#vulnerable code 
                             public static Db db() {
		assert U.must(defaultDb != null, "Database not initialized!");
		return defaultDb;
	} 
                            #location 3 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                             public static Db db() {
		assert U.must(db != null, "Database not initialized!");
		return db;
	}