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| <a name="Differences: Velocity vs. WebMacro"><strong>Differences: Velocity vs. WebMacro</strong></a> |
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| <blockquote> |
| <p> |
| On the mailing lists, we often get asked to explain the differences |
| between Velocity and WebMacro. Instead of answering the same question |
| over and over, we have started this document to describe some of the |
| differences in our approaches. In reality, both are excellent tools and |
| we feel that it is up to the user base to decide which one they would |
| like to use. |
| </p> |
| <p> |
| The language syntax is very close but not exactly the same. The syntaxes |
| were kept close for a few reasons: |
| </p> |
| <p> |
| 1) The syntactical approach of $<foo> for references and |
| #<bar> for directives is a good one! It's easy for designers to |
| work with, people understand it, and there was no reason to reinvent the |
| wheel. |
| </p> |
| <p> |
| 2) Since so many people were using the syntax, why invent another one? |
| Also, it makes the WebMacro to Velocity conversion much easier :) We |
| even have a <a href="migration.html">tool</a> to help with that. |
| </p> |
| <p> |
| Velocity was built new from the ground up. There isn't a snippet of |
| shared code. This was originally done because WebMacro was released |
| under a license (GPL) that is not compatible with the BSD/ASF license |
| and we (the original authors) needed a solution that was not under a GPL |
| license. Since then, WebMacro has been released under a dual GPL/ASF |
| license, however, that was too late, this project was already well |
| underway and we also felt that we have a better technical solution by |
| going with a generated parser instead of a hand coded one. |
| </p> |
| <p> |
| By starting new and fresh, we are less encumbered with some of the |
| baggage that WebMacro must deal with due to backwards compatibility |
| issues. We have fixed a few of the template language inconsistencies |
| that we saw creeping into WebMacro such as the fact that we do not |
| support using {} for code blocks (because we do not need to) and that we |
| use () around all #directives on the right hand side. Examples: |
| </p> |
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| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
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| <tr> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#ffffff"><pre>#set ($bar = "foo") |
| |
| #if ($bar.length() > 0) |
| stuff here |
| #end |
| |
| </pre></td> |
| <td bgcolor="#023264" width="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
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| <td bgcolor="#023264" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| <td bgcolor="#023264" width="1" height="1"><img src="/images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"/></td> |
| </tr> |
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| <p> |
| Architecturally, the biggest difference until recently was the parser. |
| The Velocity parser is written using a tool called javacc, a very |
| popular Java-based parser generator. It was originally developed for |
| Sun by Metamata and is now maintained and managed by Metamata as well : |
| <a href="http://www.metamata.com/">www.metamata.com</a>. |
| </p> |
| <p> |
| The WebMacro parser was written by hand until recently and was the cause |
| of much pain when using WebMacro (due to many bugs in it) - their new |
| parser, for their 0.95 release is also written using javacc and promises |
| to be better (however, support for a lot of the baggage still exists). |
| </p> |
| <p> |
| Velocity is very simple in its architecture, as it is meant to be |
| nothing more than a template engine. The idea is to do one thing only |
| and do it well. Much in the spirit of the Unix philosophy - have a good |
| set of simple tools, and create bigger things using them. This has |
| enabled Velocity to be used as the basis for several different tools, |
| such as <a href="anakia.html">Anakia</a> and <a href="texen.html">Texen</a>. |
| </p> |
| <p> |
| Velocity is recognized that it isn't going to try to be a 'development |
| framework' - there are plenty around, such as <a href="http://jakarta.apache.org/turbine/">Turbine</a>. |
| </p> |
| <p> |
| Therefore, all facilities like 'context tool' and 'resource brokers' are |
| purposely left out of Velocity, acknowledging that not only will |
| frameworks do it, they will do it better anyway. Further, by leaving out |
| of the Velocity core much that should be done at the application / user |
| / framework level, the Velocity core remains smaller and therefore, in |
| our opinion, more maintainable and extensible. |
| </p> |
| <p> |
| Internally, Velocity is pretty straightforward. There are no threads |
| created or managed for you. Templates are parsed and caches, kept in |
| their internal form of an 'Abstract Syntax Tree', a tree structure that |
| is then rapidly walked when your template is merged. Each node |
| corresponds to on of the syntactical elements in your template. |
| </p> |
| <p> |
| Internally within WebMacro, many of the methods and classes are marked |
| as private and/or final in order to achieve better speed. However, this |
| has prevented people from easily extending or overriding some of the |
| core funtionality and re-using some of the code. We would rather |
| sacrifice a few ms in favor of allowing more people to use our software |
| easily. |
| </p> |
| <p> |
| The biggest feature difference between the two are the '<a href="user-guide.html">Velocimacros</a>'. Velocimacros can be thought of |
| as snippets of re-usable data that can contain anything. They are parsed |
| just like the data in a #parse() statement would be and you can pass in |
| variables that can optionally have scope only within the Velocimacro. |
| Velocimacro's help keep the core #directive set small and |
| clean : you can extend the designers toolkit with libraries of Velocimacros |
| rather than adding elements to the syntax or having two write tools that generate |
| HTML (or other specific) markup. |
| </p> |
| <p> |
| We hope this helps. The best way to understand Velocity is to use it, |
| and of course nothing beats looking at the source code. |
| </p> |
| </blockquote> |
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