http://www.bouncycastle.org/java.html: The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 to JDK 1.8. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.
  • The Legion of the Bouncy Castle Inc. 
package org.bouncycastle.est;

import java.io.IOException;


ESTSourceConnectionListener is called when the source is
is connected to the remote end point but no application
data has been sent.

/**
 * ESTSourceConnectionListener is called when the source is
 * is connected to the remote end point but no application
 * data has been sent.
 */
public interface ESTSourceConnectionListener<T, I>
{
    ESTRequest onConnection(Source<T> source, ESTRequest request)
        throws IOException;
}