执行到HttpClientUtil.doGet(url, param); 的时候HttpClientUtil中42行 response = httpclient.execute(httpGet);报错是怎么回事？

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