HttpClient - ssl - unable to find valid certification path to requested target

The 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 usually stands for issues with accepting/validating SSL/TLS certificate given by web server. ClientHandshaker class is responsible for handshaking from the client side. It shares common logic such control flow and key generation with its counterpart ServerHandshaker though common parent class called Handshaker.

void processMessage(byte type, int messageLen) throws IOException {
 ...
 switch (type) {
 ...
 case HandshakeMessage.ht_certificate:
       if (keyExchange == K_DH_ANON || keyExchange == K_ECDH_ANON
                    || keyExchange == K_KRB5 || keyExchange == K_KRB5_EXPORT) {
                fatalSE(Alerts.alert_unexpected_message,
                    "unexpected server cert chain");
                // NOTREACHED
            }
            this.serverCertificate(new CertificateMsg(input));
            serverKey =
                session.getPeerCertificates()[0].getPublicKey();
            break;

Let's go briefly to the details 1. Creating CerificateMsg just takes input as a HandshakeInStream, reads first 24 bytes as a chainLenght, so can read the cert as a byte array and finally instantiates X.509 certificate, as below

cf = CertificateFactory.getInstance("X.509");
cf.generateCertificate(new ByteArrayInputStream(cert)

2. serverCertificate method will deletegate cerificate validation to the Trust Manager taken from sslContext. To solve the error the sslcontext have to be feeded with TrustStrategy that accepts self-sighed cerificates as trusted. Apache Http Client comes with org.apache.http.conn.ssl.TrustSelfSignedStrategy to do so.

HttpClients.custom()
                .setSSLSocketFactory(new SSLConnectionSocketFactory(SSLContextBuilder.create()
      .loadTrustMaterial(TrustSelfSignedStrategy.INSTANCE)
      .build(), NoopHostnameVerifier.INSTANCE))
                .build();