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.pkcs.jcajce;

import java.io.IOException;
import java.security.PrivateKey;
import java.security.cert.CertificateEncodingException;
import java.security.cert.X509Certificate;

import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.asn1.x509.Certificate;
import org.bouncycastle.operator.OutputEncryptor;
import org.bouncycastle.pkcs.PKCS12SafeBagBuilder;
import org.bouncycastle.pkcs.PKCSIOException;

public class JcaPKCS12SafeBagBuilder
    extends PKCS12SafeBagBuilder
{
    public JcaPKCS12SafeBagBuilder(X509Certificate certificate)
        throws IOException
    {
        super(convertCert(certificate));
    }

    private static Certificate convertCert(X509Certificate certificate)
        throws IOException
    {
        try
        {
            return Certificate.getInstance(certificate.getEncoded());
        }
        catch (CertificateEncodingException e)
        {
            throw new PKCSIOException("cannot encode certificate: " + e.getMessage(), e);
        }
    }

    public JcaPKCS12SafeBagBuilder(PrivateKey privateKey, OutputEncryptor encryptor)
    {
        super(PrivateKeyInfo.getInstance(privateKey.getEncoded()), encryptor);
    }

    public JcaPKCS12SafeBagBuilder(PrivateKey privateKey)
    {
        super(PrivateKeyInfo.getInstance(privateKey.getEncoded()));
    }
}