Everybody loves build.xml (test-driven Ant)

February 16, 2013 [Ant, Java, Tech, Test Driven, Videos]

In the previous post we looked at how it is possible to write reasonable code in Ant, by writing small re-usable blocks of code.

Of course, if you're going to have any confidence in your build file you're going to need to test it. Now we've learnt some basic Ant techniques, we're ready to do the necessary magic that allows us to write tests.

Slides: Everybody loves build.xml slides.

First, let me clear up what we're testing:

What do we want to test?

We're not testing our Java code. We know how to do that, and to run tests, if we've written them using JUnit, just needs a <junit> tag in our build.xml. (Other testing frameworks are available and some people say they're better).

The things we want to test are:

build artifacts - the "output" of our builds i.e. JAR files, zips and things created when we run the build,
build logic - such as whether dependencies are correct, whether the build succeeds or fails under certain conditions, and
units of code - checking whether individual macros or code snippets are correct.

Note, if you're familiar with the terminology, that testing build artifacts can never be a "unit test", since it involves creating real files on the disk and running the real build.

Below we'll see how I found ways to test build artifacts, and some ideas I had to do the other two, but certainly not a comprehensive solution. Your contributions are welcome.

Before we start, let's see how I'm laying out my code:

Code layout

build.xml      - real build file
asserts.xml    - support code for tests
test-build.xml - actual tests

I have a normal build file called build.xml, a file containing support code for the tests (mostly macros allowing us to make assertions) called asserts.xml, and a file containing the actual tests called test-build.xml.

To run the tests I invoke Ant like this:

ant -f test-build.xml test-name

test-build.xml uses an include to get the assertions:

<include file="asserts.xml"/>

Tests call a target inside build.xml using subant, then use the code in asserts.xml to make assertions about what happened.

Simple example: code got compiled

If we want to check that a <javac ...> task worked, we can just check that a .class file was created:

Here's the test, in test-build.xml:

<target name="test-class-file-created">
    <assert-target-creates-file
        target="build"
        file="bin/my/package/ExampleFile.class"
    />
</target>

We run it like this:

ant -f test-build.xml test-class-file-created

The assert-target-creates-file assertion is a macrodef in asserts.xml like this:

<macrodef name="assert-target-creates-file">
    <attribute name="target"/>
    <attribute name="file"/>
    <sequential>
        <delete file="@{file}" quiet="true"/>
        <subant antfile="build.xml" buildpath="." target="@{target}"/>
        <assert-file-exists file="@{file}"/>
    </sequential>
</macrodef>

It just deletes a file, runs the target using subant, then asserts that the file exists, which uses this macrodef:

<macrodef name="assert-file-exists">
    <attribute name="file"/>
    <sequential>
        <echo message="Checking existence of file: @{file}"/>
        <fail message="File '@{file}' does not exist.">
            <condition>
                <not><available file="@{file}"/></not>
            </condition>
        </fail>
    </sequential>
</macrodef>

This uses a trick I've used a lot, which is the fail task, with a condition inside it, meaning that we only fail if the condition is satisfied. Here we use not available which means fail if the file doesn't exist.

Harder example: JAR file

Now let's check that a JAR file was created, and has the right contents. Here's the test:

<target name="test-jar-created-with-manifest">

    <assert-target-creates-file
        target="build"
        file="dist/MyProduct.jar"
    />
    <assert-file-in-jar-contains
        jarfile="dist/MyProduct.jar"
        filename="MANIFEST.MF"
        find="Main-Class: my.package.MyMain"
    />

This just says after we're run the target, the file MyProduct.jar exists, and it contains a file called MANIFEST.MF that has the right Main-Class information in it.

assert-file-in-jar-contains looks like this:

<macrodef name="assert-file-in-jar-contains">
    <attribute name="jarfile"/>
    <attribute name="filename"/>
    <attribute name="find"/>

    <sequential>
        <!-- ... insert checks that jar exists, and contains file -->

        <delete dir="${tmpdir}/unzip"/>
        <unzip src="@{jarfile}" dest="${tmpdir}/unzip"/>

        <fail message="@{jarfile}:@{filename} should contain @{find}">
            <condition>
                <resourcecount when="equal" count="0">
                    <fileset dir="${tmpdir}/unzip">
                        <and>
                            <filename name="**/@{filename}"/>
                            <contains text="@{find}"/>
                        </and>
                    </fileset>
                </resourcecount>
            </condition>
        </fail>

        <delete dir="${tmpdir}/unzip"/>

    </sequential>
</macrodef>

Which basically unzips the JAR into a directory, then searches the directory using fileset for a file with the right name and contents, and fails if it's not found (i.e. if the resourcecount of the fileset is zero. These are the kinds of backflips you need to do to bend Ant to your will.

Or, you can choose the Nuclear Option.

The Nuclear Option

If ant tasks just won't do, since Ant 1.7 and Java 1.6 we can drop into a <script> tag.

You ain't gonna like it:

<script language="javascript"><![CDATA[
system.launchMissiles(); // Muhahahaha
]]></script>

The script tag allows us to use a scripting language as provided through the JSR 223 Java feature directly within our Ant file, meaning we can do anything.

In all the JVMs I've tried, the only scripting language actually available is JavaScript, provided by the Rhino virtual machine, which is now part of standard Java.

When using the script tag, expect bad error messages. Rhino produces unhelpful stack traces, and Ant doesn't really tell you what went wrong.

So now we know how to test the artifacts our build produces, but what about directly testing the logic in build.xml?

Testing build logic

We want to:

Confirm that targets succeed or fail under certain conditions
Check indirect dependencies are as expected
Test a unit of Ant logic (e.g. a macrodef)

Success and failure

Here's a little macro I cooked up to assert that something is going to fail:

<macrodef name="expect-failure">
    <attribute name="target"/>
    <sequential>
        <local name="ex.caught"/>
        <script language="javascript"><![CDATA[
        try {
            project.executeTarget( "@{target}" );
        } catch( e ) {
            project.setProperty( "ex.caught", "yes" )
        }
        ]]></script>
        <fail message="@{target} succeeded!!!" unless="ex.caught"/>
    </sequential>
</macrodef>

I resorted to the Nuclear Option of a script tag, and used Ant's Java API (through JavaScript) to execute the target, and catch any exceptions that are thrown. If no exception is thrown, we fail.

Testing dependencies

To check that the dependencies are as we expect, we really want to run ant's dependency resolution without doing anything. Remarkably, ant has no support for this.

But we can hack it in:

<target name="printCdeps">
    <script language="javascript"><![CDATA[

        var targs = project.getTargets().elements();
        while( targs.hasMoreElements() )
        {
            var targ = targs.nextElement();
            targ.setUnless( "DRY.RUN" );
        }
        project.setProperty( "DRY.RUN", "1" );
        project.executeTarget( "targetC" );

    ]]></script>
</target>

(See Dry run mode for Ant for more.)

Now we need to be able to run a build and capture the output. We can do that like this:

<target name="test-C-depends-on-A">
    <delete file="${tmpdir}/cdeps.txt"/>
    <ant
        target="printCdeps"
        output="${tmpdir}/cdeps.txt"
    />
    <fail message="Target A did not execute when we ran C!">
        <condition>
            <resourcecount when="equal" count="0">
                <fileset file="${tmpdir}/cdeps.txt">
                    <contains text="targetA:"/>
                </fileset>
            </resourcecount>
        </condition>
    </fail>
    <delete file="${tmpdir}/cdeps.txt"/>
</target>

We use ant to run the build, telling it to write to a file cdeps.txt. Then, to assert that C depends on A, we just fail if cdeps.txt doesn't contain a line indicating we ran A. (To assert a file contains a certain line we use a load of fail, condition, resourcecount and fileset machinery as before.)

So, we can check that targets depend on each other, directly or indirectly. Can we write proper unit tests for our macrodefs?

Testing ant units

To test a macrodef or target as a piece of logic, without touching the file system or really running it, we will need fake versions of all the tasks, including <jar>, <copy>, <javac> and many more.

If we replace the real versions with fakes and then run our tasks, we can set up our fakes to track what happened, and then make assertions about it.

If we create a file called real-fake-tasks.xml, we can put things like this inside:

<macrodef name="jar">
    <attribute name="destfile"/>
    <sequential>
        <property name="jar.was.run" value="yes"/>
    </sequential>
</macrodef>

and, in build.xml we include something called fake-tasks.xml, with the optional attribute set to true:

<include file="fake-tasks.xml" optional="true"/>

If the target we want to test looks like this (in build.xml):

<target name="targetA">
    <jar destfile="foo.jar"/>
</target>

Then we can write a test like this in test-build.xml:

<target name="test-A-runs-jar" depends="build.targetA">
    <fail message="Didn't jar!" unless="jar.was.run"/>
</target>

and run the tests like this:

cp real-fake-tasks.xml fake-tasks.xml
ant -f test-build.xml test-A-runs-jar
rm fake-tasks.xml

If fake-tasks.xml doesn't exist, the real tasks will be used, so running your build normally should still work.

This trick relies on the fact that our fake tasks replace the real ones, which appears to be an undocumented behaviour of my version of Ant. Ant complains about us doing this, with an error message that sounds like it didn't work, but actually it did (on my machine).

If we wanted to avoid relying on this undocumented behaviour, we'd need to write our real targets based on special macrodefs called things like do-jar and provide a version of do-jar that hands off to the real jar, and a version that is a fake. This would be a lot of work, and pollutes our production code with machinery needed for testing, but it could work with Ant's documented behaviour, making it unlikely to fail unexpectedly in the future.

Summary

You can write Ant code in a test-driven way, and there are even structures that allow you to write things that might be described as unit tests.

At the moment, I am using mostly the "testing artifacts" way. The tests run slowly, but they give real confidence that your build file is really working.

Since I introduced this form of testing into our build, I enjoy working with build.xml a lot more, because I know when I've messed it up.

But I do spend more time waiting around for the tests to run.