为什么在处理贝宝付款时突然出现“ PKIX路径构建失败”错误

基于Java的网站使用Paypal处理付款。该错误始于昨晚格林尼治标准时间晚上11点左右，没有代码更改，如果问题出在我的托管服务提供商或Paypla上，我将无法解决。

引发异常的代码是

                // Send back to PayPal system to validate using POST
                URL u = new URL(URL_PAYPAL_VALIDATE);
                HttpURLConnection uc = (HttpURLConnection) u.openConnection();
                uc.setDoOutput(true);
                uc.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
                PrintWriter pw = new PrintWriter(uc.getOutputStream());
                pw.println(str);
                pw.close();

并出现在uc.getOuputStream（）上

这是例外

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:975)
        at com.sun.net.ssl.internal.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:123)
        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:405)
        at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:166)
        at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:832)
        at sun.net.www.protocol.https.HttpsURLConnectionImpl.getOutputStream(HttpsURLConnectionImpl.java:230)
        at com.jthink.store.action.VerifyPayment.handleRequest(VerifyPayment.java:160)
        at com.jthink.store.JThinkStoreServlet.doGet(JThinkStoreServlet.java:45)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1002)
        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:585)
        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:885)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
        at java.lang.Thread.run(Thread.java:619)
Caused by: 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.validator.PKIXValidator.doBuild(PKIXValidator.java:285)
        at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:191)
        at sun.security.validator.Validator.validate(Validator.java:218)
        at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:126)
        at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:209)
        at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:249)
        at com.sun.net.ssl.internal.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:954)
        ... 31 more
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
        at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:174)
        at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:238)
        at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:280)
        ... 37 more