8296656: java.lang.NoClassDefFoundError exception on running fully legitimate code #12754

archiecobbs · 2023-02-25T21:22:57Z

This bug and a few others fall into the "output file clash" bucket. This is when the compiler thinks it's writing out two separate files, but due to the way the O/S filesystem maps Path's to actual files, it's really writing the same file twice.

This is usually due to case-insensitive filesystems, but can also be due to how a filesystem "normalizes" file names. For example, on MacOS, compiling this class will generate such a clash:

public class Test {
    interface Cafe\u0301 {
    }
    interface Caf\u00e9 {
    }
}

The reason is that \u0301 is the Unicode character "Combining Acute Accent" which means "stick an accent over the previous character". So MacOS normalizes a e followed by a \u0301 into a Unicode \u00e9, that is, é. However, the Java language treats these the two names Cafe\u0301 and Caf\u00e9 as distinct.

It's infeasible to truly "fix" this problem, so we resort here to a salve, which is to add a new compiler flag --detect-output-file-clashes that enables detection of output file clashes. When the flag is enabled, and a clash is detected, an error is immediately thrown. For example, compiling the example above gives this:

$ javac --help-extra
  ...
  --detect-output-file-clashes
        Generate an error if any output file is overwritten during compilation. This can occur, for example,
        on case-insensitive filesystems. This applies to class files, native header files, and source files.
  ...
$ javac --detect-output-file-clashes Test.java
Test.java:4: error: error while writing Café: output file clash: /Users/archie/test/Test$Café.class
    interface Caf\u00e9 {
    ^
1 error

This at least gives people at risk of encountering this problem a way to turn a runtime error into a compile-time error.

Outstanding Questions

Is making this optional via an (extended) flag --detect-output-file-clashes the best way to address this?
Does the new field BaseFileManager.outputFilesWritten ever need to be cleared? Can the file manager be used for multiple compilations? If so, is there some "reset" step we should hook into?

Progress

Change must be properly reviewed (1 review required, with at least 1 Reviewer)
Change must not contain extraneous whitespace
Commit message must refer to an issue
Change requires CSR request JDK-8304533 to be approved

Issues

JDK-8296656: java.lang.NoClassDefFoundError exception on running fully legitimate code
JDK-8287885: Local classes cause ClassLoader error if the type names are similar but not same
JDK-8304533: Add new lint flag "output-file-clash" (CSR)

Reviewers

Vicente Romero (@vicente-romero-oracle - Reviewer)

Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/12754/head:pull/12754
$ git checkout pull/12754

Update a local copy of the PR:
$ git checkout pull/12754
$ git pull https://git.openjdk.org/jdk.git pull/12754/head

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Checkout this PR locally:
$ git pr checkout 12754

View PR using the GUI difftool:
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https://git.openjdk.org/jdk/pull/12754.diff

bridgekeeper · 2023-02-25T21:23:59Z

👋 Welcome back archiecobbs! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2023-02-25T21:26:50Z

@archiecobbs The following label will be automatically applied to this pull request:

compiler

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

archiecobbs · 2023-02-25T21:35:54Z

/solves 8287885

openjdk · 2023-02-25T21:36:21Z

@archiecobbs
Adding additional issue to solves list: 8287885: Local classes cause ClassLoader error if the type names are similar but not same.

archiecobbs · 2023-03-06T23:02:04Z

/csr

openjdk · 2023-03-06T23:04:33Z

@archiecobbs has indicated that a compatibility and specification (CSR) request is needed for this pull request.

mlbridge · 2023-03-20T14:04:51Z

Webrevs

…tten".

vicente-romero-oracle

looks good to me

openjdk · 2023-03-27T18:28:43Z

@archiecobbs This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8296656: java.lang.NoClassDefFoundError exception on running fully legitimate code
8287885: Local classes cause ClassLoader error if the type names are similar but not same

Reviewed-by: vromero

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 76 new commits pushed to the master branch:

80e2d52: 8302558: Editable JComboBox's popup blocks user from seeing characters in Aqua look and feel
6c3b10f: 8303485: Replacing os.name for operating system customization
87b314a: 7093691: Nimbus LAF: disabled JComboBox using renderer has bad font color
2f34687: 8304689: Add hidden option to disable external spec page
6b2f34f: 8304718: GetIntArrayElements should not be passed JNI_FALSE
138cdc9: 8304694: Runtime exception thrown when break stmt is missing
46b0602: 8304547: Remove checking of -Djava.compiler in src/jdk.jdi/share/classes/com/sun/tools/jdi/SunCommandLineLauncher.java
8d423f7: 8304723: Statically allocate global mutexes
2083088: 8304828: Lots of constant static data not declared static const in cpu/x86
0712adc: 7169951: SwingSet2 throws NullPointerException with Nimbus L&F
... and 66 more: https://git.openjdk.org/jdk/compare/42723dcb1862da598092bb499056940d78a8bdac...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

As you do not have Committer status in this project an existing Committer must agree to sponsor your change. Possible candidates are the reviewers of this PR (@vicente-romero-oracle) but any other Committer may sponsor as well.

➡️ To flag this PR as ready for integration with the above commit message, type /integrate in a new comment. (Afterwards, your sponsor types /sponsor in a new comment to perform the integration).

vicente-romero-oracle · 2023-03-27T20:05:40Z

tier1-4 green

archiecobbs · 2023-03-27T20:49:23Z

/integrate

openjdk · 2023-03-27T20:51:22Z

@archiecobbs
Your change (at version 73ab9af) is now ready to be sponsored by a Committer.

vicente-romero-oracle · 2023-03-27T21:31:48Z

/sponsor

openjdk · 2023-03-27T21:33:13Z

Going to push as commit 14b970d.
Since your change was applied there have been 76 commits pushed to the master branch:

80e2d52: 8302558: Editable JComboBox's popup blocks user from seeing characters in Aqua look and feel
6c3b10f: 8303485: Replacing os.name for operating system customization
87b314a: 7093691: Nimbus LAF: disabled JComboBox using renderer has bad font color
2f34687: 8304689: Add hidden option to disable external spec page
6b2f34f: 8304718: GetIntArrayElements should not be passed JNI_FALSE
138cdc9: 8304694: Runtime exception thrown when break stmt is missing
46b0602: 8304547: Remove checking of -Djava.compiler in src/jdk.jdi/share/classes/com/sun/tools/jdi/SunCommandLineLauncher.java
8d423f7: 8304723: Statically allocate global mutexes
2083088: 8304828: Lots of constant static data not declared static const in cpu/x86
0712adc: 7169951: SwingSet2 throws NullPointerException with Nimbus L&F
... and 66 more: https://git.openjdk.org/jdk/compare/42723dcb1862da598092bb499056940d78a8bdac...master

Your commit was automatically rebased without conflicts.

openjdk · 2023-03-27T21:33:29Z

@vicente-romero-oracle @archiecobbs Pushed as commit 14b970d.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

davidalayachew · 2023-04-21T13:52:43Z

Thanks @archiecobbs and @vicente-romero-oracle

Sorry for having sent this so late, it's hard to keep up with the traffic of the mailing lists. I only found this because I looked up my old JBS submission from last year on a whim.

What's the best way I should go about testing this? I see instructions on how to build the JDK from the source code. Is that the best way? Or are there any prebuilt versions that contain this fix that you all can think of from the top of your head?

Thanks again for your time, help, and patience!
David Alayachew

davidalayachew · 2023-04-21T13:53:15Z

And just for context, I am the original submitter of 8287885

archiecobbs · 2023-04-21T15:39:05Z

What's the best way I should go about testing this?

Using whatever your original example was. Or some simple example like this on a case-insensitive filesystem:

public class Test {
    interface Inner {
    }
    interface INNER {
    }
}

I see instructions on how to build the JDK from the source code. Is that the best way?

As far as I know that's the best way... but I could be unaware of pre-built preview images somewhere.

davidalayachew · 2023-04-21T15:47:28Z

What's the best way I should go about testing this?

Using whatever your original example was. Or some simple example like this on a case-insensitive filesystem:
public class Test {
    interface Inner {
    }
    interface INNER {
    }
}
I see instructions on how to build the JDK from the source code. Is that the best way?

As far as I know that's the best way... but I could be unaware of pre-built preview images somewhere.

Ok, beautiful. I'll get on that now. Thanks again to you both!

davidalayachew · 2023-04-24T16:14:38Z

Finished building the jdk from source and tested the fix. It worked!

Here is a demonstration for those who are following along.

$ cat abc.java
public class abc
{

        public static class XYZ {}
        public static class xyz {}

        public static void main(String[] args)
        {

                System.out.println("abc");

                XYZ a = new XYZ();

                System.out.println("tuv");

                xyz b = new xyz();

                System.out.println("xyz");

        }

}

$ javac abc.java -Xlint:output-file-clash
warning: [output-file-clash] output file written more than once: C:\Users\david\_WORKSPACE\_PROGRAMMING\_JAVA\abc$xyz.class
1 warning

As you can see, if I use the -Xlint warning above, I now get a warning if any of my classes generate the same filename multiple times. This is perfect, as I now know at compile time that this is something to watch out for.

Thank you for fixing this Archie/Vicente!

Add new javac flag "--detect-output-file-clashes".

ac02eea

openjdk bot added the compiler compiler-dev@openjdk.org label Feb 25, 2023

openjdk bot added the csr Pull request needs approved CSR before integration label Mar 6, 2023

archiecobbs marked this pull request as ready for review March 20, 2023 13:58

openjdk bot added the rfr Pull request is ready for review label Mar 20, 2023

archiecobbs added 8 commits March 21, 2023 17:23

Merge branch 'master' into JDK-8296656

c1a18ca

Remove obsolete regression test for 7016187 handled now in e339e18.

22fbfeb

Synchronize consistently (and not redundantly) around "outputFilesWri…

f22ae87

…tten".

Flush "outputFilesWritten" on close() for good measure.

51134dd

Use a lint flag "output-file-clash" instead of a command line flag.

b255ed8

Convert "output-file-clash" from an error into a warning.

7d2af14

Improve warning wording to "output file written more than once".

26b758e

Add "compiler.warn.output.file.clash" to message example exclusion file.

73ab9af

openjdk bot removed the csr Pull request needs approved CSR before integration label Mar 27, 2023

vicente-romero-oracle approved these changes Mar 27, 2023

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8296656: java.lang.NoClassDefFoundError exception on running fully legitimate code #12754

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8296656: java.lang.NoClassDefFoundError exception on running fully legitimate code #12754

8296656: java.lang.NoClassDefFoundError exception on running fully legitimate code #12754

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