HTTPBuilder & Session ID - groovy

Question

I have the following code to connect to a REST API service, authenticate, retrieve a session ID then make further requests passing the session ID to authenticate. The initial request works and I get a HTTP 200 OK plus the session ID in the response, however when I try to make a second request passing the session ID in the header, I get
Caught: groovyx.net.http.HttpResponseException: Bad Request
I know the script can be written much better with the use of classes and try / catch etc. I am still learning both java and groovy so I start by just trying to do everything within the same class.
Any help much appreciated.
import groovyx.net.http.HTTPBuilder
import groovyx.net.http.URIBuilder
import static groovyx.net.http.Method.POST
import static groovyx.net.http.ContentType.*
def url = 'https://1.1.1.1/web_api/'
def uri = new URIBuilder(url)
String CHKPsid
uri.path = 'login'
def http = new HTTPBuilder(uri)
http.ignoreSSLIssues()
http.request(POST,JSON ) { req ->
headers.'Content-Type' = 'application/json'
body = [
"user":"username",
"password":"password"
]
response.success = { resp, json ->
println (json)
CHKPsid = (json.sid)
println "POST Success: ${resp.statusLine}"
}
}
uri.path = 'show-changes'
http.request(POST,JSON ) { req ->
headers.'Content-Type' = 'application/json'
headers.'X-chkp-sid' = '${CHKPsid}'
body = [
"from-date" : "2017-02-01T08:20:50",
"to-date" : "2017-10-21"
]
response.success = { resp, json ->
println (json)
println "POST Success: ${resp.statusLine}"
}
}

Answer 1

String interpolation does not work with single (or triple single quotes). When groovy will evaluate '${CHKPsid}' (single quotes), its value will be ${CHKPsid} (this string). In order to use the value of the variable, you should use double quotes: "${CHKPsid}" or simply just the variable: headers.'X-chkp-sid' = CHKPsid.
So the output of this:
String CHKPsid = "abc123"
println '${CHKPsid}'
println "${CHKPsid}"
will be:
${CHKPsid}
abc123
In order to quickly test what the server receives, you can use httpbin.org or requestb.in

Answer 2

So as well as the correct assignment of the value of the session ID, I found that calling the same HTTPbuilder - http.request the second time even with a change of uri, header and body was the problem. The listening server still saw this as part of the same login API call. My workaround / resolution was to define a 2nd HTTPbuilder with a different name and this now works. I'm interested to know if this is normal behaviour and how others approach this. Thanks.