build.xml

<project name="blabla">

	<condition property="composerDoesNotExist">
		<not>
			<available file="${basedir}/bin/composer.phar" />
		</not>
	</condition>

	<target name="composer-download" description="Downloading Composer" if="composerDoesNotExist">
		<mkdir dir="${basedir}/bin" />
		<exec executable="bash">
			<arg value="-c" />
			<arg value="curl -sS https://getcomposer.org/installer | php -- --install-dir=${basedir}/bin" />
		</exec>
	</target>

	<target name="composer-self-update" depends="composer-download">
		<exec executable="${basedir}/bin/composer.phar" failonerror="true">
			<arg value="--no-interaction" />
			<arg value="self-update" />
		</exec>
	</target>

	<target name="composer-install" description="Installing dependencies with Composer" depends="composer-self-update">
		<exec executable="${basedir}/bin/composer.phar" failonerror="true">
			<arg value="install" />
			<arg value="--no-interaction" />
		</exec>
	</target>

	<target name="composer-update" description="Updates dependencies with Composer" depends="composer-self-update">
		<exec executable="${basedir}/bin/composer.phar" failonerror="true">
			<arg value="validate" />
		</exec>
		<exec executable="${basedir}/bin/composer.phar" failonerror="true">
			<arg value="update" />
			<arg value="--no-interaction" />
		</exec>
	</target>

</project>