Make Java Properties available across classes?

Once you initialized defaultProps, you can make its contents available to other objects in your app e.g. via a public static accessor method, e.g.:

public class Config {
  private static Properties defaultProps = new Properties();
  static {
    try {
        FileInputStream in = new FileInputStream("custom.properties");
        defaultProps.load(in);
        in.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
  }
  public static String getProperty(String key) {
    return defaultProps.getProperty(key);
  }
}

This is the simplest approach, however it creates an extra dependency which makes unit testing harder (unless you provide a method in Config to set a mock property object for unit testing).

An alternative is to inject defaultProps (or individual configuration values from it) into each object which needs it. However, this may mean you need to add extra parameter(s) to lots of methods if your call hierarchies are deep.

Why not use a static ResourceBundle ?

static final ResourceBundle myResources = 
          ResourceBundle.getBundle("MyResources", currentLocale);

If you only need one instance of your properties class you can use the singleton (anti?)-pattern.

It would look like a class like this:

public class MyProperties extends Properties {
    private static MyProperties instance = null;

    private MyProperties() {
    }

    public static MyProperties getInstance() {
        if (instance == null) {
            try {
                instance = new MyProperties();
                FileInputStream in = new FileInputStream("custom.properties");
                instance.load(in);
                in.close();
            } catch (Exception e) {
                e.printStackTrace();
                return null;
            }
        }
        return instance;
    }
}