System.LimitException：当前不允许DML Class.Calloutcontroller.getperformcallout：第49行，第1列

Question

public class Calloutcontroller {
    List<Tutor__c> TutorList ;

    // public Calloutcontroller() {
    //  }
    //  public Calloutcontroller(Tutor controller) {
    //  }
    //public Calloutcontroller(ApexPages.StandardController controller) {

    public List<consolewrap> ConsoleWrapperList{get;set;}
    public List<consolewrap> getperformcallout(){
        TutorList = new List<Tutor__c>();
        ConsoleWrapperList = new List<consolewrap>();
        HttpRequest req = new HttpRequest();
        HttpResponse res = new HttpResponse();
        Http http = new Http();
        req.setEndpoint('http://www.itutorindia.com/angularjs/api/smallWidget/tutorHourSalesForce');
        //req.setEndpoint('http://52.24.111.14/angularjs/api/smallWidget/tutorHourSalesForce');
        req.setMethod('GET');
        req.setHeader('Content-Type', 'application/json');
        req.setHeader('Accept','application/json');
        res = http.send(req);
        if(res.getstatusCode() == 200 && res.getbody() != null){
            system.debug('Response body result is+++: ' + res.getBody());
            ConsoleWrapperList=(List<consolewrap>)json.deserialize(res.getbody(),List<consolewrap>.class);

            /* To save Aoi data in custom object tutor*/

            Tutor__c tutorObj;
            system.debug('test1++++++++');
            for (integer i = 0; i<ConsoleWrapperList.size(); i++)
            {
                system.debug('test2++++++++');
                tutorObj = new Tutor__c();
                tutorObj.Class_ID__c = Integer.valueOf(ConsoleWrapperList[i].class_id);
                tutorObj.Tutor_ID__c = Integer.valueOf(ConsoleWrapperList[i].tutor_id);
                tutorObj.Class_Time__c =date.ValueOf(ConsoleWrapperList[i].class_time);
                //system.debug('time is++++ '+tutorObj.Class_Time__c);
                //system.debug('Time values entered');
                tutorObj.Name = ConsoleWrapperList[i].tutor_name;
                tutorObj.E_Mail__c = ConsoleWrapperList[i].tutor_email;
                tutorObj.Tutor_Phone__c = ConsoleWrapperList[i].tutor_phone;
                TutorList.add(tutorObj);
               // system.debug('test3++++++++');
            }
            //system.debug('TutorList++++++++'+TutorList);
        }
        //system.debug('Time values entered222');
        upsert TutorList;
        system.debug('Time values entered+++++++');
        return ConsoleWrapperList;
    }
}

Wrapper class

    public class consolewrap {

    public String class_id{get;set;}

    public String class_time{get;set;}

    public String tutor_id{get;set;}

    public String tutor_name{get;set;}

    public String tutor_email{get;set;} 

    public String tutor_phone{get;set;}
}

VF PAGE

<apex:form >

<apex:pageBlock >

<apex:pageBlockTable value="{!performcallout}" var="wrap">

<apex:column headerValue="Class Id" value="{!wrap.class_id}"/>

<apex:column headerValue="Class Time" value="{!wrap.class_time}"/>

<apex:column headerValue="Tutor Id" value="{!wrap.tutor_id}"/>

<apex:column headerValue="Tutor Name" value="{!wrap.tutor_name}"/>

<apex:column headerValue="Tutor Email" value="{!wrap.tutor_email}"/>

<apex:column headerValue="Tutor Phone" value="{!wrap.tutor_phone}"/>

</apex:pageBlockTable>

</apex:pageBlock>

</apex:form>
</apex:page>