Java Javadoc – Linking to javadoc.io Using Javadoc -link Option

javajavadoc

I am trying to link into some Javadocs hosted at javadoc.io (specifically, PowerMock's Javadocs) using the @link option. I have tried to add the URL to PowerMock's Javadocs to my -link flag, but can't get Javadoc to recognize it. I am using external links to other Javadocs just fine (e.g. Guava, Java SE 7) with Gradle as my build system. I have tried the following options:

-link http://static.javadoc.io/org.powermock/powermock-core/1.6.3/

^ I have confirmed that there is a package-list file in this directory

-link http://static.javadoc.io/org.powermock/powermock-core/

-link http://javadoc.io/doc/org.powermock/powermock-core/1.6.3/

-link http://javadoc.io/doc/org.powermock/powermock-core/

All of these result in the following error (URL changed accordingly):

javadoc: warning - Error fetching URL: http://static.javadoc.io/org.powermock/powermock-core/1.6.3/

Does anyone have advice on how to make this work?

As far as I can tell this is some sort of javadoc.io specific problem, though likely a usage issue on my end – for example I am currently using -link http://junit.org/javadoc/latest/ without issue, but -link http://static.javadoc.io/junit/junit/4.12/ doesn't work.

Best Answer

From the command line, use an argument like -J-Dhttp.agent=javadoc.

In Maven, use something like:

<additionalJOption>-J-Dhttp.agent=maven-javadoc-plugin-${pom‌​.name}</additionalJO‌​ption>

The background: As Danilo Pianini suggests in another answer, the problem is the User-Agent header. However, the problem isn't an empty User-Agent; it's the default Java User-Agent, which looks something like "Java/1.8.0_112":

$ URL=https://static.javadoc.io/org.checkerframework/checker-qual/2.2.2/package-list

# default Java User-Agent:
$ wget -U Java/1.8.0_112 "$URL" 2>&1 | grep response
HTTP request sent, awaiting response... 403 Forbidden

# no User-Agent:
$ wget -U '' "$URL" 2>&1 | grep response
HTTP request sent, awaiting response... 200 OK

# custom User-Agent:
$ wget -U javadoc "$URL" 2>&1 | grep response
HTTP request sent, awaiting response... 200 OK

So the fix is to tell Javadoc to use a different User-Agent. Java won't let you omit the User-Agent, so you'll have to provide a value, which Java will prepend to its default agent.

As best I can tell, the blocking of Javadoc isn't intentional: Javadoc just (probably unwisely) uses the default Java User-Agent, and the content delivery network that javadoc.io uses blocks that by default.

(One more note about Maven: Everything works fine with -link. It also works fine with -linkoffline if you download the package-list file and tell Javadoc to read it from disk. However, if you use -linkoffline but tell Javadoc to fetch package-list from the javadoc.io URL (this is an unusual thing to do), it may fail. The problem: Maven tries to pre-validate the package-list file but, under some versions of Java, fails because it rejects the SSL certificate of javadoc.io, a certificate that Javadoc itself accepts.)

(Oh, and it appears to be important to use a URL specifically from static.javadoc.io, not javadoc.io. Also, I would recommend https, not http, in case http://static.javadoc.io someday starts issuing redirects to https://static.javadoc.io, as Javadoc currently doesn't handle such redirects. Also, https is a good thing :))

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