I am integrating Google Drive API with salesforce. I have created 2 classes here, one is the controller class for the page and another one is helper class. As soon as my page gets loaded I can see access token(after entering the crendials) in debug log but, when I click on a disp button then it gets nulled. I dont understand why is this happening and how to resolve it. Following is my code:

Controller code:

public class GDrive {
	public static String AccessToken;

	public GDrive(){
		System.debug('GDrive constructor called ');
	}

	public PageReference authentication(){
		System.debug('Authentication called');
		PageReference pg = GoogleDriveHelper.DriveAuth();
		return pg;
	}

	public void Files(){
		System.debug('Gdrive access token in gd is '+GDrive.AccessToken);
		System.debug('Files called in GDrive');
		//GoogleDriveHelper.AccessToken();
		//list<String> fList = new list<String>();
		/*String fList = GoogleDriveHelper.displayFiles();
		System.debug('>>> l is ' + fList);
		//return fList;*/
	}
}

Helper class code:

public class GoogleDriveHelper {
	// Stores authorization code.
	private static String code;
    //Client ID.
    private static string key = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com';
    //Client secret key.
    private static string secret = 'xxxxxxxxxxxxxxxxxxxxxxxxxxx';
    // Url to be redirected when authentication is done.
    private static string redirect_uri = 'https://c.ap5.visual.force.com/apex/googleDrive';
    private static String access_token;
    public static list<String> files;
    public GoogleDriveHelper()
    {
    	System.debug('Constructor of Helper class called ');
    	/*//code = ApexPages.currentPage().getParameters().get('code');
    	System.debug('constructor for helper class called');
        
        //Get the access token once we have code
        if(code != '' && code != null)
        {
            AccessToken() ;
        }*/
    }
    
    public static PageReference DriveAuth()
    {
        //Authorization
        code = ApexPages.currentPage().getParameters().get('code');
        if(code != '' && code != null){
        	System.debug('<<>>>><');
        	GDrive.AccessToken = GoogleDriveHelper.AccessToken();
        	System.debug('GDrive.AccessToken is '+GDrive.AccessToken);
        	return null;
        }
        else{
        	PageReference pg = new PageReference(GoogleDriveAuthUri (key , redirect_uri)) ;
       		return pg;
        }
        
    }
    
    // To redirect to OAuth server, google drive api here.
    public static String GoogleDriveAuthUri(String Clientkey,String redirect_uri)
    {
    	System.debug('Entered authorization ');
        String key = EncodingUtil.urlEncode(Clientkey,'UTF-8');
        String uri = EncodingUtil.urlEncode(redirect_uri,'UTF-8');
        String authuri = '';
        // Call the Google OAuth 2.0 endpoint directly, you'll generate a URL and set the parameters on that URL.
        authuri = 'https://accounts.google.com/o/oauth2/auth?'+
        'client_id='+key+
        '&response_type=code'+
        '&scope=https://www.googleapis.com/auth/drive'+	//Scope to access data on drive. This gives full access
        '&redirect_uri='+uri+
        '&state=security_token%3D138r5719ru3e1%26url%3Dhttps://oa2cb.example.com/myHome&'+
        //'&login_hint=jsmith@example.com&'+
        'access_type=offline';	//Indicates whether your application can refresh access tokens when the user is not present at the browse
        return authuri;
    }
    
    
    public static String AccessToken()
    {
    	code = ApexPages.currentPage().getParameters().get('code');
        //Getting access token from google drive api app
        System.debug('code in access_token is ' + code);
        HttpRequest req = new HttpRequest();
        req.setMethod('POST');	//http method of POST is used.
        req.setEndpoint('https://accounts.google.com/o/oauth2/token');	//URL from which you have to extract resources.
        req.setHeader('content-type', 'application/x-www-form-urlencoded');	//file type:application, extension:x-www-form-urlencoded
        String messageBody = 'code='+code+'&client_id='+key+'&client_secret='+secret+'&redirect_uri='+redirect_uri+'&grant_type=authorization_code';
        req.setHeader('Content-length', String.valueOf(messageBody.length()));
        req.setBody(messageBody);
        req.setTimeout(60*1000);	//Time out is for 60 secs.

        Http h = new Http();
        String resp;
        HttpResponse res = h.send(req);
        System.debug('access token request is ' + res);
        resp = res.getBody();
        Map<String,object> m = (Map<String, Object>)Json.deserializeuntyped(resp);	// Stores JSON string values in a map
        access_token = (String)(m.get('access_token'));	//Retrives the value of access token from map and stores it.
        //We have to pass this access token everytime when a hit is made.
        System.debug(' access Token is ' + access_token);
        return access_token;
        
   	}

   	public static String displayFiles(){
   		System.debug('access_token is >>>> '+ access_token);
   		HttpRequest req = new HttpRequest();
   		req.setMethod('GET');	// Http method of get is used here.
   		// setEndPoint url parameters are passed 
   		req.setEndpoint('https://www.googleapis.com/drive/v3/files?corpora=user,allTeamDrives&includeTeamDriveItems=true&supportsTeamDrives=true');
   		// to pass access token.
   		System.debug('AccessToken in disp is ' + access_token);
   		req.setHeader('Authorization', 'Bearer ' + access_token);
   		req.setTimeout(60*1000);

   		Http h = new Http();
   		String resp;
   		HttpResponse res = h.send(req);
   		System.debug('Send request is ' + res);
   		resp = res.getBody();	// JSON string values are returned here.
   		Map<String,object> m = (Map<String, Object>)Json.deserializeuntyped(resp);
   		System.debug(m.size());
   		String str = (String)m.get('files.Name');
   		//files = str.split(',');
   		//System.debug('?????? '+files[0]);
   		System.debug(resp);
   		return str;
   	}
}

​