Allow evaluation of Java expressions that does not return text or value!
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Key: VELOCITY-444
URL: http://issues.apache.org/jira/browse/VELOCITY-444
Project: Velocity
Type: Improvement
Components: Source
Versions: 1.4
Environment: WindowsXP
Reporter: Colbert Philippe
Priority: Critical
I have started to use Velocity as a file preprocessor with my own macros in
separate files. I quickly realized that there are some features missing in
Velocity.
I want to create Velocity macros with optional parameters, which is essential
for my application. When a user calls my macros, she/he should not be obliged
to pass all the necessary parameters to my macro. The macro should be
callable with no parameter or with partial or full set of parameters. All
parameters will have default values.
I think the best way to do this is to have a Map (like a java.util.Map as in
HashMap). As far as I know, Velocity does not support a Map objects directly.
Such object as a Map has to be created in Java and used from the Velocity
macro language. Therein lies the problem.
Velocity does not have a reserved word for evaluating a Java expression without
returning a value or text. In the documentation, using #set( $result =
$myObj.method() ), method() can have side effects but it must return a value.
I want to be able to call methods that return nothing (void). I have tried
many things to try circumventing this but nothing works. Velocity always
outputs the text of the expression without evaluating it.
The solution would be to have a reserve construct similar like #set(....) but
it would be called #eval(....). Such expression #eval(....) would take a Java
expression and would evaluate it without generating any text. The #eval(....)
would be used to set the value of my Map.
// Utility Java class that creates and keeps HashMap(s)
public static class Utility {
public static final byte MAP_COUNT = 16;
private final Stack<Map<String,String>> maps =
new Stack<Map<String,String>>();
public Utility() { this(MAP_COUNT); }
public Utility(int count) {
for(int i=0;i<count;i++) {
maps.push(new HashMap<String,String>());
}
}
public Map<String,String> createMap() { return maps.pop(); }
public void discardMap(Map<String,String> map) { map.clear();
maps.push(map); }
}
protected final Utility util = new Utility();
engine.setProperty("util", util); // ...later passed as property of
VelocityEngine
##=======================
## Macro to create a Map and use it inside the macro
##=======================
#set( $myMap = $util.createMap() )
$myMap.size()
#$myMap.put("first", "alpha")
#$myMap.put("second", "beta")
#$myMap.put("third", "kapa")
#$myMap.put("fourth", "gamma")
#foreach( $myKey in [ "first", "second", "third", "fourth" ] )
$myKey ${myMap.get($myKey)}
#end
##=======================
## Output text file ...DOES NOT WORK AS PLANNED!!
##=======================
$myMap.size()
#$myMap.put("first", "alpha")
#$myMap.put("second", "beta")
#$myMap.put("third", "kapa")
#$myMap.put("fourth", "gamma")
first ${myMap.get($myKey)}
second ${myMap.get($myKey)}
third ${myMap.get($myKey)}
fourth ${myMap.get($myKey)}
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