ant depends vs. antcall

Question

When defining sequential build steps I use the depends attribute of the target element. I have recently seen an ant file, where the build sequence was defined by antcall elements inside the targets. To illustrate :

<target name="a" depends="b"> ...</target> 

vs

<target name="a"> <antcall target="b"/> ...</target> 

Is there a real difference between the two approaches? Is one of them preferable?

Answer 1

The biggest difference is that Ant will ensure that dependencies declared via depends are called at most once. For example:

<target name="a" />  <target name="b" depends="a" />  <target name="c" depends="a" />  <target name="d" depends="b, c" /> 

If I call target d, b and c are called. However, a is only called once (even though both b and c depends on it).

Now suppose we decide to use antcall instead of depends for target d:

<target name="d">    <antcall target="b" />    <antcall target="c" /> </target> 

Calling target d will now call targets b and c; however, target a will get called twice, once for b and then again for c.

In other words, antcall sidesteps the normal dependency rules that are the cornerstone of Ant.

I don't think antcall should be used as a substitute for normal Ant-like dependencies; that's what depends is for. So when would you use it? The antcall task does allow you to control what properties and references are defined (which is why a new Ant environment is created--and why it's so slow) so it can be used to create variants of the same thing; e.g., maybe two jars, one with and one without debug symbols.

Overusing antcall, however, creates slow, brittle, and hard to maintain build scripts. Think of it as the goto of Ant--it's evil. Most well-written build scripts simply don't need it except in unusual cases.