What is the meaning of different timeout properties of Apache FTPClient (and DefaultFtpSessionFactory in Spring)?

Question

There are following timeout properties used for FTP communication:

• connectTimeout : connection timeout in milliseconds, which will be passed to the Socket object's connect() method
• defaultTimeout : default timeout in milliseconds to use when opening a socket
• dataTimeout : timeout in milliseconds to use when reading from the data connection

Could you explain to Java/Kotlin developer why to set them and what bad things could happen if you do not set them?

To add some context: My server app needs to connect to FTP and list/upload/download/delete files. I would like to be sure my attempts to connect or transfer file won't hang forever in case something goes wrong.

Answer 1

All those properties from the AbstractFtpSessionFactory are propagated down to an FTPClient :

    if (this.connectTimeout != null) {
        client.setConnectTimeout(this.connectTimeout);
    }
    if (this.defaultTimeout != null) {
        client.setDefaultTimeout(this.defaultTimeout);
    }
    if (this.dataTimeout != null) {
        client.setDataTimeout(this.dataTimeout);
    }

The connectTimeout over there has a default value as private static final int DEFAULT_CONNECT_TIMEOUT = 60000; . So, it is OK to miss it. Yes, this one is used when we create a Socket via connect() : _socket_.connect(new .netSocketAddress(host, port), connectTimeout);

The defaultTimeout is propagated down to _socket_.setSoTimeout(_timeout_); which has an effect on socket read operations. See Socket.setSoTimeout() JavaDocs. Yes, its default value is 0 . So, also OK to miss its configuration.

The dataTimeout is used for the server Socket created in a ACTIVE_LOCAL_DATA_CONNECTION_MODE for similar setSoTimeout() option. Default is the same: 0 - infinite wait for read operation answer.

I derived all of that from Apache Commons Net source code.

Doesn't look like the project provides some docs on the matter by itself: https://commons.apache.org/proper/commons.net/

So, yeah, a rule of thumb: always configure those props with a reasonable values.