I am trying to do a OAuth2 login with java to connect salesforce,when i send the post request to this url :https://login.salesforce.com/services/oauth2/token 

an error occured:
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

does anybody meet this problem before? who can help me to solve this problem ,thanks a lot in advance