Callout.java

public with sharing class CallOutUtil {
	
	public String data {get;set;}    

    public CallOutUtil(){    	        
        data = getContent(ApexPages.currentPage().getParameters().get('url'));
        //encoding data so as to avoid any data loss       
        data = EncodingUtil.base64Encode(Blob.valueOf(data));
    }

    public String getContent(String url) {    
        Http h = new Http();
        
        HttpRequest req = new HttpRequest();
        req.setEndpoint(url);
        req.setMethod('GET');
    
        HttpResponse res = h.send(req);
        return res.getBody();
    } 

	public static String doCallout(String url){
		PageReference pageRef = Page.Callout;
        pageRef.getParameters().put('url',url);        
        System.debug(pageRef.getContent().toString());
        Dom.Document doc = new Dom.Document();
        doc.load(pageRef.getContent().toString());
        Dom.XMLNode root = doc.getRootElement();                                
        String response = root.getChildElement('body',null).getChildElement('customTag',null).getText();        
        response = EncodingUtil.base64Decode(response).toString();        
        return response;
	}
	
}