get Json Array from URL by java

Our Central Bank provides currency exchange rates in many ways. Ex. one currency on date is easy to get: http://api.nbp.pl/api/exchangerates/rates/a/usd/2020-08-20?format=json
(It begins with simple brace { as typical Json)

but another table - each currency on date: http://api.nbp.pl/api/exchangerates/tables/a/2020-08-20?format=json returns 'nullSession' after line of the the code

	if (httpURLConnection.getResponseCode() == okRespCode) { ...

despite the fact that this Json is presented in webrowser without any problem. It begins with square bracket [ as an array.

Have you ever met and solved such an issue?

Please find below exception stack trace from my iDempiere development IDE.

> PM org.compiere.process.SvrProcess process SEVERE: nullSession java.lang.NoSuchFieldError: nullSession at
> sun.security.ssl.ClientHandshaker.getKickstartMessage(ClientHandshaker.java:1343)
> at sun.security.ssl.Handshaker.kickstart(Handshaker.java:1117) at
> sun.security.ssl.SSLSocketImpl.kickstartHandshake(SSLSocketImpl.java:1500)
> at
> sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1367)
> at
> sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1416)
> at
> sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1400)
> at
> sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:559)
> at
> sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:185)
> at
> sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1570)
> at
> sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1498)
> at
> java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:480)
> at
> sun.net.www.protocol.https.HttpsURLConnectionImpl.getResponseCode(HttpsURLConnectionImpl.java:352)
> at one.stabilis.utils.JsonUtils.readJsonToString(JsonUtils.java:75)
> at
> one.stabilis.utils.JsonUtils.readArrayOfExchangeRatesTable(JsonUtils.java:108)
> at
> one.stabilis.utils.JsonUtils.publishedOnDateTable(JsonUtils.java:55)
> at
> one.stabilis.utils.JsonUtils.getPublishedOnDateTable(JsonUtils.java:32)
> at
> one.stabilis.impconvrates.ImportCurrencyRates.doIt(ImportCurrencyRates.java:84)
> at org.compiere.process.SvrProcess.process(SvrProcess.java:201) at
> org.compiere.process.SvrProcess.startProcess(SvrProcess.java:147) at
> org.adempiere.util.ProcessUtil.startJavaProcess(ProcessUtil.java:172)
> at
> org.adempiere.util.ProcessUtil.startJavaProcess(ProcessUtil.java:139)
> at
> org.adempiere.util.ProcessUtil.startJavaProcess(ProcessUtil.java:128)
> at
> org.compiere.interfaces.impl.ServerBean.process(ServerBean.java:78)
> at
> org.compiere.apps.AbstractProcessCtl.startProcess(AbstractProcessCtl.java:429)
> at
> org.compiere.apps.AbstractProcessCtl.run(AbstractProcessCtl.java:234)
> at org.adempiere.webui.apps.WProcessCtl.process(WProcessCtl.java:197)
> at
> org.adempiere.webui.apps.AbstractProcessDialog$ProcessDialogRunnable.doRun(AbstractProcessDialog.java:1083)
> at org.adempiere.util.ContextRunnable.run(ContextRunnable.java:38)
> at
> org.adempiere.webui.apps.DesktopRunnable.run(DesktopRunnable.java:40)
> at
> java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
> at java.util.concurrent.FutureTask.run(FutureTask.java:266) at
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
> at
> java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
> at
> java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
> at
> java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
> at java.lang.Thread.run(Thread.java:748)

You can either map the response as List<JsonNode> or List<CustomMapperObject>.

Since the response is itself an array rather than an object, you can do something like this

String responseJson = "some array of objects";
List<CustomMapperObject> rates = objectMapper.readValue(responseJson, 
             ParameterizedTypeReference<List<CustomMapperObject>>() {
});

or

ResponseEntity<List<CustomMapperObject>> responseObject = 
                       restTemplate.exchange(uri.toUriString(), HttpMethod.GET,
			           httpEntity,
			           new ParameterizedTypeReference<List<CustomMapperObject>>() {
			           });

I would recommend to have a look at the Jackson Library for reference