google app engine - access denied ("java.lang.RuntimePermission" "modifyThreadGroup") -

i'm developing server application using google appengine , goodle cloud sql. have problem when trying connect cloud sql when running server on local machine. error:

java.security.accesscontrolexception: access denied ("java.lang.runtimepermission" "modifythreadgroup")

here servlet code:

package com.example.example;  import com.google.appengine.api.utils.systemproperty;  import java.io.ioexception; import java.io.printwriter; import java.sql.connection; import java.sql.drivermanager; import java.sql.sqlexception;  import javax.servlet.http.*;  public class myservlet extends httpservlet {     @override     public void doget(httpservletrequest req, httpservletresponse resp) throws ioexception {         resp.setcontenttype("text/plain");         resp.getwriter().println("please use form post url");     }      @override     public void dopost(httpservletrequest req, httpservletresponse resp) throws ioexception {         string url = null;         try {             if (systemproperty.environment.value() == systemproperty.environment.value.production) {                 // load class provides new "jdbc:google:mysql://" prefix.                 class.forname("com.mysql.jdbc.googledriver");                 url = "jdbc:google:mysql://app_id:instance_id/data?user=root";             } else {                 // local mysql instance use during development.                 class.forname("com.mysql.jdbc.driver");                 url = "jdbc:mysql://instance-ip:3306/data?user=root";             }         } catch (exception e) {             e.printstacktrace();             return;         }          printwriter out = resp.getwriter();         try {             connection conn = drivermanager.getconnection(url);             system.out.println("connection established");             out.println("connection established");             try {             } {                 conn.close();             }         } catch (sqlexception e) {             out.append(e.getmessage());             e.printstacktrace();         }     }  } 

i have added following line java.policy file:

permission java.lang.runtimepermission "modifythreadgroup";

but did not fix problem.