Java: why does it take several seconds to check if a file exists on an inaccessible Windows drive?

Question

Checking for an existing file using any available method is usually very fast, but if the file path includes an inaccessible (network) drive, the check takes several seconds.

Checking for a non-existent drive is no problem ("R:" in the code sample), but checking a known drive that is no longer accessible ("S:" in the code sample) takes very long time.

Edit: this has a significant effect both when validating a list of recent files (multiply each inaccessible file by 2-4 seconds) and when opening the standard JFileChooser(), which can take 1-2 minutes with a few disconnected drives.

Questions:

1. Is this a known problem?
2. Is there a workaround?

Context: Java 11 or 17 on Windows 10 with an existing "C:" drive, a non-existent "R:" drive, and a network drive "S:" that is not accessible.

Sample code:

import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.LinkOption;
import java.nio.file.Path;
import java.nio.file.attribute.BasicFileAttributes;

public class FileSystemExperiment {

   private static final String[] FILES = {
      System.getProperty("user.home"), // home dir exists
      "/no/such/file",                 // missing file on current drive
      "C:\\no\\such\\file",            // missing file on existing drive
      "R:\\",                          // an unknown drive
      "S:\\",                          // a known but inaccessible drive
   };

   private static long started;

   private static void exists(String... pathStrings) {
      for (String pathString : pathStrings) {
         File file = new File(pathString);
         Path path = file.toPath();
         System.out.printf("%nFile '%s' ...%n", file);

         started = System.currentTimeMillis();
         time("file.exists()", file.exists());
         time("file.lastModified()>0", file.lastModified() > 0);
         time("Files.exists(path)", Files.exists(path));
         time("Files.readAttributes(path)", readAttributes(path) != null);
      }
   }

   private static void time(String msg, boolean exists) {
      System.out.printf("%-30s | %-5s | time=%s ms%n", msg, exists, System.currentTimeMillis() - started);
      started = System.currentTimeMillis();
   }

   private static BasicFileAttributes readAttributes(Path path) {
      try {
         return Files.readAttributes(path, BasicFileAttributes.class, LinkOption.NOFOLLOW_LINKS);
      }
      catch (IOException e) {
         return null;
      }
   }

   public static void main(String[] args) {
      exists(FILES);
   }
}

Output:

File 'C:\Users\p2r' ...
file.exists()                  | true  | time=1 ms
file.lastModified()>=0         | true  | time=0 ms
Files.exists(path)             | true  | time=2 ms
Files.readAttributes(path)     | true  | time=0 ms

File '\no\such\file' ...
file.exists()                  | false | time=0 ms
file.lastModified()>=0         | false | time=0 ms
Files.exists(path)             | false | time=1 ms
Files.readAttributes(path)     | false | time=0 ms

File 'C:\no\such\file' ...
file.exists()                  | false | time=0 ms
file.lastModified()>=0         | false | time=0 ms
Files.exists(path)             | false | time=1 ms
Files.readAttributes(path)     | false | time=1 ms

File 'R:\' ...
file.exists()                  | false | time=0 ms
file.lastModified()>=0         | false | time=0 ms
Files.exists(path)             | false | time=0 ms
Files.readAttributes(path)     | false | time=0 ms

File 'S:\' ...
file.exists()                  | false | time=7802 ms
file.lastModified()>=0         | false | time=10067 ms
Files.exists(path)             | false | time=7171 ms
Files.readAttributes(path)     | false | time=10047 ms

Answer 1

Q1: Yes, this is a known problem, reported in 1999 but still not resolved. See https://bugs.java.com/bugdatabase/view_bug.do?bug_id=4260746:

Situation: You have connections to other computers in Win NT and these computers are not online. You start a JAVA-application (jdk1.2.1) with a JFileChooser. The dialog tries to "see" all the connected computers. Because some other computers are not avaiable, the filechooser-dialog appears after reaching the timeout. It may take 60 to 120 seconds until the dialog appears.

WORK AROUND: Close the network-connections to the computers that are not avaiable. The dialog appears just after invoking.

EVALUATION: This is an important performance problem and should be addressed in a not too distant release.

Q2: No, there are no feasible workarounds.

See How can I make JFileChooser behave properly with disconnected network drives?

It seems like it would be a common problem, but I can't find a way to work around it. Possible options I've seen are:

• use JFileDialog; replace FileSystemView with one where getRoots() returns a fixed list of drives
• switch to a WAIT cursor before trying to open JFileChooser(s)
• create a JFileChooser at startup and keep it around forever
• try using xfiledialog

In addition to the options mentioned above:

• Check and cache availability of all drives using a background thread with a timeout
• Wrap all calls to file operations (e.g. file.exists()) in methods that check the availability of the drive before delegating the call to the file
• Instantiate the JFileChooser with a custom FileSystemView that wraps the default FileSystemView (FileSystemView.getFileSystemView()) and checks the drive status before delegating calls to the wrapped view