Access Token

Is there Any way to generate Access Token without Authorization Code. Actually I was Implementing it in Salesforce without using Visual force Page.... And what is Expiration of the Access Token... Here is my  code

 

public class DropboxController
{
//Fetched from URL
String code;
String accesstoken;
public DropboxController() {
code = ApexPages.currentPage().getParameters().get('code');
//Get the access token once we have code
if(code != '' && code != null) {
AccessToken();
}
}

public PageReference DropAuth() {
//Authenticating
PageReference pg = new PageReference('https://www.dropbox.com/1/oauth2/authorize?response_type=code&client_id=<REDACTED>&redirect_uri=https://c.ap1.visual.force.com/apex/DropboxPage&state=Mytesting');
return pg;
}

public String AccessToken() {
//Getting access token from dropbox
String tokenuri = 'https://api.dropbox.com/1/oauth2/token?grant_type=authorization_code&code='+code+'&redirect_uri=https://c.ap1.visual.force.com/apex/DropboxPage';
HttpRequest req = new HttpRequest();
req.setEndpoint(tokenuri);
req.setMethod('POST');
req.setTimeout(60*1000);

Blob headerValue = Blob.valueOf('<REDACTED>' + ':' + '<REDACTED>');
String authorizationHeader = 'Basic ' + EncodingUtil.base64Encode(headerValue);
req.setHeader('Authorization', authorizationHeader);
Http h = new Http();
String resp;
HttpResponse res = h.send(req);
resp = res.getBody();

JSONParser parser = JSON.createParser(resp);
while (parser.nextToken() != null) {
if ((parser.getCurrentToken() == JSONToken.FIELD_NAME)) {
String fieldName = parser.getText();
parser.nextToken();
if(fieldName == 'access_token') {
accesstoken = parser.getText();
}
}
}
//system.debug('accessToken = '+accessToken );
//System.debug(' You can parse the response to get the access token :: ' + resp);
string txtdata='';
try {
string data = 'https://content.dropboxapi.com/2/files/download';
HttpRequest re = new HttpRequest();
re.setEndpoint(data);
re.setHeader('Authorization' , 'Bearer ' +accesstoken);
re.setHeader('Dropbox-API-Arg', '{ "path":"/Test.txt" }');
re.setMethod('POST');
re.setTimeout(60000);
Http h2 = new Http();
HttpResponse res2 = h2.send(re);
system.debug('body----->'+res2.getBody());
txtdata = res2.getBody();
}
catch(exception e) {
system.debug('error-----'+e.getMessage()+'line number--------->'+e.getLineNumber());
}
return txtdata;
}