Saturday, 8 February 2014

Turning off doclint in JDK 8 Javadoc

JDK 8 includes many updates, but one is I suspect going to cause quite a few complaints - doclint for Javadoc.

Javadoc doclint

Documentation is not something most developers like writing. With Java, we were fortunate to have the Javadoc toolset built in and easy to access from day one. As such, writing Javadoc is a standard part of most developers life.

The Javadoc comments in source code use a mixture of tags, starting with @, and HTML to allow the developer to express their comment and format it nicely.

Up to JDK 7, the Javadoc tool was pretty lenient. As a developer, you could write anything that vaguely resembled HTML and the tool would rarely complain beyond warnings. Thus you could have @link references that were inaccurate (such as due to refactoring) and the tool would simply provide a warning.

With JDK 8, a new part has been added to Javadoc called doclint and it changes that friendly behaviour. In particular, the tool aim to get conforming W3C HTML 4.01 HTML (despite the fact that humans are very bad at matching conformance wrt HTML).

With JDK 8, you are unable to get Javadoc unless your tool meets the standards of doclint. Some of its rules are:

  • no self-closed HTML tags, such as <br /> or <a id="x" />
  • no unclosed HTML tags, such as <ul> without matching </ul>
  • no invalid HTML end tags, such as </br>
  • no invalid HTML attributes, based on doclint's interpretation of W3C HTML 4.01
  • no duplicate HTML id attribute
  • no empty HTML href attribute
  • no incorrectly nested headers, such as class documentation must have <h3>, not <h4>
  • no invalid HTML tags, such as List<String> (where you forgot to escape using &lt;)
  • no broken @link references
  • no broken @param references, they must match the actual parameter name
  • no broken @throws references, the first word must be a class name

Note that these are errors, not warnings. Break the rules and you get no Javadoc output.

In my opinion, this is way too strict to be the default. I have no problem with such a tool existing in Javadoc, but given the history of Javadoc, errors like this should be opt-in, not opt-out. Its far better to get slightly broken Javadoc than no Javadoc.

I also haven't been able to find a list of the rules, which makes life hard. At least we can see the source code to reverse engineer them.

Turning off doclint

The magic incantation you need is -Xdoclint:none. This goes on the command line invoking Javadoc.

If you are running from maven, you need to use the additionalparam setting, as per the manual. Either add it as a global property:

  <properties>
    <additionalparam>-Xdoclint:none</additionalparam>
  </properties>

or add it to the maven-javadoc-plugin:

  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-javadoc-plugin</artifactId>
      <configuration>
        <additionalparam>-Xdoclint:none</additionalparam>
      </configuration>
    </plugin>
  </plugins>

Ant also uses additionalparam to pass in -Xdoclint:none, see the manual.

Gradle does not expose additionalparam but Tim Yates and Cedric Champeau advise of this solution:

  if (JavaVersion.current().isJava8Compatible()) {
    allprojects {
      tasks.withType(Javadoc) {
        options.addStringOption('Xdoclint:none', '-quiet')
      }
    }
  }

See also the Gradle manual.

Summary

I don't mind doclint existing, but there is no way that it should be turned on to error mode by default. Getting some Javadoc produced without hassle is far more important than pandering to the doclint style checks. In addition, it is very heavy handed with what it defines to be errors, rejecting plenty of HTML that works perfectly fine in a browser.

I've asked the maven team to disable doclint by default, and I'd suggest the same to Ant and Gradle. Unfortunately, the Oracle team seem convinced that they've made the right choice with errors by default and their use of strict HTML.

Comments welcome, but please note that non-specific "it didn't work for me" comments should be at Stack Overflow or Java Ranch, not here!

11 comments:

  1. How can they possibly consider this an acceptible change? Ths amount of currently valid documentation that will fail now will be immense. I know that much of my own documentation will be invalid as I write XHTML (I.e. I always close all tags). Some editors will not format Javadoc properly if the tags aren't closed.

    So now, many, possibly most, developers will turn doclint off and never turn it back on again negating its benefits.

    ReplyDelete
  2. Surely they should be enforcing a subset of HTML5, not HTML4.01?

    As all modern browsers support a fair chunk of HTML5.

    ReplyDelete
  3. I think it's a good thing they decided to fail-fast on invalid javadocs. But since when are "self-closed HTML tags, such as <br />" invalid?

    ReplyDelete
    Replies
    1. <br /> is invalid in HTML4, and has always been. HTML is not XML. Browsers are very lenient, of course.

      This stems from a complicated relationship between HTML and XHTML. An old but relevant article on the subject: http://hixie.ch/advocacy/xhtml

      HTML5 fixes this issue (both <br> and <br /> are fine, among thousands of other such things), so it's a mystery why Javadoc insists on HTML4.

      Delete
  4. Agreed: they need to support a subset of HTML 5, not HTML 4, especially since they fail-fast by default and HTML 4 doesn't support
    .

    But the decision to fail-fast by default on invalid input is a good decision I think.

    ReplyDelete
  5. Looks like it won't work with Maven 3.0.4, what is the minimum Maven and Java 8 build to actually accept -Xdoclint:none?

    Besides styling and font of Java 8 JavaDoc look extremely bad in every browser, even the Spec Leads of some Platform JSRs avoid it for that reason...

    ReplyDelete
    Replies
    1. I applied both in a POM and it ended up working with a Maven 3.x version and Java 8 close to Final.
      JavaDoc is still bad and someone from the Frontend team at Oracle shared the issue. The problem is, that almost every major browser shows many characters like "a" as an ugly, unreadable chunk (the upper part of the letter running into the lower one like it's melting ;-/ ) So far Firefox seems to handle it best, but all other browsers I checked (Chrome, IE, Opera) look horrible.

      Delete
  6. I've seen on StackOverflow (link) that with JDK8 you must now add either a caption or a summary if you use HTML tables inside Javadoc comments. Why would this be required? Isn't a <table> without a caption or a summary perfectly legal HTML?

    ReplyDelete
    Replies
    1. Looks like its an accessibility feature. http://www.w3schools.com/tags/att_table_summary.asp . Big corporates like Oracle tend to worry about such things.

      Delete
  7. There is an XML flavor of HTML 4 (but not HTML5) called XHTML 1.0, so something like
    might well be what you intended to output....

    ReplyDelete
  8. I wish they'd ditch HTML in favor of markdown or wiki syntax or something. I have to be able to read the docs in source before they get stuffed into a full-blown HTML page, and raw HTML is not human-friendly at all. I see too much un- and under-commented code as it is: all this does is create a disincentive to write Javadoc at all. It would make more sense to fail when public classes lack Javadocs entirely than to do what they've done.

    ReplyDelete