HttpsURLConnection https:// 时好时异常！什么情况

Java code

 public static Image GetCodeImage() throws IOException  {
         
          try {
              
            Image img;
            String urlStr = "https://login.sina.com.cn/cgi/pin.php?r=46111694";

            URL url = new URL(urlStr);
            HttpsURLConnection urlConn = (HttpsURLConnection) url.openConnection();   //打开网站链接s
            urlConn.setRequestMethod("GET");
             
           
            img = ImageIO.read(urlConn.getInputStream());

            
            String cookie = urlConn.getHeaderField("set-cookie");
            System.out.println(cookie);

            

            return img;
            

        } catch (MalformedURLException ex) {
            Logger.getLogger(SinaBlog.class.getName()).log(Level.SEVERE, null, ex);
        }
          return null;
     }

  

run：
2012-8-10 12:28:03 com.from.Frame1 jButton5ActionPerformed
严重: null
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 com.sun.net.ssl.internal.ssl.Alerts.getSSLException(Alerts.java:174)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1591)
at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Handshaker.java:187)
at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Handshaker.java:181)
at com.sun.net.ssl.internal.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1035)
at com.sun.net.ssl.internal.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:124)
at com.sun.net.ssl.internal.ssl.Handshaker.processLoop(Handshaker.java:516)
at com.sun.net.ssl.internal.ssl.Handshaker.process_record(Handshaker.java:454)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:884)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1096)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1123)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1107)
at sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:415)
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:166)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:979)
at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:234)
at com.net.post.SinaBlog.GetCodeImage(SinaBlog.java:34)
at com.from.Frame1.RunTaskCall(Frame1.java:359)
at com.from.Frame1.jButton5ActionPerformed(Frame1.java:383)
at com.from.Frame1.access$200(Frame1.java:23)
at com.from.Frame1$4.actionPerformed(Frame1.java:197)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6134)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
at java.awt.Component.processEvent(Component.java:5899)
at java.awt.Container.processEvent(Container.java:2023)
at java.awt.Component.dispatchEventImpl(Component.java:4501)
at java.awt.Container.dispatchEventImpl(Container.java:2081)
at java.awt.Component.dispatchEvent(Component.java:4331)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4301)