如何找出哪个对象在 Java 中打开了文件？

Question

I need to find out which object in my Java app has a file open. This is for debugging, so tools or utilities are welcome.

If finding which object is too specific, which class would also be very helpful.

Answer 1

That could be tricky. You could start by using a profiler, like VisualVM (free) or YourKit (not free) to inspect java.io.File objects in memory. If you can find one with the path you're interested in, that might point you in the right direction. The problem is that a File object doesn't necessarily have to exist for an app to still have the file open.

Your next step might be to set breakpoints in both FileInputStream and FileOutputStream (unless you know whether the file is being read or written to), attach a debugger, and watch for your file to be opened by one of those. But of course, there are other ways of opening files as well. You might also need to stake out things like Class.getResourceAsStream() and URL.openConnection() , for example.

Your last resort might be to try out an omniscient debugger, like ODB or TOD . I only have limited experience with them, but they claim to "know everything". You'll just have to figure out how to ask the right question.

Answer 2

Use AspectJ and intercept calls to various flavors of opening file streams. Use an around aspect. Note this can only tell you which object opens the stream, but obviously the object can pass around references to the stream, so ...

Answer 3

The following is Windows and IntelliJ specific, though it is likely similar for other operating systems and IDEs.

Additionally it relies on JDK internals, the following has been written for JDK 11.

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1. Find out the handle of the file. For this you can use the Windows Sysinternals Handle tool (see also this Server Fault question ):

    .\\handle64.exe -p java

2. In the output search for the relevant file, eg:

    D7C: File C:\\Users\\Someone\\myfile.txt

   (Note: Most terminals provide a search function, for cmd.exe it is Ctrl + F ; for Windows Terminal it is Ctrl + ⇧ + F .)
   The value before the colon is important (in this example D7C ), note it down.
3. In IntelliJ pause execution with the debugger.
4. Open the Memory tab
5. Search for java.io.FileDescriptor
6. Right-click the result and select "Show Instances"
7. As condition enter handle == 0x , with handle being the value you noted down in step 2 (in this example handle == 0xD7C ).
   (Note: Under Unix you might have to compare fd == instead.)
   ➔ You should see a single result
8. Right-click the result and select "Show Referring Objects..."
9. On the results you can then continue selecting "Show Referring Objects..." until you found the relevant result (the reference in FileCleanable can most likely be ignored)

This won't always answer which caller forgot to close the file, but it might give a hint. You can then for example set further breakpoints in the relevant classes to find out who opened the file.