What causes SunCertPathBuilderException: unable to find valid certification path to requested target

I am trying to run a very simple server/client ssl code with a self generated keystore file. I got the following exception. I am sure the code has no problem because I can run it several weeks ago. But now I just get this exception: Could anyone tell me what's wrong?

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:1649)
    at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Handshaker.java:241)
    at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Handshaker.java:235)
    at com.sun.net.ssl.internal.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1206)
    at com.sun.net.ssl.internal.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:136)
    at com.sun.net.ssl.internal.ssl.Handshaker.processLoop(Handshaker.java:593)
    at com.sun.net.ssl.internal.ssl.Handshaker.process_record(Handshaker.java:529)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:893)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1138)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.writeRecord(SSLSocketImpl.java:632)
    at com.sun.net.ssl.internal.ssl.AppOutputStream.write(AppOutputStream.java:59)
    at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:202)
    at sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:272)
    at sun.nio.cs.StreamEncoder.implFlush(StreamEncoder.java:276)
    at sun.nio.cs.StreamEncoder.flush(StreamEncoder.java:122)
    at java.io.OutputStreamWriter.flush(OutputStreamWriter.java:212)
    at java.io.BufferedWriter.flush(BufferedWriter.java:236)
    at EchoClient.main(EchoClient.java:32)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:115)
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:323)
    at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:217)
    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:1185)
    ... 19 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:318)
    ... 25 more

Following is my Code:

public
class EchoServer {
    public static void main(String[] arstring) {
        System.setProperty("javax.ne开发者_运维问答t.ssl.keyStore", "c:\\projects\\sandbox\\SSLServer\\mySrvKeyStore");
        System.setProperty("javax.net.ssl.keyStorePassword", "delphi");
        try {
            SSLServerSocketFactory sslserversocketfactory =
                    (SSLServerSocketFactory) SSLServerSocketFactory.getDefault();
            SSLServerSocket sslserversocket =
                    (SSLServerSocket) sslserversocketfactory.createServerSocket(9999);
            SSLSocket sslsocket = (SSLSocket) sslserversocket.accept();

            InputStream inputstream = sslsocket.getInputStream();
            InputStreamReader inputstreamreader = new InputStreamReader(inputstream);
            BufferedReader bufferedreader = new BufferedReader(inputstreamreader);

            String string = null;
            while ((string = bufferedreader.readLine()) != null) {
                System.out.println(string);
                System.out.flush();
            }
        } catch (Exception exception) {
            exception.printStackTrace();
        }
    }
}


public
class EchoClient {
    public static void main(String[] arstring) {
        System.setProperty("javax.net.ssl.keyStore", "c:\\projects\\sandbox\\SSLClient\\mySrvKeyStore");
        System.setProperty("javax.net.ssl.keyStorePassword", "delphi");
        try {
            SSLSocketFactory sslsocketfactory = (SSLSocketFactory) SSLSocketFactory.getDefault();
            SSLSocket sslsocket = (SSLSocket) sslsocketfactory.createSocket("localhost", 9999);

            InputStream inputstream = System.in;
            InputStreamReader inputstreamreader = new InputStreamReader(inputstream);
            BufferedReader bufferedreader = new BufferedReader(inputstreamreader);

            OutputStream outputstream = sslsocket.getOutputStream();
            OutputStreamWriter outputstreamwriter = new OutputStreamWriter(outputstream);
            BufferedWriter bufferedwriter = new BufferedWriter(outputstreamwriter);

            String string = null;
            while ((string = bufferedreader.readLine()) != null) {
                bufferedwriter.write(string + '\n');
                bufferedwriter.flush();
            }
        } catch (Exception exception) {
            exception.printStackTrace();
        }

    }
}

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The client needs to know how to establish a trust relationship with the server's certificate. That means that the client can find a certificate authority, or chain of them, that are responsible for signing the certificate that the server presents. By default, this is the cacerts file buried in the java runtime directory, but that probably won't work in your case.

Try setting this property: javax.net.ssl.trustStore to the keystore that holds the cert that signed the server's cert. If it is self-signed, then you are basically telling the client that the you trust the server to identify itself.

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It may mean you are using a self-signed certificate and it is not "trusted". It is discussed here. That link explains how to add the certificate to the key store (if that's really what you want to do).