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.mime;

import java.io.IOException;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

public abstract class MimeWriter
{
    protected final Headers headers;

    protected MimeWriter(Headers headers)
    {
        this.headers = headers;
    }

    public Headers getHeaders()
    {
        return headers;
    }

    public abstract OutputStream getContentStream()
        throws IOException;


    protected static List<String> mapToLines(Map<String, String> headers)
    {
        List hdrs = new ArrayList(headers.size());

        for (Iterator<String> it = headers.keySet().iterator(); it.hasNext();)
        {
            String key = (String)it.next();

            hdrs.add(key + ": " + headers.get(key));
        }

        return hdrs;
    }
}