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;

import java.io.IOException;

import org.bouncycastle.asn1.ASN1Encodable;
import org.bouncycastle.asn1.ASN1EncodableVector;
import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.DEROctetString;
import org.bouncycastle.asn1.DERSet;
import org.bouncycastle.asn1.pkcs.Attribute;
import org.bouncycastle.asn1.pkcs.CertBag;
import org.bouncycastle.asn1.pkcs.PKCSObjectIdentifiers;
import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.asn1.pkcs.SafeBag;
import org.bouncycastle.asn1.x509.Certificate;
import org.bouncycastle.asn1.x509.CertificateList;
import org.bouncycastle.cert.X509CRLHolder;
import org.bouncycastle.cert.X509CertificateHolder;
import org.bouncycastle.operator.OutputEncryptor;

public class PKCS12SafeBagBuilder
{
    private ASN1ObjectIdentifier bagType;
    private ASN1Encodable        bagValue;
    private ASN1EncodableVector  bagAttrs = new ASN1EncodableVector();

    public PKCS12SafeBagBuilder(PrivateKeyInfo privateKeyInfo, OutputEncryptor encryptor)
    {
        this.bagType = PKCSObjectIdentifiers.pkcs8ShroudedKeyBag;
        this.bagValue = new PKCS8EncryptedPrivateKeyInfoBuilder(privateKeyInfo).build(encryptor).toASN1Structure();
    }

    public PKCS12SafeBagBuilder(PrivateKeyInfo privateKeyInfo)
    {
        this.bagType = PKCSObjectIdentifiers.keyBag;
        this.bagValue = privateKeyInfo;
    }

    public PKCS12SafeBagBuilder(X509CertificateHolder certificate)
        throws IOException
    {
        this(certificate.toASN1Structure());
    }

    public PKCS12SafeBagBuilder(X509CRLHolder crl)
        throws IOException
    {
        this(crl.toASN1Structure());
    }

    public PKCS12SafeBagBuilder(Certificate certificate)
        throws IOException
    {
        this.bagType = PKCSObjectIdentifiers.certBag;
        this.bagValue = new CertBag(PKCSObjectIdentifiers.x509Certificate, new DEROctetString(certificate.getEncoded()));
    }

    public PKCS12SafeBagBuilder(CertificateList crl)
        throws IOException
    {
        this.bagType = PKCSObjectIdentifiers.crlBag;
        this.bagValue = new CertBag(PKCSObjectIdentifiers.x509Crl, new DEROctetString(crl.getEncoded()));
    }

    public PKCS12SafeBagBuilder addBagAttribute(ASN1ObjectIdentifier attrType, ASN1Encodable attrValue)
    {
        bagAttrs.add(new Attribute(attrType, new DERSet(attrValue)));

        return this;
    }

    public PKCS12SafeBag build()
    {
        return new PKCS12SafeBag(new SafeBag(bagType, bagValue, new DERSet(bagAttrs)));
    }
}