PasswordAuthentication.Java

Login_User__c loginUserSetting = Login_User__c.getOrgDefaults();
String Session_Id;
If(loginUserSetting !=null){
	Organization orgInfo = [Select Id, Name, isSandbox From Organization LIMIT 1];
	String endPoint = '';
	If(orgInfo.isSandbox)
		endPoint = 'https://test.salesforce.com/services/oauth2/token';
	else
		endPoint = 'https://login.salesforce.com/services/oauth2/token';
	
	System.debug('#### endPoint '+endPoint);
	String requestBody = 'grant_type=password&client_id='+loginUserSetting.Client_Id__c+'&client_secret='+loginUserSetting.Client_Secret__c
							+'&username='+loginUserSetting.userName__c+'&password='+loginUserSetting.Password__c+loginUserSetting.Token__c;
	System.debug('#### requestBody '+requestBody);                        
	HttpRequest httpReq = new HttpRequest();
	HttpResponse httpRes = new HttpResponse();
	httpReq.SetEndPoint('https://test.salesforce.com/services/oauth2/token');
	httpReq.SetHeader('Content-Type','application/x-www-form-urlencoded');
	//httpReq.setTimeout(12000);
	httpReq.setMethod('POST');
	httpReq.setBody(requestBody);
	
	httpRes = new http().send(httpReq);
	System.debug('#### responseBody '+httpRes.getBody());
	IF(httpRes.getStatusCode() == 200){
		Map<String, Object> responseMap = (Map<String, Object>)JSON.deserializeUntyped(httpRes.getBody());
		String sessionId = (String)responseMap.get('access_token');
		Session_Id = sessionId;
	}
}