How to obtain the Public IP Address on EC2 launch instance request?

Question

I can't figure it out how I can do it:

runInstancesRequest.withImageId("ami-53170b32")
                .withInstanceType("t2.micro")
                .withMinCount(1)
                .withMaxCount(1)
                .withKeyName("mac")
                .withSecurityGroupIds("sg-49025d2d");

RunInstancesResult runInstancesResult =
                amazonEC2Client.runInstances(runInstancesRequest);

So far everything works fine. Now I want to get the Public IP Address from the recently started instance. How can I do that?

I tried:

runInstancesResult.getReservation().getInstances().get(0).getPublicIpAddress()

but the IP is always null.

Answer 1

When an instance is launched, it enters the Pending state and does not yet have a Public IP address. You will need to wait a little bit for it to be available.

After a few seconds, call DescribeInstances with the Instance ID originally returned, then extract the PublicIpAddress.

Here's a dump doing it from the AWS Command-Line Interface (CLI):

$ aws ec2 run-instances --image-id ami-1500742f ...
{
    "OwnerId": "123456789012", 
    "ReservationId": "r-0d8cc4a12a94faba7", 
    "Groups": [], 
    "Instances": [
        {
            "Monitoring": {
                "State": "disabled"
            }, 
            "PublicDnsName": "", 
            "KernelId": "aki-c362fff9", 
            "State": {
                "Code": 0, 
                "Name": "pending"
            }, 
            "EbsOptimized": false, 
            "LaunchTime": "2016-01-22T21:17:49.000Z", 
            "PrivateIpAddress": "172.31.12.208", 
            "ProductCodes": [], 
            "VpcId": "vpc-7d087014", 
            "StateTransitionReason": "", 
            "InstanceId": "i-0afe19e0d061b95b5", 
...
}

$ aws ec2 describe-instances --instance-ids i-0afe19e0d061b95b5
{
    "Reservations": [
        {
            "OwnerId": "123456789012", 
            "ReservationId": "r-0d8cc4a12a94faba7", 
            "Groups": [], 
            "Instances": [
                {
                    "Monitoring": {
                        "State": "disabled"
                    }, 
                    "PublicDnsName": "ec2-52-62-35-146.ap-southeast-2.compute.amazonaws.com", 
                    "RootDeviceType": "ebs", 
                    "State": {
                        "Code": 16, 
                        "Name": "running"
                    }, 
                    "EbsOptimized": false, 
                    "LaunchTime": "2016-01-22T21:17:49.000Z", 
                    "PublicIpAddress": "52.62.35.146", 
                    "PrivateIpAddress": "172.31.12.208", 
...
}