While running "aws ec2 describe-instances" in command line, It gives list of all ec2 Instances but with Java AWS-SDK it's gives zero Reservations. Please see below code snippet,
 AmazonEC2 ec2;
 if (ec2 == null) {
    AWSCredentialsProviderChain credentialsProvider = new 
    AWSCredentialsProviderChain(
        new InstanceProfileCredentialsProvider(),
        new ProfileCredentialsProvider("default"));
      ec2 = new AmazonEC2Client(credentialsProvider);
  }
 for (Reservation reservation : ec2.describeInstances().getReservations()) {
         for (Instance instance : reservation.getInstances()) {
                System.out.println("TAG" + instance.getInstanceId());
      } 
   }