DefaultFtpSessionFactory provides an abstraction over the underlying client API, which (since Spring Integration 2.0) is Apache Commons Net. This spares you from the low-level configuration details of the org.apache.commons.net.ftp.FTPClient. Several common properties are exposed on the session factory (since version 4.0, this now includes connectTimeout, defaultTimeout, and dataTimeout). However, you sometimes need access to lower level FTPClient configuration to achieve more advanced configuration (such as setting the port range for active mode). For that purpose, AbstractFtpSessionFactory (the base class for all FTP Session Factories) exposes hooks, in the form of the two post-processing methods shown in the following listing:spring-doc.cn

/**
 * Will handle additional initialization after client.connect() method was invoked,
 * but before any action on the client has been taken
 */
protected void postProcessClientAfterConnect(T t) throws IOException {
    // NOOP
}
/**
 * Will handle additional initialization before client.connect() method was invoked.
 */
protected void postProcessClientBeforeConnect(T client) throws IOException {
    // NOOP
}

As you can see, there is no default implementation for these two methods. However, by extending DefaultFtpSessionFactory, you can override these methods to provide more advanced configuration of the FTPClient, as the following example shows:spring-doc.cn

public class AdvancedFtpSessionFactory extends DefaultFtpSessionFactory {

    protected void postProcessClientBeforeConnect(FTPClient ftpClient) throws IOException {
       ftpClient.setActivePortRange(4000, 5000);
    }
}

FTPS and Shared SSLSession

When using FTP over SSL or TLS, some servers require the same SSLSession to be used on the control and data connections. This is to prevent “stealing” data connections. See scarybeastsecurity.blogspot.cz/2009/02/vsftpd-210-released.html for more information.spring-doc.cn

Currently, the Apache FTPSClient does not support this feature. See NET-408.spring-doc.cn

The following solution, courtesy of Stack Overflow, uses reflection on the sun.security.ssl.SSLSessionContextImpl, so it may not work on other JVMs. The stack overflow answer was submitted in 2015, and the solution has been tested by the Spring Integration team on JDK 1.8.0_112.spring-doc.cn

The following example shows how to create an FTPS session:spring-doc.cn

@Bean
public DefaultFtpsSessionFactory sf() {
    DefaultFtpsSessionFactory sf = new DefaultFtpsSessionFactory() {

        @Override
        protected FTPSClient createClientInstance() {
            return new SharedSSLFTPSClient();
        }

    };
    sf.setHost("...");
    sf.setPort(21);
    sf.setUsername("...");
    sf.setPassword("...");
    sf.setNeedClientAuth(true);
    return sf;
}

private static final class SharedSSLFTPSClient extends FTPSClient {

    @Override
    protected void _prepareDataSocket_(final Socket socket) throws IOException {
        if (socket instanceof SSLSocket) {
            // Control socket is SSL
            final SSLSession session = ((SSLSocket) _socket_).getSession();
            final SSLSessionContext context = session.getSessionContext();
            context.setSessionCacheSize(0); // you might want to limit the cache
            try {
                final Field sessionHostPortCache = context.getClass()
                        .getDeclaredField("sessionHostPortCache");
                sessionHostPortCache.setAccessible(true);
                final Object cache = sessionHostPortCache.get(context);
                final Method method = cache.getClass().getDeclaredMethod("put", Object.class,
                        Object.class);
                method.setAccessible(true);
                String key = String.format("%s:%s", socket.getInetAddress().getHostName(),
                        String.valueOf(socket.getPort())).toLowerCase(Locale.ROOT);
                method.invoke(cache, key, session);
                key = String.format("%s:%s", socket.getInetAddress().getHostAddress(),
                        String.valueOf(socket.getPort())).toLowerCase(Locale.ROOT);
                method.invoke(cache, key, session);
            }
            catch (NoSuchFieldException e) {
                // Not running in expected JRE
                logger.warn("No field sessionHostPortCache in SSLSessionContext", e);
            }
            catch (Exception e) {
                // Not running in expected JRE
                logger.warn(e.getMessage());
            }
        }

    }

}