Thursday, January 01, 2009

ant: BAD

Who doesn't know history is condemned to repeat it. Or not even that, as ant managed. In my option, ant started with a bad idea and then made it worse. In other words, it is Broken As Designed.

The quotes are form the aforementioned page, section 'Apache Ant'.

Apache Ant is a Java-based build tool. In theory, it is kind of like Make, but without Make's wrinkles.

Yeah, the wrinkles have been replaced by pointy brackets, and all the deep problems haven't even been addressed at all.

Why another build tool when there is already make, gnumake, nmake, jam, and others? Because all those tools have limitations that Ant's original author couldn't live with when developing software across multiple platforms.

Well, at least I don't want to live with ant. As little as possible. Granted, make isn't usable for building java projects either, but then neither is ant, without working around the buggy javac task.

Make-like tools are inherently shell-based -- they evaluate a set of dependencies, then execute commands not unlike what you would issue in a shell.

That's the point! Why invent another scripting language when you can have one for free? Instead ant goes along inventing a new scripting language (which may not even be turing complete), and it takes them years to come to a point where ant can at least do what the good old shell always could.

And still, to do a simple grep Whatever | sed -e s/XX/YY/ you need to write java programs, compile them, feed the jars to ant, and after a few hour get it actually working. Not to menting the fact that those classes can't easily be part of your project because you need them before starting ant. (Ok, that's not actually true, but in an ugly way.)

This means that you can easily extend these tools by using or writing any program for the OS that you are working on. However, this also means that you limit yourself to the OS, or at least the OS type such as Unix, that you are working on.

Gladly so. Nowadays you can easily get VMs oder cygwin if you should really need to work elsewhere.

At least it's much better than making everything complicated, everywhere. The average trivial build.xml contains about 60 lines, of which three do actually vary with the project, and the rest is always the same. Don't repeat yourself, huh? At least we hope it's the same, and there aren't any copy&paste bugs lurking.

Makefiles are inherently evil as well. Anybody who has worked on them for any time has run into the dreaded tab problem. "Is my command not executing because I have a space in front of my tab!!!" said the original author of Ant way too many times.

The original author of ant obviously had the wrong editor, or was missing a little script to check for that. I've written my share of makefiles, and I can't remember ever running into that problem.

If only the original author of ant ran into the real problems that make poses; then ant wouldn't have been such a terrible misdesign.

Ant is different. Instead of a model where it is extended with shell-based commands, Ant is extended using Java classes. Instead of writing shell commands, the configuration files are XML-based, calling out a target tree where various tasks get executed. Each task is run by an object that implements a particular Task interface.

So, instead of writing little one-liners I need to write code in a programming language that itself is known as verbose, write additional build targets to get those compiled, and finally knit it all together in XML, yet another source of bloat to the whole end. (And for the nitpickers, ant uses two extra syntactic elements: comma-separated lists and property value replacements. Guess why they don't do that as XML elements.) ant has a serious Dr. No syndrome.

Granted, this removes some of the expressive power that is inherent by being able to construct a shell command such as `find . -name foo -exec rm {}`, but it gives you the ability to be cross platform -- to work anywhere and everywhere.

...and making that a lot of work. Ant really does not fall into the category 'make the easy things easy and the hard things possible'.

And hey, if you really need to execute a shell command, Ant has an <exec> task that allows different commands to be executed based on the OS that it is executing on.

Now that't a cop-out. If our uber-tool happens to not support what we want we can still go platform-specific. Thanks; due to general disgust with XML I wrote a little shell script (of all things) that does everything I need to do in the projects I need to manage, including compiling and collecting used libraries of my own. /bin/sh isn't exactly the best way to do, but the whole thing is just five times larger than the (broken) build.xml I got for those projects (and shorter than this blog entry), and the actual build script for one of those condenses to

. "`dirname "$0"`"/../proj-a/tools/
depdir ../proj-a
mkjar proj-b

and is also a shell script.

Now what?

Ok, more details on how ant is misdesigned. The javac task is broken:
It only compiles java files that have no corresponding class file or
whose class file is older than the java file. It does not erase class
files for which there is no longer a source file, nor does it
recompile dependencies between the classes. Especially the latter
makes for interesting bugs. Workaround: Always clean before compiling,
either manually or by the 'dependencies'.

And these 'dependencies'. They aren't, really. The individual tasks sometimes execute conditionally depending on whether the destination is newer than the source, but you need to state dependencies explicitly, no matter whether ant could deduce the dependency itself. For example, when a javac task produces what a jar task consumes ant won't make them dependent automatically. Otherwise, when you put a target into the dependencies list of another, it is executed, no matter what. As opposed to the following make fragment

prog : main.c gen.c
cc -o prog main.c gen.c
tool : tool.c
cc -o tool tool.c
gen.c : tool
./tool gen.c

which tells that gen.c needs to be generated by tool which in turn needs to be compiled from tool.c. The point here is that when you call make, the tool won't be recompiled unless you modified its source, and gen.c won't be generated unless you modified either the generator or its input file.

Ant stays blissfully unaware of any of this dependency management, and thusly degenerates to the fixed execution of a number of scripts (called targets) with a number of commands (called tasks) each. If you want your task not to do work when none is needed, don't expect support from ant. Ant is not really a build tool, it is a simple script executor, however their creators talk about declarative operations and the ant way which seems pretty long-winded to me.

The 'declarative way' does not keep people from relying on the fact that the dependencies of a target are executed in the order they are specified. A dependencies="clean, compile" to work around the javac problem is all but uncommon, and clearly will break when ant decides to run the dependencies in inverse oerder.

To be fair, the dependency problem isn't exactly trivial, especially without support from the actual compiler. make doesn't do a good job for java, either. But on the other hand side we'd expect an industrial-strength build system to have invested not just a little thought?

Then ant completely ignored the lesson of the X windowing system. Those guys actually improved on make by using the C preprocessor. Their Imakefile system inspired another (proprietary) system that could just say

CProgramFromSources (prog) {
CSource (gen)
CSource (main)
CProgram (tool)
gen.c : tool
./tool gen.c

with one important difference: The macros expand so that not only make prog does the expected thing, but also that make clean removed all the temporaries, except for gen.c which was done with a plain make rule. We need to add

clean ::
rm -f gen.c

to make that work, too. This also shows another overlooked make feature: You can actually combine a target from multiple separate commands and dependencies. It's just not possible to have multiple clean targets in ant, making it even more error-prone to do the right cleanout.

Ant simply aims too low by one or two levels of abstraction.

Then what?

Unfortunately ant has gotten a lot of traction in the java community. Proves again that you need to be the first, not the best. Compounded by the fact that most programmers don't care about the build system any more than needed to make it apparently work. And everything and the kitchen sink is available as an ant task, so it's not just programmers to turn around.

And I'm not exactly in a position to get a lot of traction for a change. The most promising way is to fight the system from within; perhaps by actually having some preprocessor (again) generating a tmp/build.xml to then be included.

The most depressing thing is that ant will make the majority of people think that this is the state of the art in build system design. Far from it.

1 comment:

Br.Bill said...

Thank you. Ant is overcomplex, poorly documented, and is worse than the tool(s) it was meant to replace.