问Gmail Api在spring boot中: javax.mail.MessagingException:无法将套接字转换为TLS

EN

spring.mail.host=smtp.gmail.com
spring.mail.port=587
spring.mail.username=xxx@gmail.com
spring.mail.password=xxxpwd
spring.mail.properties.mail.smtp.auth=true
spring.mail.properties.mail.smtp.starttls.enable=true

    package interv.Web.Notifications;

import org.hibernate.validator.constraints.Email;

import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;

public class MailObject {

    @Email
    @NotNull
    @Size(min = 1, message = "Please, set an email address to send the message to it")
    private String to;
    private String subject;
    private String text;

    public String getTo() {
        return to;
    }

    public void setTo(String to) {
        this.to = to;
    }

    public String getSubject() {
        return subject;
    }

    public void setSubject(String subject) {
        this.subject = subject;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }
}

    @Controller
public class MailController {

    @Autowired
    public EmailServiceImpl emailService;

    @Autowired
    public SimpleMailMessage template;

    @RequestMapping(value = "/send", method = RequestMethod.POST)
    public String createMail(@RequestBody MailObject mailObject) {


        emailService.sendSimpleMessage(mailObject.getTo(),
                mailObject.getSubject(), mailObject.getText());

        return "that was succesfull";

    }
}

public interface EmailService {

    void sendSimpleMessage(String to,
                           String subject,
                           String text);
    void sendSimpleMessageUsingTemplate(String to,
                                        String subject,
                                        SimpleMailMessage template,
                                        String templateArgs);

}

@Component
public class EmailServiceImpl implements EmailService{

    @Autowired
    public JavaMailSender emailSender;

    @Override
    public void sendSimpleMessage(String to, String subject, String text) {
        SimpleMailMessage message = new SimpleMailMessage();
        message.setTo(to);
        message.setSubject(subject);
        message.setText(text);

        emailSender.send(message);
    }

    @Override
    public void sendSimpleMessageUsingTemplate(String to, String subject, SimpleMailMessage template, String templateArgs) {
        String text = String.format(template.getText(), templateArgs);
        sendSimpleMessage(to, subject, text);
    }
}

servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.mail.MailSendException: Mail server connection failed; nested exception is 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. Failed messages: 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; message exceptions (1) are:
Failed message 1: 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] with root cause

sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
    at sun.security.provider.certpath.SunCertPathBuilder.build(SunCertPathBuilder.java:141) ~[na:1.8.0_161]
    at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:126) ~[na:1.8.0_161]
    at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:280) ~[na:1.8.0_161]