Have some problem about the SSL connection by JDBC

I have some big problem.
I install these packages in Redhat Enterprise ES3.
postgresql-7.4.1-1PGDG.i386.rpm
postgresql-jdbc-7.4.1-1PGDG.i386.rpm
postgresql-libs-7.4.1-1PGDG.i386.rpm
postgresql-server-7.4.1-1PGDG.i386.rpm

When I use the ssl connection by JDBC,I have some problem.
--------------my program----------------
try{
  String DBDriver="org.postgresql.Driver";
  Class.forName(DBDriver);
  String DBUrl   = "jdbc:postgresql://myserver:50813/mydbname?ssl";
  String strUser = "user";
  String strPassword = "user";
  Connection conn=DriverManager.getConnection(DBUrl,strUser,strPassword);
... ....
  conn.close();
 }catch(Exception e){
  out.println(e.getMessage());
 }
-------------------Error-------------------------
An I/O error has occured while flushing the output - Exception:
javax.net.ssl.SSLHandshakeException:
sun.security.validator.ValidatorException: No trusted certificate found
Stack Trace:
javax.net.ssl.SSLHandshakeException:
sun.security.validator.ValidatorException: No trusted certificate found
 at com.sun.net.ssl.internal.ssl.BaseSSLSocketImpl.a(DashoA6275)
 at com.sun.net.ssl.internal.ssl.SSLSocketImpl.a(DashoA6275) .....
----------------server log---------------
could not initialize SSL connection: sslv3 alert certificate unknown
When I use pgdev.301.jdbc3.jar download from
http://jdbc.postgresql.org/download.html I get the same error.
Can anybody help me?