Https Web Service Client

I run a Web Service Client in Java and then a exception is shown:

"PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target"

Based on the above error mesage, it seems related to SSL certificate. Then, I do the following step:

(1) Download the Web Service certificate

(2) Install the certificate

After I install the certificate to JRE, the issue is solved.

If I install the cert in Java\jre6, tomcat under eclipse runs fine.
If I install the cert in Java\jdk\jre, tomcat runs fine.

Reference:
keytool reference