Perl LWP::UserAgent parse response JSON - json

Question

I am using the LWP::UserAgent module to issue a GET request to one of our APIs.
#!/usr/bin/perl
use strict;
use warning;
use LWP::UserAgent;
use Data::Dumper;
my $ua = LWP::UserAgent->new;
my $request = $ua->get("http://example.com/foo", Authorization => "Bearer abc123", Accept => "application/json" );
print Dumper $request->content;
The request is successful. Dumper returns the following JSON.
$VAR1 = '{
"apiVersion": "v1",
"data": {
"ca-bundle.crt": "-----BEGIN CERTIFICATE-----abc123-----END CERTIFICATE-----\\n"
},
"kind": "ConfigMap",
"metadata": {
"creationTimestamp": "2021-07-16T17:13:01Z",
"labels": {
"auth.openshift.io/managed-certificate-type": "ca-bundle"
},
"managedFields": [
{
"apiVersion": "v1",
"fieldsType": "FieldsV1",
"fieldsV1": {
"f:data": {
".": {},
"f:ca-bundle.crt": {}
},
"f:metadata": {
"f:labels": {
".": {},
"f:auth.openshift.io/managed-certificate-type": {}
}
}
},
"manager": "cluster-kube-apiserver-operator",
"operation": "Update",
"time": "2021-09-14T17:07:39Z"
}
],
"name": "kube-control-plane-signer-ca",
"namespace": "openshift-kube-apiserver-operator",
"resourceVersion": "65461225",
"selfLink": "/api/v1/namespaces/openshift-kube-apiserver-operator/configmaps/kube-control-plane-signer-ca",
"uid": "f9aea067-1234-5678-9101-9d4073f5ae53"
}
}';
Let's say I want to print the value of the apiVersion key, which should print v1.
print "API Version = $request->content->{'apiVersion'} \n";
The following is being printed. I am not sure how to print the value v1. Since HTTP::Response is included in the output, I suspect I might have to use the HTTP::Response module?
API Version = HTTP::Response=HASH(0x2dffe80)->content->{'apiVersion'}

Answer 1

Perl doesn't expand subroutine calls in a double-quoted string.
print "API Version = $request->content->{'apiVersion'} \n";
In this line of code, content() is a subroutine call. So Perl sees this as:
print "API Version = $request" . "->content->{'apiVersion'} \n";
And if you try to print most Perl objects, you'll get the hash reference along with the name of the class - hence HTTP::Response=HASH(0x2dffe80).
You might think that you just need to break up your print() statement like this:
print 'API Version = ', $request->content->{'apiVersion'}, "\n";
But that's not going to work either. $request->content doesn't return a Perl data structure, it returns a JSON-encoded string. You need to decode it into a data structure before you can access the individual elements.
use JSON;
print 'API Version = ', decode_json($request->content)->{'apiVersion'}, "\n";
But it might be cleaner to do the decoding outside of the print() statement.
use JSON;
my $data = decode_json($request->content);
In which case you can go back to something more like your original code:
print "API Version = $data->{'apiVersion'} \n";

Answer 2

The JSON content must be decoded first. There are several modules for that, like JSON:
use JSON;
# ...
my $href = decode_json $request->content;
And then use it like a normal hash reference: $href->{apiVersion}