apache velocity - oscon2007

8/11/2019 Apache Velocity - OSCON2007

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Apache VelocityA Java Template Engine

Nathan Bubna

[email protected],

Henning P. [email protected]

OSCON 2007, July 25th 2007

Portland, Oregon

http://people.apache.org/~nbubna/oscon2007
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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Template Engine? Defines a simple language

(VTLVelocity Template Language)

Templates are text containing active

elements Active elements are replaced with

values from the model

No industry standard templatelanguage

Templates are processed, notcompiled


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Velocity Background- Velocity was started in 2000

- One of the first Jakarta projects

- ASF-licensed alternative to WebMacro

- Language syntax is similar and still very stable

- 100% Pure Java (runs on Java 1.3 or better)

- Velocity became an Apache TLP in 2006

- Current release is 1.5 (as of March 07)


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Velocity Spotting- Velocity is integrated with many other projects (Turbine, Struts 2,

Maven) and others (http://wiki.apache.org/velocity/PoweredByVelocity)

- Support for IDEs and Editors available (e.g. Eclipse, IntelliJ

IDEA, emacs) (http://wiki.apache.org/velocity/VelocityEditors)

- Response content in webapp frameworks (e.g. Struts, Turbine)

- Code generation (e.g. Torque, MyEclipse)

- Documentation (e.g. Maven, Anakia)

- Email (e.g. Spring)
http://wiki.apache.org/velocity/PoweredByVelocityhttp://wiki.apache.org/velocity/VelocityEditorshttp://wiki.apache.org/velocity/VelocityEditorshttp://wiki.apache.org/velocity/PoweredByVelocity


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Velocity vs JSP- Easier to test

- No servlet container required

- Better separation of code and design

- No compilation into Java code

- Few, easy to learn language elements

- No embedded Java code!

- Easy on the eyes


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Real Template Engines- perl: HTML-Template, Mason

- php: Smarty, SmartTemplate,

TinyButStrong- Ruby: Galena, LiquidRuby

- Python: QuickSilver, Cheetah,TurboGears, Airspeed

- Java: WebMacro, FreeMarker,Apache Velocity


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Velocity Template Language- Simple constructs

- #directive() (line directives)

- #directive() #end (block directives)

- $referenceor ${reference}

- Embedded directly into template files(no open / close tags like JSP or PHP)

- References are loosely typed

- VTL can be learned in minutes


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Assignments - #set()- #set is used to create or update a reference

- #set( $foo = text ) String value

- #set( $foo = 100 ) Numeric value- #set( $foo = [ 1, 2, 3 ] ) Array

- #set( $foo = { 1 : 2, 3 : 4 } ) Map

- #set( $foo = $bar ) Reference- #set( $foo = $bar.foo ) Bean Property

- #set( $foo = $bar.doFoo() ) Method Result


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Demo #1 - References

- #set( $message = Hello World )

- This is a Velocity $message program.

- #set( $favorite = 137 )

- My favorite number is not $favorite.


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Display Control Elements- Loop

- #foreach( $foo in $bars ) ... #end

- Conditional

- #if ( ) #elseif ( ) #else #end

- Inclusion of external elements

- #include( ) Load external file

- #parse( ) Load and parse file


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Demo #2 - Loops

- #set( $list = [ a, b, c, d] )

- #foreach( $alpha in $list )

- The current letter is $alpha

- #end

- Lets count #foreach( $i in [ 1..10 ] )$i #end


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Demo #3 - Conditionals

- #if( $speaker eq Henning )##Use german

- Hallo Welt!

- #elseif( $hello )

- $hello

- #else #*Use default greeting*#

- Hi World!

- #end


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Macros- Builds a block directive on the fly

- Create: #macro (name) #end

- Use: #name() #end

- Velocity supports global and local scope

- Technically some sort of method call(please dont use it like this)

- For factoring out common template code/content


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Macros- Macros can take parameters

- Create:

- #macro( name $arg $arg2 ) $arg$arg2#end

- Use:

- #name( $foo $bar ) #end

- Arbitrary number of parameters possible

- Number of call parameters must match definition!


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Demo #4 - Macros- #macro( quote $value )"$value"#end

- We use a macro to quote #quote( 'thisphrase' ).


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Reference Syntax & Evaluation- A $referencerepresents a Java object

- Use a${formalReference}to avoid parsing ambiguities

- $!ref or $!{ref}means be quiet when null(do not confuse with !$reference)

- Evaluation alwaysinvokes toString()method


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Velocity Miscellany- A range operator exists- #set ($foo = [ 1..100 ]) List of integers 1 to 100

- Arithmetic (both integer and floating point)

- #set ($foo = 1.5+ 2 ) $foo is now 3.5

- +, -, /, *, %

- Boolean operators for conditionals

- !, &&, ||, ==, =

- not, and, or, eq, ne, gt, ge, lt, le

- For == and !=, if the operands are not of the same class,their toString()values are compared

- String interpolation for #set(), macro args and method args

- #set( $foo = foo was $foo ) $foo is now foo was 3.5


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Velocity Context- The Context is the means of importing values into the

template

- It is essentially a map of reference identifiers (i.e. thefoo in ${foo}) to Java objects

- All access to data in the template comes this way

- There is no natural or native way to access or create

Java objects


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DemoUsing A Context- public static void main(String [] args) throws Exception {

- VelocityEngine engine = new VelocityEngine();

- VelocityContext context = new VelocityContext();- context.put(speaker", Nathan);

- context.put("hello", "Hello World!");

- Template template = engine.getTemplate(args[0]);

- StringWriter out = new StringWriter();

- template.merge(context, out);

- System.out.println(out.toString());

- }


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Interacting With Context Objects- The real power of Velocity

- All public methods in public classes are available

- All works via runtime reflection

- Shortcut notation for property access ($a.b vs $a.getB())

- Type promotion as Java does

- Method parameters can not be omitted!(this is not perl!)


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VelocityTools- A tool is just a context object that is useful in a

template

- Collection of useful Java classes.

- Comes in three flavors:

- GenericTools All-purpose tools

- VelocityView Tools and more for webapps

- VelocityStruts Velocity as Struts View layer

- Other tools out there on the web


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Advanced Velocity- Custom directives possible

(org.apache.velocity.runtime.directive.Directive)

- Multiple Resource loaders to load templates fromdifferent sources(files, classpath, jars, database, remote URLs)

- Event Handlers for Exception handling, XML escaping,etc

- Custom Introspector for method and property accesspossible


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Advanced Velocity- Velocity Template Language (VTL) is defined in JavaCC

grammar

- VTL itself can be changed or extended(needs recompilation of velocity.jar)

- More Advanced Velocity athttp://wiki.apache.org/velocity/HackingVelocityincluding theexcellent ApacheCon 2004 session slides by Will Glass-Husain
http://wiki.apache.org/velocity/HackingVelocityhttp://wiki.apache.org/velocity/HackingVelocity


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Demo #5 - VelocityView- Uses VelocityTools 2.0 (only alpha release available)

- VelocityViewServlet does the work

- Uses LinkTool, ParameterTool, EscapeTool- (aka $link, $params, and $esc)

- One custom tool: AddressBook is a ~30 line POJO

- One template: index.vm is just ~50 lines

- ~20 lines of config (including web.xml)


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Velocity with Struts 1.x- VelocityTools provides Struts tools for integration

- Seamless, not mutually exclusive with JSP

-

- (Image from http://velocity.apache.org/tools/devel/struts/!)


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Velocity with Struts 1.x- ActionMessagesTool - Action Messages

- ErrorsTool - Error Messages

- FormTool - Forms handling- MessageTool - i18n Message Support

- StrutsLinkTool,SecureLinkTool - Links

- TilesTool - Struts-Tiles Framework

- ValidatorTool - Validator Framework


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Velocity with Turbine- Preferred View Layer of Turbine

- No separate Servlet

- Turbine fully integrates Velocity

- Turbine provides infrastructure

- Tools

- Template Loaders

- Configuration and Logging


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Velocity with- Velocity is supported as a first-class view layer in

- Struts 2.x (uses custom directives instead of tools)

- Spring MVC (also supports VelocityTools 1.x well)- Click (http://click.sourceforge.net/)

- Restlet (http://www.restlet.org/)

- Mentawai (http://www.mentaframework.org/)- And at least a dozen others

- (http://wiki.apache.org/velocity/PoweredByVelocity)
http://click.sourceforge.net/http://www.restlet.org/http://www.restlet.org/http://click.sourceforge.net/


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Other Velocity Uses- Texenfor generic text generation

- Anakiafor XML to documentation

- Torquegenerate Java and SQL code

- VelocityViewTagEasily embed Velocity in JSP- (Part of the upcoming VelocityTools 2.0)

- VelosurfTool for abstracting database interaction


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Any questions?


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Where to go from here?- Velocity Homepage

- http://velocity.apache.org/

- Velocity Mailing Lists

- http://velocity.apache.org/contact.html

- Velocity Wiki

- http://wiki.apache.org/velocity/

- These Slides and Demo Code

- http://people.apache.org/~nbubna/oscon2007

- Velocity 1.5 Release Notes

- http://wiki.apache.org/jakarta-velocity/Velocity15ReleaseNotes
http://velocity.apache.org/http://velocity.apache.org/contact.htmlhttp://wiki.apache.org/velocity/http://people.apache.org/~nbubna/oscon2007http://wiki.apache.org/jakarta-velocity/Velocity15ReleaseNoteshttp://wiki.apache.org/jakarta-velocity/Velocity15ReleaseNoteshttp://wiki.apache.org/jakarta-velocity/Velocity15ReleaseNoteshttp://wiki.apache.org/jakarta-velocity/Velocity15ReleaseNoteshttp://wiki.apache.org/jakarta-velocity/Velocity15ReleaseNoteshttp://people.apache.org/~nbubna/oscon2007http://wiki.apache.org/velocity/http://velocity.apache.org/contact.htmlhttp://velocity.apache.org/


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Thanks for your attention!

apache velocity - oscon2007

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