apache fluent client POST blocks forever when underlying call actually returned - my thread stuck - spring-boot

Question

I'm calling the Post method of org.apache.http.client.fluent.Request class, and it's blocking - the next line of code is not executed. I'm getting logging that says that I got a successful response back too! My log messages (below) confirm that the message came back with an OK status (200).
>> ...with body JSON:
<< status: 200
So what happened? More importantly, what can I do to protect myself?
P.S. I've used this code thousands of times and this has, to my knowledge, never happened before.
Context
This is a Spring BOOT application (version 1.3.5). I'm using the RESTTemplate to communicate internally among microservices, but I'm using the apache fluent client to talk to an outside web service. I'm using org.apache.httpcomponents (version 4.5.2).
My code:
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;
import java.util.stream.Collectors;
import org.apache.commons.io.IOUtils;
import org.apache.commons.lang.StringUtils;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.StatusLine;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpResponseException;
import org.apache.http.client.ResponseHandler;
import org.apache.http.client.fluent.Request;
import org.apache.http.entity.ContentType;
import org.json.JSONObject;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import com.eneura.devcom.microservices.exceptions.ExternalWebServiceCommunicationFailureException;
import com.eneura.devcom.microservices.exceptions.InvalidArgumentException;
import com.eneura.devcom.microservices.exceptions.PreconditionsFailureException;
final private String internalDoPostSingleCall(
String subUrlPath, String jsonString, List<String> parmList
)
throws InvalidArgumentException, ExternalWebServiceCommunicationFailureException, HttpResponseException
{
String bodyArg = (StringUtils.isNotBlank(jsonString)) ? jsonString : ""; //don't pass in null or blanks or ... just fold down to ""
String url = computeUrl(subUrlPath, parmList);
logger.info(">> sending POST " + url);
logger.info(">> ...with body JSON: " + bodyArg);
String res;
try {
res = Request
.Post( url )
.bodyString(
bodyArg, ContentType.APPLICATION_JSON
).execute()
.handleResponse(new ResponseHandler<String>() {
public String handleResponse(
final HttpResponse response) throws IOException {
StatusLine statusLine = response.getStatusLine();
HttpEntity entity = response.getEntity();
logger.info("<< status: " + statusLine.getStatusCode() );
if (statusLine.getStatusCode() >= 300) {
throw new HttpResponseException(statusLine
.getStatusCode(), statusLine
.getReasonPhrase());
}
String content =
(null == entity) ? (
"" // ok not to have any content back.
) : (
IOUtils.toString(entity.getContent(), "UTF-8")
);
return content;
}
});
} catch (HttpResponseException e) {
throw e;
} catch (IOException e) {
String msg =
"failed to perform POST to ExternalWebService - exception: " + e.getLocalizedMessage();
throw new ExternalWebServiceCommunicationFailureException(msg);
}
logger.info("<< body returned for POST (len="+ res.length() +"): " + res);
return res;
}
Log messages:
2020-03-14 18:46:14.119 INFO 15213 --- [http-nio-6444-exec-9] c.e.d.m.s.avcourier.AVRestDelegate : >> sending POST https://externalwebservice.com/api/v1/operations/a0a01212445566770011001130304040/cancel
2020-03-14 18:46:14.119 INFO 15213 --- [http-nio-6444-exec-9] c.e.d.m.s.avcourier.AVRestDelegate : >> ...with body JSON:
2020-03-14 18:46:14.393 INFO 15213 --- [http-nio-6444-exec-9] c.e.d.m.s.avcourier.AVRestDelegate : << status: 200
2020-03-14 18:50:07.589 INFO 15213 --- [AsyncResolver-bootstrap-executor-0] c.n.d.s.r.aws.ConfigClusterResolver : Resolving eureka endpoints via configuration
2020-03-14 18:55:07.590 INFO 15213 --- [AsyncResolver-bootstrap-executor-0] c.n.d.s.r.aws.ConfigClusterResolver : Resolving eureka endpoints via configuration
2020-03-14 19:00:07.591 INFO 15213 --- [AsyncResolver-bootstrap-executor-0] c.n.d.s.r.aws.ConfigClusterResolver : Resolving eureka endpoints via configuration