Detect or prevent Commons Net FTPClient from blocking on read.

Question

I'm using the Commons Net FTPClient class to periodically poll an ftp site and attempt to download a file. Occasionally the thread is blocking indefinitely when trying to read from the site and I wondered if anyone else had encountered this problem and could offer any workarounds?

I'm using FTPClient's retrieveFileStream(String) method to get a handle to an InputStream, which I then attempt to read from. Below is a partial stack trace of the blocked thread.

"FTP File Poll Thread" prio=10 tid=0x083b0800 nid=0x6324 runnable [0x65b5b000..0x65b5beb0]
   java.lang.Thread.State: RUNNABLE
        at java.net.SocketInputStream.socketRead0(Native Method)
        at java.net.SocketInputStream.read(SocketInputStream.java:129)
        at java.io.BufferedInputStream.fill(BufferedInputStream.java:218)
        at java.io.BufferedInputStream.read(BufferedInputStream.java:237)
        - locked <0x5c851928> (a java.io.BufferedInputStream)
        at java.io.FilterInputStream.read(FilterInputStream.java:66)
        at java.io.PushbackInputStream.read(PushbackInputStream.java:122)
        at org.apache.commons.net.io.FromNetASCIIInputStream.__read(FromNetASCIIInputStream.java:75)
        at org.apache.commons.net.io.FromNetASCIIInputStream.read(FromNetASCIIInputStream.java:170)
        at java.io.FilterInputStream.read(FilterInputStream.java:116)

Answer 1

java.io is blocking, you need to use the java.nio package. Never used ftp with java.nio, but assume it must work because http can work with java.nio.

Regards

Answer 2

The kludge I ended up using here was to schedule a task to raise an alert if the FTP operation took longer than X seconds. I did actually find javanioftp on Sourceforge but in my situation it doesn't warrant the development overhead in switching clients.