    Document document = docBuilder.parse(new StringInputStream(samlResponse));

    Element element = document.getDocumentElement();
    Unmarshaller unmarshaller = unmarshallerFactory.getUnmarshaller(element);
    XMLObject responseXmlObj = unmarshaller.unmarshall(element);
    Response response = (Response) responseXmlObj;
    return response.getAssertions().get(0);

  }

My POM-

         <dependency>
            <groupId>org.opensaml</groupId>
            <artifactId>opensaml-core</artifactId>
            <version>3.2.0</version>
        </dependency>


        <dependency>
            <groupId>org.opensaml</groupId>
            <artifactId>opensaml-saml-api</artifactId>
            <version>3.2.0</version>
        </dependency>

The problem is that I am getting null for unmarshaller . I've verified that the samlResponse is valid

Copyright Notice：Content Author:「gauravphoenix」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/40558996/how-to-use-opensaml-v3-there-is-little-to-no-documentation-and-v2-is-eol