Javamail Could not convert socket to TLS GMail

Question

I want to send an email using JavaMail through Gmail SMTP Server, but I am getting an error.
This is the code:

final String username = "mygmail@gmail.com";
final String password = "mygmailpassword";

Properties props = new Properties();
props.put("mail.smtp.auth", true);
props.put("mail.smtp.starttls.enable", true);
props.put("mail.smtp.host", "smtp.gmail.com");
props.put("mail.smtp.port", "587");

Session session = Session.getInstance(props,
    new javax.mail.Authenticator() {
        protected PasswordAuthentication getPasswordAuthentication() {
            return new PasswordAuthentication(username, password);
        }
    }
);

try {
    Message message = new MimeMessage(session);
    message.setFrom(new InternetAddress("no-reply@gmail.com"));
    message.setRecipients(Message.RecipientType.TO,
                        InternetAddress.parse("test@gmail.com"));
    message.setSubject("Testing Subject");
    message.setText("Dear Mail Crawler," + "\n\n No spam to my email, please!");
    Transport.send(message);
    System.out.println("Done");
} catch (MessagingException e) {
    throw new RuntimeException(e);
}

This is the error:

Could not convert socket to TLS;

The complete stack trace:

Exception in thread "main" java.lang.RuntimeException: javax.mail.MessagingException: Could not convert socket to TLS; nested exception is: 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 Test.main(Test.java:43) Caused by: javax.mail.MessagingException: Could not convert socket to TLS; nested exception is: 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.mail.smtp.SMTPTransport.startTLS(SMTPTransport.java:1907) at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:666) at javax.mail.Service.connect(Service.java:317) at javax.mail.Service.connect(Service.java:176) at javax.mail.Service.connect(Service.java:125) at javax.mail.Transport.send0(Transport.java:194) at javax.mail.Transport.send(Transport.java:124) at Test.main(Test.java:38) Caused by: 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.startHandshake(SSLSocketImpl.java:1165) at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1149) at com.sun.mail.util.SocketFetcher.configureSSLSocket(SocketFetcher.java:549) at com.sun.mail.util.SocketFetcher.startTLS(SocketFetcher.java:486) at com.sun.mail.smtp.SMTPTransport.startTLS(SMTPTransport.java:1902) ... 7 more 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) ... 17 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) ... 23 more

Can someone explain to me why I am facing this issue and how can I solve this?