Velocity or FreeMarker: Looking at 5 Years of Practical Experience
This month's Javaworld contains an article by Jeroen Van Bergen entitled "Velocity or FreeMarker". While it is, of course, nice to get such good publicity for our project, I have certain misgivings about the article and I feel it is necessary to make certain points about it. I have chosen to do so on this blog because this entry will provide a basic link to refer people to in the future, any time we feel these points need to be made.
Here is my central concern about the aforementioned article: typical readers, not very familiar with the template space, will come away with the idea that Velocity and FreeMarker are broadly comparable in functionality, and that it does not matter very much which one you choose. However, they would be extremely mistaken. There is now five years of practical experience that shows that, in the general case, Velocity is not a sufficiently capable tool for professional use. This is why so many projects -- among them some of the most visible projects in the java world, things like Hibernate, Webwork/Struts 2, and Netbeans -- used Velocity at some earlier stage, but have since migrated to FreeMarker.
Quite amazingly, Mr. Van Bergen's article makes no mention of any of this. So let me start by filling in some of this background.
Max's story about FreeMarker and Velocity
One of the better known cases of a project switching from Velocity to FreeMarker is Hibernate tools. In early 2006, the lead developer, Max Andersen, wrote a blog entry about this that generated some buzz. While Max outlined various issues he had with Velocity, his biggest problem, the central message, was that he simply could not get Velocity to provide useful error messages. If he misspelt a variable somewhere in a template, the default behavior was just to keep going as if nothing happened. Even if he configured Velocity to throw an exception at this point, he had no information about the line number or even the file in which the error occurred.
Now, to be fair, the most recent version of Velocity has some improvements on this front. However, Velocity is still quite weak in error reporting compared to FreeMarker, and real practical experience with both tools shows that the lack of good error messages can be a huge drag on developer productivity.
Error messages were also one of the two stated reasons that the Webwork project, in early 2005, switched from using Velocity by default to FreeMarker. The other was FreeMarker's support for JSP taglibs. Note that Webwork later was donated to the Apache Software Foundation and was renamed "Struts 2". At around this time, Matthew Porter wrote a blog entry describing his switch from Velocity to FreeMarker for all his development.
Now, meanwhile, other people undertook the same migration but for other reasons. Consider Matt Ward's blog entry from mid-2005. One of Matt's main problems with Velocity was that Velocity simply did not address the whole issue of outputting numbers, dates, and currencies, for different audiences, despite the fact that the information to do this is embedded in the core java libraries. Another issue for Matt was Velocity's lack of robust support for macro libraries. FreeMarker supports the notion of namespaces so that different sets of macros can define variables without any fear of them clobbering one another.
More recently, it has come to our attention that Netbeans 6 leverages FreeMarker for templating functionality. (Previous versions were using Velocity.) I have not seen any public explanation of their reasons, but I infer that they were running into various limitations of Velocity, and were looking for a better tool and opted to migrate towards FreeMarker. Consider this blog entry outlining how to adapt to the change to FreeMarker.
What about changes in the other direction?
Now, so far, I have provided examples of people switching from Velocity to Freemarker, but none of anybody going in the opposite direction. Well, feel free to look for examples of this. Google is your friend.
I can only find one example. In this blog entry, Howard Abrams, a long-time FreeMarker user, describes his experience switching to Velocity. Except that he finds Velocity to be too limited, and switches back to FreeMarker!
In the foregoing, I highlighted examples of 5 years of practical experience that the java development community has had with Velocity and FreeMarker. Jeroen Van Bergen's article simply omits all of this. While that is surely the central problem, I should still point out other issues with the comparison carried out in the article.
For example, on the last page is a feature checklist chart, showing features that FreeMarker has and Velocity has. FreeMarker allows iteration, Velocity allows iterations, FreeMarker has macros, Velocity has macros. And so on. The chart is outrageously misleading, because it suggests that the two tools are comparable functionally. But that is simply because it omits all the features that FreeMarker has that Velocity lacks!
A more complete list would contain things like:
- Support for transparent i18n with numbers and dates shown according to the relevant locale: FreeMarker, yes. Velocity, no.
- Support for JSP taglibs: FreeMarker, yes. Velocity, no.
- Support for Jython and Rhino: FreeMarker, yes. Velocity, no.
- Special tags for stripping of extraneous whitespace: FreeMarker, yes. Velocity, no.
- Support for auto-escaping interpolations on blocks of text (converting problematic characters to HTML entities, for example): FreeMarker, yes. Velocity, no.
Also, just consider the statement that FreeMarker has macros and Velocity has macros, with no further elaboration. It is actually true, as far as that goes, but it is certainly quite misleading. Now, it is true that in paragraph below that feature checklist chart, the author does mention that FreeMarker's macro system has some additional capabilities. However, the basic impression given from the way the information is presented is that the difference is at most marginal. Surely, a more complete feature checklist of the tools' respective macro capabilities is called for and that would be something like:
- Support for importing a set of macros (macro library) in a separate namespace to avoid variable naming clashes: FreeMarker, yes, Velocity, no.
- Support for macros with an associated template block: FreeMarker, yes, Velocity, no.
- Support for optional and default parameters in macro calls: FreeMarker, yes. Velocity, no.
- Output of the macro call stack when an error is hit: FreeMarker, yes. Velocity, no.
Right below this, Mr. Van Bergen mentions Anakia, which is an add-on to Velocity for processing XML. He neglects to mention that FreeMarker provides similar XML processing functionality (though the implementation is much more complete, since it supports XML namespaces, for example) as part of its core feature set. Declarative XML processing is supported in FreeMarker via the #visit and #recurse directives, which are core directives in the FreeMarker language. One would infer from what the article says that XML processing is a point in favor of Velocity, when, really, quite the opposite is the case. The XML processing functionality available for Velocity is add-ons like Anakia and DVSL that are basically abandonware, where the XML processing support in FreeMarker is a core part of the product, and is clearly supported.
Still, the central problem with this article and the comparison it undertakes is that it ignores this huge body of evidence from real-world experience that all points to a very clear conclusion: quite frequently, people switch to FreeMarker after hitting Velocity's limitations. In Max Andersen's case, it was the poor to nonexistent error reporting, where for other people, it is Velocity's half-baked, rather buggy implementation of macros. And then, in other cases, the deal breaker is Velocity's lack of transparent support for internationalization. But regardless of the reasons in the specific case, it is quite clear that there has been a large migration from Velocity to FreeMarker and virtually no movement in the opposite direction. In the foregoing, I mentioned cases that are highly visible like Netbeans or Hibernate. We can only speculate about how many projects are out there, most of them not visible at all, because they are in a company's closed codebase, that have ended up undertaking a similar migration. In any of these cases, the people involved could have saved themselves a lot of time and effort by opting for FreeMarker in the first place, rather than starting with Velocity, discovering its limitations, and then having to migrate. So, it serves the readers of a publication like Javaworld rather poorly to have articles that foster the misconception that FreeMarker and Velocity are broadly comparable in functionality. They are not. There is an overwhelming body of evidence from real-world praxis that shows this quite clearly.