A new release of Ant is just around the corner, so it's a good time to introduce one of the coolest new features that Java developers will soon be able to play with: antlibs. These are a better way for Java developers to create and distribute custom Ant tasks, types, and macros, and a much better way for the Ant developers to distribute the optional tasks included with the Ant distribution.
So, what was wrong with normal taskdefs that desperately needed
to be fixed?
taskdef classes--people still have
problems getting JUnit to work properly with Ant.Antlibs help to alleviate these problems:
If you already have an antlib, it can be used in a build file in
a few different ways. Use <typedef
file="example-antlib.xml"/> if the antlib is just a
directory of classes and an example-antlib.xml descriptor. This
is the most similar to a normal external Ant task.
Another approach is to use:
<typedef resource="com/mycompany/ant/example-antlib.xml"
uri="example:/mycompany.com"/>This is appropriate if the antlib is a .jar file of classes and
the antlib descriptor. This can then be used with an xmlns
declaration as follows:
<example:task xmlns:example="example:/mycompany.com">
...
</example>This assumes that there's a task called task
defined in the example-antlib.xml file.
Finally, there's the most convenient way of using antlibs. If the following conditions are met, Ant will automatically load all definitions of tasks and types declared in the antlib:
<project xmlns:example="antlib:com.mycompany.ant">
<target name="test">
<example:task/>
</target>
</project>Before we start creating our own antlib, let's ask ourselves:
what exactly is an antlib, and what are the ingredients of one? To
begin with, an antlib is simply a collection of classes bundled
with an XML descriptor file. Typically, an antlib is distributed as
a .jar file, but this isn't a strict requirement. The root element
of the XML file is <antlib>. Any classes can go
into the makeup of an antlib, but only some classes can be declared
in the antlib.xml file. The following are allowed to be
declared in the antlib.xml:
<typedef><taskdef><macrodef><presetdef><scriptdef>org.apache.tools.ant.taskdefs.AntLibDefinitionSo now that we have an idea of what goes into an antlib, let's get our hands dirty and make one.
Let's start with a simple goal: wrap a command-line executable to
give us a nicer interface than <exec>. For this
I've decided to pick one of the many open source version control
systems, Arch.
First, let's get Arch installed (if it isn't already): find the
correct package for your system and follow the install instructions.
Finally, type tla help and you should see something
similar to Figure 1.
Figure 1. GNU Arch installed
For Ant tasks that wrap around a command-line tool, a good
starting point is to build a base class that handles the main
setup of the environment, and then create a subclass for each
command you want to support. For example, the Ant SVN library has
an AbstractSvnTask, and then a Svn task
that handles any commands, along with SvnRevisionDiff
and SvnTagDiff, which are specialized for performing
the svn diff command.
Because GNU Arch is a similar tool to Subversion (ignoring the whole
distributed versus client/server argument here), the same strategy
of writing a base class and then creating specialized subclasses
should work just as well. The only problem is that the Ant
Svn doesn't (yet) ship with ant.
However, we don't need to re-invent the wheel, since Ant already
contains a task for CVS,
which behaves in an identical manner to the Svn task
(indeed, the Svn task was modeled closely on the
Cvs task). It also wraps a command line tool and it
deals with version control, an almost perfect fit!
With the strategy of "baseclass + specialized subclasses" chosen, and an example to model our code on, we can start writing our antlib.
Because we are creating an antlib, we should put the code in a
separate Java package/namespace; indeed an entirely new project
seems appropriate here. I'm using Eclipse, but the choice of IDE (or to even use an IDE) has no bearing on the way an antlib is developed.
For our package, I've decided on org.apache.ant.tla,
and following the example of the AbstractCvsTask, I'm
naming my base class AbstractTlaTask.
The most important part of the code is shown below, the
runCommand() method; all of the other code in
AbstractTlaTask sets the environment up before calling
this method to actually perform the work. It isn't essential for
all antlibs to have a runCommand() method, but in our
case, as we have to interface with a command-line tool, this is one
way to achieve that level of interaction, which works well with the
AbstractCvsTask. As you can see, when executing system
commands we have to deal with a lot of potential
Exceptions. Check out the sample
code to see the full source of this Task.
protected void runCommand(Commandline toExecute)
throws BuildException {
Environment env = new Environment();
Execute exe = new Execute(
getExecuteStreamHandler(), null
);
exe.setAntRun(getProject());
exe.setCommandline(
toExecute.getCommandline()
);
exe.setEnvironment(env.getVariables());
try {
String actualCommandLine =
executeToString(exe);
log(
actualCommandLine,
Project.MSG_VERBOSE
);
int retCode = exe.execute();
log(
"retCode=" + retCode,
Project.MSG_DEBUG
);
if (failOnError &&
Execute.isFailure(retCode)) {
throw new BuildException(
"tla exited with error code "
+ retCode
+ StringUtils.LINE_SEP
+ "Command line was ["
+ actualCommandLine + "]",
getLocation()
);
}
} catch (IOException e) {
if (failOnError) {
throw new BuildException(
e, getLocation()
);
} else {
log(
"Caught exception: " +
e.getMessage(),
Project.MSG_WARN
);
}
} catch (BuildException e) {
if (failOnError) {
throw (e);
} else {
Throwable t = e.getException();
if (t == null) {
t = e;
}
log(
"Caught exception: " +
t.getMessage(),
Project.MSG_WARN
);
}
} catch (Exception e) {
if (failOnError) {
throw new BuildException(
e, getLocation()
);
} else {
log(
"Caught exception: " +
e.getMessage(),
Project.MSG_WARN
);
}
}
}
|
Now that we have code that can handle passing a command to the
system, let's look at one of the commands that we must implement to
have a semi-useful wrapper for Arch. The first thing
that the antlib must be able to do is to
register-archive. Figure 2 shows the format of the
command we need to pass to the system. The archive is in the form
of a URL. We can also see that passing an archive name is an
optional parameter that our task should support.
Figure 2. The format for the register-archive
command
Because we have most of the support code written in
AbstractTlaTask, the RegisterArchive is
fairly straightforward. As you can see in the code below, we
override the execute method and include the parameters
that are required. Before running the command,
validate is called to ensure that the required
parameters are present.
package org.apache.ant.tla;
import org.apache.tools.ant.BuildException;
import org.apache.tools.ant.types.Commandline;
public class RegisterArchive
extends AbstractTlaTask {
private String archive;
private void validate()
throws BuildException {
if (null == getRepoURL() ||
getRepoURL().length() == 0) {
throw new BuildException(
"You must specify a url for the repo."
);
}
if (null != archive &&
archive.indexOf("@") == -1) {
throw new BuildException(
"If you specify an archive name" +
",you must specify it correctly" +
"see the GNU arch documentation"
);
}
}
public void execute()
throws BuildException {
validate();
Commandline c = new Commandline();
c.setExecutable("tla");
c.createArgument(true).setValue(
"register-archive"
);
if (null != archive &&
archive.length() > 0) {
c.createArgument().setValue(archive);
}
c.createArgument().setValue(
this.getRepoURL()
);
this.addConfiguredCommandline(c);
super.execute();
}
public String getArchive() {
return archive;
}
public void setArchive(String archive) {
this.archive = archive;
}
}Now that we have a basic antlib prepared, we need to add the "special sauce" that makes it an antlib, and then we can get down to testing our creation! For all antlibs, we must include an XML document with our code (in the package with the class files).
<?xml version="1.0" encoding="utf-8"?>
<antlib>
<taskdef
name="registerarchive"
classname="org.apache.ant.tla.RegisterArchive"
/>
</antlib>Since we currently only have one task defined in our antlib, we
only need one entry in the antlib.xml. We don't
include the AbstractTlaTask as it is only a support
class and we don't want the user to be able to access it directly.
Finally, we need to jar the antlib and place it into
our $ANT_HOME/lib directory or in our classpath.
Note although the antlib facility is being used extensively for Ant 1.7+, the ability to load an antlib is available in the current release version, Ant 1.6.5.
So we're done right? We've got our new antlib .jar on our
classpath and we can use the defined task just like any other
<taskdef>. But does it really work?
Traditionally, Java developers use JUnit to create their unit tests. However, testing Ant tasks and antlibs is easier if you use the facilities provided by Ant.
Instead of subclassing TestCase, Ant provides a
wrapper BuildFileTest, which allows us to test an Ant
task using a build file as the driver. This is a much better test
of the code since it is being exercised in a similar way to how it will operate in real life. However, some people may object
to calling these tests unit tests, as they are more like
integration or system tests.
For our RegisterArchive task, we'll just bang out a
quick build file to test it by eye-balling (don't worry, the
sample code does include a real BuildFileTest).
For a quick test and a BuildFileTest, you can use the
same build file. For the tests, I've selected the Arch repository
mentioned in the Arch documentation.
<?xml version="1.0" encoding="utf-8"?>
<project name="tla-test"
basedir="../../../"
default="register"
xmlns:tla="antlib:org.apache.ant.tla">
<target name="register">
<tla:registerarchive
repoURL=
"http://www.atai.org/archarchives/atai@atai.org--public/"
/>
</target>
</project>The output of running Ant with our new antlib and the build file above looks like this:
Spikefish:~/projects/ant-tla/trunk kj$ ant -f src/etc/testcases/registerarchive.xml
Buildfile: src/etc/testcases/registerarchive.xml
register:
[tla:registerarchive] Registering archive: atai@atai.org--public
BUILD SUCCESSFUL
Total time: 3 secondsAnother new facility provided by Ant is the AntUnit
antlib. Unlike a JUnit TestCase or a
BuildFileTest, AntUnit allows you to specify your
tests without using any Java code. AntUnit itself is provided as an
antlib, so it must be placed in your $ANT_HOME/lib
directory, or specified on the classpath. In a recursively friendly
way, AntUnit is used to test both Ant and AntUnit!
Since we have a build file that we have already used for ad hoc testing, let's modify it to use AntUnit and introduce a much more repeatable test. First, we must declare the AntUnit namespace, so the header of our build file becomes:
<project name="tla-test"
basedir="../../../"
default="go"
xmlns:tla="antlib:org.apache.ant.tla"
xmlns:au="antlib:org.apache.ant.antunit">AntUnit works in a similar way to JUnit and looks for targets
which start with test, so we must change our
register target to test-register. Like
JUnit, AntUnit also needs some way of asserting if the test passed.
In this case, we'll use the assertLogContains macro to
check if the output is what we are expecting. Here's what our new
test-register looks like:
<target name="test-register">
<tla:registerarchive
repoURL=
"http://www.atai.org/archarchives/atai@atai.org--public/"
/>
<au:assertLogContains text="Registering archive:"/>
</target>The final addition is the driving target go. We
need to set up AntUnit in this target:
<target name="go">
<au:antunit>
<fileset dir="src/etc/testcases"
includes="au-registerarchive.xml"/>
<au:plainlistener/>
</au:antunit>
</target>As you can see below, our test passes. Adding another test is
simply a case of adding a new target with a name beginning with
test. No compilation, no Java: a much simpler way of
testing Ant tasks.
Spikefish:~/projects/ant-tla/trunk kj$ ant -f src/etc/testcases/au-registerarchive.xml
Buildfile: src/etc/testcases/au-registerarchive.xml
go:
[au:antunit] Build File: /Users/kj/projects/ant-tla/trunk/src/etc/testcases/au-registerarchive.xml
[au:antunit] Tests run: 1, Failures: 0, Errors: 0, Time elapsed: 3.378 sec
[au:antunit] Target: test-register took 3.329 sec
BUILD SUCCESSFUL
Total time: 7 secondsFinally, lets look at how much work there is involved in refactoring a standard custom task to change it into an antlib.
Let's start with the code of the original VSS task. What is the minimum amount of code required to change it into an antlib? Actually, just adding one file, antlib.xml, is all that is required:
<antlib>
<taskdef
name="vss"
classname="org.apache.ant.vss.MSVSS"
/>
<taskdef
name="add"
classname="org.apache.ant.vss.MSVSSADD"
/>
<taskdef
name="checkin"
classname="org.apache.ant.vss.MSVSSCHECKIN"
/>
... (code elided)
</antlib>Of course, we also want to decouple the optional task from the main Ant source so that we can ship fixes to the optional tasks (now antlibs) independently from releasing a full Ant distribution. In reality, all this requires is a change of package namespace and, along with the antlib.xml file, the optional task becomes a decoupled antlib!
We have seen just two of the main new features of Ant 1.7: antlibs and AntUnit.
The antlib feature allows developers of Ant tasks to ship fixes and updates independently of the main Ant distribution, and the AntUnit antlib is a much quicker method of creating unit tests for Ant tasks.
In this article we've written the start of an antlib for the SCM system Arch, showing how easy it is to develop new tasks with the antlib mechanism. We also tested this antlib with an AntUnit test.
Kev Jackson is a software developer and a committer on the Apache Ant project.
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