How to configure client authentication with apache

I implemented a FTPS client (FTP over SSL/TLS) in java using the apache.commons.net-framework. It is configured to do explicit security on the default port 21.

ftpsClient = new FTPSClient(false);
ftpsClient.setTrustManager(getConfiguration().getCertificatesManager());
ftpsClient.connect(getConfiguration().getHostName(), getConfiguration().getPort());

As long as I don't enforce client-authentication on the server, everything works fine. But I need to enable client-authentication, so I enforce it on the server and configured the client-system properties:

-Djavax.net.ssl.keyStore="D:/.../ftps-client-auth.keystore"
-Djavax.net.ssl.keyStorePassword="*****"
-Djavax.net.ssl.keyStoreType=JKS

What I got was the same as if I did not set the system properties:

javax.net.ssl.SSLHandshakeException: Received fatal alert: handshake_failure
    at com.sun.net.ssl.internal.ssl.Alerts.getSSLException(Alerts.java:174)
    at com.sun.net.ssl.internal.ssl.Alerts.getSSLException(Alerts.java:136)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.recvAlert(SSLSocketImpl.java:1806)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:986)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1170)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1197)
    at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1181)
    at org.apache.commons.net.ftp.FTPSClient.sslNegotiation(FTPSClient.java:265)
    at org.apache.commons.net.ftp.FTPSClient._connectAction_(FTPSClient.java:207)
    at org.apache.commons.net.SocketClient.connect(SocketClient.java:172)
    at org.apache.commons.net.SocketClient.connect(SocketClient.java:192)

The server-log says:

DEBUG: Client "<my ip address>", "SSL_accept failed: error:140890C7:SSL routines:SSL3_GET_CLIENT_CERTIFICATE:peer did not return a certificate"

Seems right - I enabled -Djavax.net.debug=all, what shows that the server sends a list of CNs it accepts, but the client sends an empty certificate chain.

What have I done wrong?
Do I need to do some configuration programatically?
Do the certificates or the private key need to support anything special for SSL/TLS?

Figured it out: you need to programmatically set a KeyManager. Setting the system properties (-Djavax.net.ssl.keyStore, ...) is not sufficient, because the the framework does not use Suns SSLSocketFactory.

Example:

ftpsClient = new FTPSClient(false);
ftpsClient.setTrustManager(TrustManagerUtils.getAcceptAllTrustManager());
KeyManager keyManager = org.apache.commons.net.util.KeyManagerUtils.createClientKeyManager(new File(keystorePath), keystorePass);
ftpsClient.setKeyManager(keyManager);
ftpsClient.connect(getConfiguration().getHostName(), getConfiguration().getPort());

You may want to choose a different Trust-Manager, e.g. one that is based on a Java-keystore. The utils provide a method for that, too: TrustManagerUtils.getDefaultTrustManager(keystore)