Java URL连接https URL上的奇怪行为

public class URLConnectionReader { 
     public static void main(String[] args) throws Exception { 
      URL urlObj = new URL("https://www.paypal.com"); // enter url here 
      HttpURLConnection connection = (HttpURLConnection) urlObj.openConnection(); 
      connection.setRequestMethod("GET"); 
      connection.connect(); 
      int code = connection.getResponseCode(); 
      System.out.println(code); 
     } 
}

我正在创建一个URL测试器，它检查URL是否正常工作。上面是我的代码。当我用https://www.paypal.com测试它时，它返回200，当我用https://www.facebook.com测试它时，它会抛出异常。下面是异常堆栈跟踪Java URL连接https URL上的奇怪行为

Exception in thread "main" 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.https.HttpsURLConnectionImpl.connect(HttpsURLConnectionImpl.java:153) 
    at URLConnectionReader.main(URLConnectionReader.java:27) 
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:387) 
    at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:292) 
    at sun.security.validator.Validator.validate(Validator.java:260) 
    at sun.security.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:324) 
    at sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerImpl.java:229) 
    at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:124) 
    at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1491) 
    ... 11 more 
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target 
    at sun.security.provider.certpath.SunCertPathBuilder.build(SunCertPathBuilder.java:141) 
    at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:126) 
    at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:280) 

at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:382) 
... 17 more

有谁能够解释一下我为什么https://www.paypal.com返回200，但https://www.facebook.com抛出异常。

来源

2017-07-04 Darshan