tomaswolf commented on issue #743:
URL: https://github.com/apache/mina-sshd/issues/743#issuecomment-4308565800
I finally got around to debug this. I'm sorry about the long delay; I had
too many other things on my plate.
Long story short: don't use `FileSystems.newFileSystem(URI,
Map<String,?>,ClassLoader)`.
Instead use this work-around:
```
public class SftpClientNoLeak {
private static final FileSystemProvider SFTP_FILE_SYSTEMS = new
SftpFileSystemProvider();
public static void main(String[] args) throws Exception {
var uri = args.length == 1 ? URI.create(args[0]) : URI.create(
// use: docker run -p 2200:22 -v path-to-file:/home/demo/sftp --rm
-it emberstack/sftp
"sftp://localhost:2200/sftp/anyfile.txt";
);
for (int i = 0; i < 100; i++) {
var data = fetchData(uri, "demo", "demo");
System.out.println(Thread.getAllStackTraces().size() + " threads");
}
}
public static byte[] fetchData(URI url, String user, String pw) throws
Exception {
URI uri = SftpFileSystemProvider.createFileSystemURI(url.getHost(),
url.getPort(), user, pw);
try (FileSystem fs = SFTP_FILE_SYSTEMS.newFileSystem(uri, Map.of())) {
Path remotePath = fs.getPath(url.getPath());
return Files.readAllBytes(remotePath);
}
}
}
```
Full story:
Apache MINA SSHD assumes that the `SftpFileSystemProvider ` used by
`FileSystems.newFileSystem()` was a singleton. But that works only if the class
can be found via the system class loader or the platform class loader, and Java
treats it as an "installed file system provider". See the javadoc of class
`FileSystems`. In an executable JAR built by the spring-boot-maven-plugin, this
is not the case.
If not present as an "installed file system provider",
`FileSystems.newFileSystem()` then tries to find the provider though the
service loader using the given class loader, in this case, the thread context
classloader. That does find our `SftpFileSystemProvider` all right, and the
file system gets created. But `FileSystems.newFileSystem()` then does _not_
remember that, and the next call to `FileSystems.newFileSystem()` will create
another instance, and so on.
Each `SftpFileSystemProvider` instance creates an `SshClient` that will
remain until the application exits. And each `SshClient` will create its own IO
threads. So that's why the threads accumulate.
The solution for Apache MINA SSHD could be to publish only a stateless
facade via Java SPI and let these facade instances use a true singleton
provider behind the scenes.
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