RFR: 8353440: Disable FTP fallback for non-local file URLs by default [v12]

[Shaojin Wen]

On Fri, 25 Apr 2025 18:56:36 GMT, Eirik Bjørsnøs <eirbjo at openjdk.org> wrote:

>> Please help review this PR which disables the unspecified but long-standing feature where an `FtpURLConnection` is opened as a fallback for non-local file URLs.
>> 
>> Before this change, if a file URL has a non-local host component, say `file://remotehost/folder/data.txt`, then the  implementation would attempt opening an FTP connection to `remotehost`. After this change, such URLs will be rejected with a `MalformedURLException`, unless the FTP fallback feature is explicitly re-enabled via a system property.
>> 
>> This change was initially discussed here: https://mail.openjdk.org/pipermail/net-dev/2025-March/025988.html
>> 
>> See the above discussion and CSR draft JDK-8354678 for the motivation for this change.
>> 
>> This PR:
>> 
>> * Changes file URL `Handler::openConnection` implementation for unix/windows to throw `MalformedURLException`, unless the FTP fallback feature is explicitly enabled by configuration.
>> * Introduces a new system property `jdk.net.file.ftpenabled` which when set to `true` re-enables the feature.
>> * Documents the new property in `net-properties.html` 
>> * Updates the existing test `NonLocalFtpFallback` to enable the feature via said system property.
>> * Adds a new test `NonLocalFtpFallbackDisabled` verifying that a `MalformedURLException` is thrown by default for a non-local URL host component.
>> * Moves testing of exceptional behavior in FtpConnection when using non-local file URLs with FTP fallback enabled from `OpenStream` to `NonLocalFtpFallback`
>> 
>> I have added a Release Note as a subtask in the JBS issue, this also needs a review.
>
> Eirik Bjørsnøs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Move URL construction out of assertThrows

src/java.base/share/classes/sun/net/www/protocol/file/FileURLConnection.java line 53:

> 51:     // by default and can be re-enabled by setting a system property
> 52:     private static boolean FTP_FALLBACK_ENABLED =
> 53:             Boolean.getBoolean("jdk.net.file.ftpfallback");

Suggestion:

    private static final boolean FTP_FALLBACK_ENABLED =
            Boolean.getBoolean("jdk.net.file.ftpfallback");

Add final here

test/jdk/sun/net/www/protocol/file/NonLocalFtpFallback.java line 182:

> 180:     @Test
> 181:     public void verifyFtpUnknownHost() throws IOException {
> 182:         URL url  = new URL("file://nonexistinghost/not-exist.txt");

Suggestion:

        URL url = new URL("file://nonexistinghost/not-exist.txt");

One more space

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PR Review Comment: https://git.openjdk.org/jdk/pull/24657#discussion_r2072029391
PR Review Comment: https://git.openjdk.org/jdk/pull/24657#discussion_r2072031757