On 29.05.25 12:21, Kevin Walls wrote:
Hi --
Just to be clear, is this an actual persistent leak that we can observe, or is
it that we could close earlier with try-with-resources?
I'm not seeing a leak when calling a line like this over and over in a tight
loop:
final var cmdline = Files.lines(path).findFirst();
I believe it is persistent until the Cleaner of FileChannel closes it.
The #line Javadoc [1] mentions the need to close
> This method must be used within a try-with-resources statement or
> similar control structure to ensure that the stream's open file is
> closed promptly after the stream's operations have completed.
The Stream class Javadoc [2] explicitly mentions #lines as well
> Generally, only streams whose source is an IO channel, such as those
> returned by Files.lines(Path), will require closing. If a stream does
> require closing, it must be opened as a resource within a
> try-with-resources statement or similar control structure to ensure
> that it is closed promptly after its operations have completed.
If I run something like the code below I get
java.nio.file.FileSystemException: marker: Too many open files. You can
also lower the number, set a breakpoint at System.out.println and check
/proc/<pid>/fd
Path marker = Path.of("marker");
if (!Files.exists(marker)) {
Files.writeString(marker, "line1");
}
// usually over the max fd limit
int fileCount = 100_000;
// prevent streams from being garbage collected and cleaner to run and
close fd
List<Stream<?>> streams = new ArrayList<>(fileCount);
for (int i = 0; i < fileCount; i++) {
Stream<String> stream = Files.lines(marker);
streams.add(stream);
Optional<String> firstLine = stream.findFirst();
if (firstLine.isPresent()) {
if (firstLine.get().hashCode() == 42) {
System.out.println('x');
}
}
}
System.out.println(streams.hashCode());
[1]
https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/nio/file/Files.html#lines(java.nio.file.Path)
[2]
https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/util/stream/Stream.html
Regards
Philippe
