Avoid assigning to servlet static fields without using a shared lock??

Question

Hi
i received the following warning on my code, does anyone give me a help on this? Thanks
"Avoid assigning to servlet static fields from javax.servlet.Service.service () without using a shared lock."
And my code are as follow:
public class MyDataSource {
    private static DataSource ds = null;
    public static DataSource getDataSource() throws NamingException, Exception
        if (ds==null){
               InitialContext ctx = new InitialContext();
            try{
                synchronized(getDataSource()){
                if(ds==null){
                    ds = (DataSource) ctx.lookup(SysProperties.ds);
            } catch (Exception ex) {
                throw new Exception("DBConnection Exception: " +ex.toString());
            }finally{
                    if(ctx !=null){
                        ctx.close();
        return ds;       
}

Answer 1

The service method of a servlet is multi-threaded to handle multiple requests. Hence,any static or public fields accessed by the service method should be restricted to a single thread access (for example using synchronized keyword) to make it thread-safe. That is why you are getting the warning message.