Can't Upload Files

Hello,

I succeed to create folders with my token in Java application, and also succeed to download files, but when I try to upload files, I encontered this exception:

Exception in thread "main" com.box.sdk.BoxAPIException: Couldn't connect to the Box API due to a network error.
at com.box.sdk.BoxMultipartRequest.writeBody(BoxMultipartRequest.java:154)
at com.box.sdk.BoxAPIRequest.trySend(BoxAPIRequest.java:389)
at com.box.sdk.BoxAPIRequest.send(BoxAPIRequest.java:209)
at com.box.sdk.BoxAPIRequest.send(BoxAPIRequest.java:184)
at com.box.sdk.BoxFolder.uploadFile(BoxFolder.java:387)
at com.box.sdk.BoxFolder.uploadFile(BoxFolder.java:334)
at com.soft.test.BoxSDKSample.uploadFile(BoxSDKSample.java:120)
at com.soft.test.BoxSDKSample.listFolder(BoxSDKSample.java:150)
at com.soft.test.BoxSDKSample.listFolder(BoxSDKSample.java:147)
at com.soft.test.BoxSDKSample.main(BoxSDKSample.java:57)
Caused by: javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.ssl.Alerts.getSSLException(Alerts.java:192)
at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1949)
at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:302)
at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:296)
at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1509)
at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:216)
at sun.security.ssl.Handshaker.processLoop(Handshaker.java:979)
at sun.security.ssl.Handshaker.process_record(Handshaker.java:914)
at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1062)
at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1375)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1403)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1387)
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.getOutputStream0(HttpURLConnection.java:1283)
at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:1258)
at sun.net.www.protocol.https.HttpsURLConnectionImpl.getOutputStream(HttpsURLConnectionImpl.java:250)
at com.box.sdk.BoxMultipartRequest.writeBody(BoxMultipartRequest.java:126)
... 9 more

I guess that when I'm trying to upload a file, the upload URL goes to https://upload.box.com/api/2.0/files/content, it seems that I need to import a certificate, BUT I don't

know where to get it.

Can anyone give me a hand?Thanks in advance.