RFR: 8216497: javadoc should auto-link to platform classes

[Hannes Wallnöfer]

On Tue, 15 Sep 2020 12:56:13 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

>> This pull request is identical with the RFR previously sent for the Mercurial repository:
>> 
>> https://mail.openjdk.java.net/pipermail/javadoc-dev/2020-August/001796.html
>> 
>> I'm copy-pasting the comments from the original RFR below.
>> 
>> Most of the new code is added to the Extern class where it fits in quite nicely and can use the existing supporting
>> code for setting up external links.
>> The default behaviour is to generate links to docs.oracle.com for released versions and download.java.net for
>> prereleases. Platform documentation URLs can be configured using the new --link-platform-properties <url> option to
>> provide a properties file with URLs pointing to to alternative locations. See the CSR linked above for more details on
>> the new options.  The feature can be disabled using the --no-platform-link option, generating the same output as
>> previously.   One problem I had to solve was how to handle the transition from prerelease versions to final releases,
>> since the location of the documentation changes in this transition. For obvious reasons we don’t want to make that
>> switch via code change at a time shortly before release.  The way it is done is that we determine if the current
>> javadoc instance is a prerelease version as indicated by the Version returned by BaseConfiguration#getDocletVersion(),
>> and then check whether the release/source version of the current javadoc execution uses the same (latest) version. This
>> means that that only the latest version will ever generate prerelease links (e.g. running current javadoc 16 with
>> source version 15 will generate links to the final release documentation) but I think this is acceptable.  Another
>> issue I spent some time getting right was tests. New releases require a new element-list resource*), so tests have to
>> pick up new releases. On the other hand, we don’t want hundreds of tests to fail when a new release is added, ideally
>> there should be one test  with a clear message. Because of this, when a release is encountered for which no
>> element-list is available a warning is generated instead of an error, and the documentation is generated without
>> platform links as if running with --no-platform-link option. This allows most existing tests to pass and just the new
>> test to fail with a relatively clear message of what is wrong.
>> *) I also thought about generating the element-list for the current release at build time. It’s quite involved, and we
>>  still need to maintain element-lists for older versions, so I’m not sure it’s worth it.
>> 
>> For existing tests that check output affected by the new option I added  the --no-platform-link option to disable the
>> feature. Otherwise we’d have to update those tests with each new release (or else freeze the tests to use some static
>> release or source version, which we don’t want either).  I updated the CSR to the new code. It also needs to be
>> reviewed: https://bugs.openjdk.java.net/browse/JDK-8251181
>> 
>> Thanks,
>> Hannes
>
> Build changes look good.

Converted to draft as @lahodaj has offered to enhance the feature as described in the comments above.

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PR: https://git.openjdk.java.net/jdk/pull/171