getResourceAsStream returns null randomly and infrequently

Question

I'm getting a null pointer exception when using getResourceAsStream very rarely like once for every 10000 runs. This is how the class looks like

public class ConfigLoader{
  private Properties propies;
  private static ConfigLoader cl = null;
  private ConfigLoader(){
        propies = new Properties;
  }
  public static ConfigLoader getInstance(){
    if(cl == null){
          cl = new ConfigLoader();
    }
  }

  public boolean Load(){
   try{
         propies.load(this.getClass().getResourceAsStream("/config.properties"));          
   }
   catch(NullPointerException e){
         System.out.println("File Does Not Exist");
         return false;
   }
   return true;
   }
}

As can be seen here, the class is implemented as a singleton. The resource clearly exists and is detected most of the time but I'm not sure why its failing once in a while which seems really strange to me.

Answer 1

1. It would help to find out what is null (propies? the value returned by getResourceAsStream?)
2. My guess is that you call getInstance from several threads and one of them gets a ConfigLoader that has not been properly initialised because your singleton is not thread safe

So I would first confirm with logging that when it fails, it is because propies is null, and if it is the case, make the singleton thread safe, for example:

private static final ConfigLoader cl = new ConfigLoader;
public static ConfigLoader getInstance() { return cl; }

or even better use an enum:

public enum ConfigLoader{
  INSTANCE;
  private Properties propies;
  private ConfigLoader(){
    propies = new Properties;
  }

  public static ConfigLoader getInstance(){ return INSTANCE; }

  public boolean Load(){
   try{
         propies.load(this.getClass().getResourceAsStream("/config.properties"));          
   }
   catch(NullPointerException e){
         System.out.println("File Does Not Exist");
         return false;
   }
   return true;
   }
}

Alternatively, if getResourceAsStream("/config.properties") returns null, could it be a packaging issue due to the resource not having been included in your jar?