Hi,
there are two subtle differences between Maven 2.x and 3.x regarding POM
interpolation I would like to discuss.
a) Ordering of Interpolation Sources
According to section 6.1 of the POM spec for Maven 3.x, the precedence
of interpolation sources is:
1. Maven Props
2. System Props
3. POM Props
4. Environment Props
In particular, system properties dominate POM values, i.e. "mvn package
-Dproject.version=0.1" will override the project version as given in the
POM. This is different compared to Maven 2.x where model values can't be
overriden by system properties.
As far as I understand 3.x, the reasoning for the above ordering is that
"POM Props" include both reflective data from the model and the values
from the <properties> section (which, I agree, should be dominated by
system props).
IMHO, there should be no way to override model values to ensure a
reliable degree of independence from the environment. Hence I suggest
the handle model values and the <properties> section as independent
interpolation sources, i.e.
1. Maven Props (e.g. basedir)
2. Model Values
3. System Props
4. <properties> section
5. Environment Props
b) Resolution of Expressions
Consider this POM snippet:
<properties>
<project.prop>A</project.prop>
<prop>B</prop>
<foo>C</foo>
</properties>
<name>${project.prop} - ${prop} - ${foo}</name>
If there was a "Maven certified developer", a nice question for his exam
could be "What is the effective value of the <name> element?".
Well, in Maven 2.x it's "B - B - C". In Maven 3.x, it's "A - B - C".
The first "B" in the Maven 2.x answer results from the interpolator
stripping the "project.|pom." prefix from "project.prop" when querying
the <properties> section.
IMHO, here is Maven 3.x making a nicer job. "Nice" here means more
intuitive and more consistent. With "consistent" I refer to the behavior
of the PluginParameterExpressionEvaluator that does not strip any
prefixes from expressions when resolving them from the <properties>
section.
If we agree that the current Maven 2.x way to handle an expression that
is prefixed with project.|pom. but doesn't match a model value is odd,
can we change/fix this behavior for Maven 2.1.x and even Maven 2.0.x?
Benjamin
P.S.: For those that sill wonder what YAPII means: "Yet Another POM
Interpolation Issue". Sorry, but I was in strange mood.
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