[SalesForce] What can cause “System.CalloutException: You have uncommitted work pending. Please commit or rollback before calling out”

Problem

Actually, webservice or http callouts and a DML won't take place in the same transaction. And that's why you are getting the error. Read about this problem.

Solution

1. You could break your code into two parts, as per the suggestion in the above article, Webservice part and the DML part.
2. Use @Future annotation. I had such problem and resolved using that.

Code Sameple This is just a out-of-mind code. but you need make changes or test this as per your need.

//global variable at the top
Map<String,Object> mapObject;
//start ()
execute () {
      CallWebService();
      Map<String,Object> rawObj = mapObject;
      // Your remaining code
}

@future
public PageReference CallWebService() {
    // initialize the global variable.
    mapObject = new Map<String,Object>();


    HttpRequest req = new HttpRequest();
    req.setMethod('GET');
    req.setEndpoint('https://www.publicstuff.com/api/2.0/request_view?request_id='+newReq');
    Http http = new Http();
    HTTPResponse res = http.send(req);

    String resonseBody = res.getBody();
    String jsonString = resonseBody;
    mapObject = (Map<String,Object>) JSON.deserializeUntyped(jsonString);
}

I Identified the issue in Batch Class code. There was a DML operation between callouts which was causing this.

Thanks for your help @Eric.

believe since you are implementing database.stateful that the DML persits and thus you cannot make a callout in the finish method

Database.stateful is to persist values of instance variables and not for DML operations. Issue was in Finish method code, while doing 4 difffenent APEX callouts code was inserting records(DML) inbetween callouts. I just moved all the DML statements after callouts to resolve the issue.