THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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POM
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Expression
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evaluation
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Expressions
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in
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the
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pom
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generally
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look
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like
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this:
| Code Block |
|---|
{code} ${foo}{code} |
or
| Code Block |
|---|
{code} ${bar.baz}{code} |
The
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bar
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in
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the
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last
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example
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can
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be
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a
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special
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token:
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pom
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,
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env
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,
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or
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the
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legacy
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project
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which
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is
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deprecated
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and
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means
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the
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same
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as
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pom
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.
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In
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this
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case
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bar
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is
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called
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a
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prefix
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and
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the
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baz
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expression
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is
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evaluated
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against
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the
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project
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model
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using
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reflection,
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so
| Code Block |
|---|
{code} ${pom.version}{code} |
evalueates
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to
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the
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value
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of
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the
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<project><version>
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tag.
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If
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the
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prefix
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is
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env
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,
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the
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expression
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evaluates
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to
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the
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value
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of
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System.getEnv(
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"baz"
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);
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In
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all
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other
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cases,
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if
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the
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prefix
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is
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NOT
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pom
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or
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env
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,
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the
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current
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context
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is
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consulted.
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It
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typically
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contains
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the
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basedir
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value,
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the
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system
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properties,
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and
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any
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key/value
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pairs
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specified
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with
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the
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-Dkey=value
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commandline
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option.
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When
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the
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context
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does
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not
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contain
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the
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value
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for
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the
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expression,
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the
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POM
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properties
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are
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consulted.
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Maven
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2.0.x
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behaviour
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In
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Maven
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2.0.x
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the
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prefix
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was
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discarded
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if
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it
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was
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pom
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,
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project
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,
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or
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env
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.
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The
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remaining
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expression
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was
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evaluated
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using
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the
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following
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algorithm:
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- check
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- the
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- context;
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- if
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- the
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- key/value
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- exists,
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- use
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- that.
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- If
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- the
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- key
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- exists
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- but
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- the
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- value
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- is
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- null,
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- skip
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- this
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- term,
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- and
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- leave
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- the
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- _$
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- {expression
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- }
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- as
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- is.
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- check
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- the
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- model
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- properties.
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- If
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- that
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- value
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- exists,
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- use
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- that.
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- check
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- the
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- model
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- using
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- reflection.
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- If
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- that
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- yields
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- a
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- value,
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- use
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- that.
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- check
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- the
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- environment
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- variables
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- if
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- the
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- expression
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- refers
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- to
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- itself
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- abort.
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- This
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- check
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- is
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- broken
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- in
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- several
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- ways:
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- it
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- tests
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- for
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- string
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- equality
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- but
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- both
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- pom.*
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- and
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- project.*
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- refer
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- to
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- the
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- same
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- object.
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- so
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- "${pom.foo}"
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- evaluating
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- to
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- "${project.foo}
"
- "${project.foo}
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- will
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- introduce
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- recursion:
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- since
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- it
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- discards
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- the
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- prefix,
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Code Block
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<scm><connection>${scm.connection}</connection></scm>
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will introduce recursion.- it only checks 1 level deep:
Code Block <properties><foo>${bar}</foo><bar>${foo}</foo></properties>
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.
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- Even
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- if
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- this
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- works,
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- try
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- adding
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- another
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- level,
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or
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- even
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- this:
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Code Block
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<properties><foo>${pom.bar}</foo><bar>${foo}</bar></properties>
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.
Some unexpected results can occur, and have caused some bugs. Poms containing _${pom.version
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}
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or
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even
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_$
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{version
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}
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can
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break
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the
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build
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if
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-Dversion=foo
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is
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supplied
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on
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the
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commandline.
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So,
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expressions
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like
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$
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{version
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}
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or
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$
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{artifactId
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}
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,
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or
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even
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$_
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{pom.version
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}
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can
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cause
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all
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sorts
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of
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problems.
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Final
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algorithm.
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The
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algoritm
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is
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simple,
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were
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it
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not
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for
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backwards
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compatibility
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check
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due
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to
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the
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enormous
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amount
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of
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faulty
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poms
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in
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the
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repository.
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Therefore,
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we'll
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use
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the
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new
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approach
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but
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still
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support
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legacy
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(broken)
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poms,
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though
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the
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behaviour
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may
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be
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a
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little
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bit
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different.
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The
...
first
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problem
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we
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need
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to
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solve
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is
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the
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fact
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that
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both
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$
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{pom.version
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}
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and
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_$
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{version
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}
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are
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resolved
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using
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the
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same
...
algorithm.
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This
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will
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solve
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the
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problem
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of
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commandline
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arguments
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and
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system
...
properties
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inadvertedly
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overriding
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pom
...
attributes,
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like _${version}.
Only if the expression has a pom. or project. prefix, reflection will be used.
Here's the complete algorithm:
- determine the prefix: there is only a prefix if the expresssion starts with pom., project., or env.. In all other cases there is no prefix (it equals the empty string).
- if the prefix is project., log warning about project. prefix deprecation and replace it with the pom. prefix.
- if the expression has a prefix of pom., evaluate using reflection. If no result found, continue.
- if the expression has a prefix of env., evaluate using environment vars. If no result found, continue.
- check the context for the full expression, including prefix. If there was a key, but the value was null, don't touch this expression, but leave it as an expression in the original string, and continue to the next expression. If the key didn't exist, continue.
- check the model properties for the full expression, including prefix. If the value is null, continue.
- if all this still didn't yield a result, fall back to legacy behaviour: apply the prefix-stripped expression on the model; both ${version} and _${pom.version} will result in _<project><version>.