Avoiding re-building prerequisites in Ant - ant

Question

I have an existing Ant project and would like to speed up the build process
by avoiding re-building components that are already up to date.
Ant permits you to specify that one target depends on another, but by
default every prerequisite is always rebuilt, even if it is already up to
date. (This is a key difference between Ant and make. By default, make
only re-builds a target when needed -- that is, if some prerequisite is
newer.)
<uptodate property="mytarget.uptodate"> // in set.mytarget.uptodate task
...
</uptodate>
<!-- The prerequisites are executed before the "unless" is checked. -->
<target name="mytarget" depends="set.mytarget.uptodate" unless="mytarget.uptodate">
...
</target>
To make Ant re-build prerequisites only if necessary, there seem to be two
general approaches within Ant.
The first approach is to use the uptodate task to set a property. Then,
your task can test the property and build only if the property is (not)
set.
<uptodate property="mytarget.uptodate"> // in set.mytarget.uptodate task
...
</uptodate>
<!-- The prerequisites are executed before the "unless" is checked. -->
<target name="mytarget" depends="set.mytarget.uptodate" unless="mytarget.uptodate">
...
</target>
An alternate first approach is to use the outofdate task from ant contrib.
It's nicer in that it is just one target without a separate property being
defined; by contrast, outofdate requires separate targets to set and to
test the property.
The second approach is to create a <fileset> using the <modified>
selector. It calculates MD5 hashes for files and selects files whose MD5
differs from earlier stored values. It's optional to set
<param name="cache.cachefile" value="cache.properties"/>
inside the selector; it defaults to "cache.properties". Here is
an example that copies all files from src to dest whose content has
changed:
<copy todir="dest">
<fileset dir="src">
<modified/>
</fileset>
</copy>
Neither of these is very satisfactory, since it requires me to write Ant
code for a process (avoiding re-building) that ought to be automatic.
There is also Ivy, but I can't tell from its documentation whether it
provides this feature. The key use case in the Ivy documentation seems to
be downloading subprojects from the Internet rather than avoiding wasted
work by staging the parts of a single project. Maven provides similar
functionality, with the same use case highlighted in its documentation.
(Moving an existing non-trivila project to Maven is said to be a nightmare;
by contrast, starting greenfield development with Maven is more palatable.)
Is there a better way?

Answer 1

This conditional compilation of a large build is a feature of make that I initally missed in ANT. Rather than use target dependencies, I'd suggest dividing your large project into smaller modules, each publishing to a common shared repository.
Ivy can then be used to control the component versions used by the main module of the project.
<ivy-module version="2.0">
<info organisation="com.myspotontheweb" module="multi_module_project"/>
<publications>
<artifact name="main" type="jar"/>
</publications>
<dependencies>
<dependency org="com.myspotontheweb" name="component1" rev="latest.integration"/>
<dependency org="com.myspotontheweb" name="component2" rev="latest.integration"/>
<dependency org="com.myspotontheweb" name="component3" rev="latest.integration"/>
<dependency org="com.myspotontheweb" name="component4" rev="latest.integration"/>
</dependencies>
</ivy-module>
The ivy:retrieve task will only download/copy one of the sub-modules if they have changed (published from their build files)
It all sounds more complicated but maybe you're already sub-dividing the project within your build file.... For example, if your ANT uptodate task is being made dependent on one the build artifacts.