Get AWS EC2 instance details using RunInstancesRequest - amazon-web-services

Question

I am trying to get AWS EC2 instance details using RunInstancesRequest. For that I followed AWS doc https://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/examples-ec2-instances.html.
RunInstancesRequest runInstancesRequest = new RunInstancesRequest();
runInstancesRequest.withImageId(imageId).withInstanceType(instanceType).withMinCount(1).withMaxCount(count).withSecurityGroups(securityGroupName);
RunInstancesResult runInstancesResult = amazonEC2.runInstances(runInstancesRequest);
String instance_id = runInstancesResult.getReservation().getReservationId();
//waiting for 2 minute
DescribeInstancesRequest describeInstancesRequest = new DescribeInstancesRequest();
describeInstancesRequest.setInstanceIds(Arrays.asList(instance_id));
DescribeInstancesResult describeInstancesResult = amazonEC2.describeInstances(describeInstancesRequest);
for(Reservation reservation : describeInstancesResult.getReservations()){
for(Instance instance : reservation.getInstances()) {
System.out.println(instance.getPublicDnsName());
}
}
Here I am able to get AWS EC2 instance up and running but the problem I am facing is I am not able to get the EC2 details using the RunInstancesResult object. As per AWS documentation it seems like instance_id is reservation_id but I believe it is not so. As instance_id start with "i-" and reservation_id with "r-".
How I can get the details of only one EC2 which I created using API? As I got RunInstancesResult object as output of the previous API hence the question: How I can get AWS EC2 instance details using RunInstancesRequest?

Answer 1

Reservations are the request to launch instances. For example, you could use one launch request to create two instances. Thus, the Reservation contains multiple Instances.
If you look in the response object, you will see that the Reservation does indeed contain multiple instances, eg:
{
"OwnerId": "123456789012",
"ReservationId": "r-08626e73c547023b1",
"Groups": [
{
"GroupName": "MySecurityGroup",
"GroupId": "sg-903004f8"
}
],
"Instances": [
{
"Monitoring": {
"State": "disabled"
},
"PublicDnsName": null,
"RootDeviceType": "ebs",
"State": {
"Code": 0,
"Name": "pending"
},
"EbsOptimized": false,
"LaunchTime": "2013-07-19T02:42:39.000Z",
"ProductCodes": [],
"StateTransitionReason": null,
"InstanceId": "i-1234567890abcdef0",
"ImageId": "ami-1a2b3c4d",
"PrivateDnsName": null,
"KeyName": "MyKeyPair",
etc.

Answer 2

There is small confusion created in AWS doc. They are refering reservation id as instance_id. After changing following changes in my code, was able to filter out the instances:
String instance_id = runInstancesResult.getReservation().getInstances()..get(0).getInstanceId();