I'd like to set up an Ant build that pulls down its plugins at buildtime. So for instance, if my build used findbugs and checkstyle JARs and called their tasks from inside build.xml, then ideally I could:

Run an ivy-resolve and ivy-retrieve task to pull findbugs and checkstyle JARs down into, say, gen/lib/buildtime.
Add gen/lib/buildtime to Ant's classpath
Define <taskdefs> that define the various findbugs and checkstyle tasks
Execute the findbugs and checkstyle tasks in a downstream target

Is this possible? If so, how? I think the only show-stopper here is the fact that I don't think you can dynamically add <taskdefs>... thanks in advance!

The best approach is to use the cachepath task to populate an ANT path with the required jars. Your taskdef then becomes:

<ivy:cachepath pathid="build.path" conf="build"/>
<taskdef uri="antlib:org???" resource="???" classpathref="build.path"/>

An ivy configuration can be used to control which jars should populate the path.

The following example uses Sonar to run findbugs and checkstyle automatically. It demonstrates using ivy to pull in the following ANT plugins:

junit
sonar
jacoco

Example

├── build.properties
├── build.xml
├── ivy.xml
└── src
    ├── main
    │   └── java
    │       └── org
    │           └── demo
    │               └── App.java
    └── test
        └── java
            └── org
                └── demo
                    └── AppTest.java

ivy.xml

<ivy-module version="2.0">
    <info organisation="com.myspotontheweb" module="demo"/>

    <configurations defaultconfmapping="compile->default">
        <conf name="compile" description="Required to compile application"/>
        <conf name="test"    description="Required for test only" extends="compile"/>
        <conf name="build"   description="Build dependencies"/>
    </configurations>

    <dependencies>
        <!-- compile dependencies -->

        <!-- test dependencies -->
        <dependency org="junit" name="junit" rev="4.10" conf="test->default"/>

        <!-- build dependencies -->
        <dependency org="org.codehaus.sonar-plugins" name="sonar-ant-task" rev="2.1" conf="build->default"/>
        <dependency org="org.jacoco" name="org.jacoco.ant" rev="0.6.3.201306030806" conf="build->default"/>

        <!-- Global exclusions -->
        <exclude org="org.apache.ant"/>
    </dependencies>

</ivy-module>

build.xml

<project name="demo" default="test" xmlns:ivy="antlib:org.apache.ivy.ant">

    <property file="build.properties"/>

    <target name="bootstrap" description="Install ivy">
        <mkdir dir="${user.home}/.ant/lib"/>
        <get dest="${user.home}/.ant/lib/ivy.jar" src="http://search.maven.org/remotecontent?filepath=org/apache/ivy/ivy/2.3.0/ivy-2.3.0.jar"/>
    </target>

    <target name="resolve" description="Download dependencies and setup classpaths">
        <ivy:resolve/>
        <ivy:report todir='${reports.dir}/ivy' graph='false' xml='false'/>

        <ivy:cachepath pathid="compile.path" conf="compile"/>
        <ivy:cachepath pathid="test.path"    conf="test"/>
        <ivy:cachepath pathid="build.path"   conf="build"/>
    </target>

    <target name="init" depends="resolve" description="Create build directories">
        <mkdir dir="${classes.dir}"/>
        <mkdir dir="${test.classes.dir}"/>
        <mkdir dir="${reports.dir}/junit"/>
    </target>

    <target name="compile" depends="init" description="Compile source code">
        <javac srcdir="${src.dir}" destdir="${classes.dir}" includeantruntime="false" debug="true" classpathref="compile.path"/>
    </target>

    <target name="compile-tests" depends="compile" description="Compile test source code">
        <javac srcdir="${test.src.dir}" destdir="${test.classes.dir}" includeantruntime="false" debug="true">
            <classpath>
                <path refid="test.path"/>
                <pathelement path="${classes.dir}"/>
            </classpath>
        </javac>
    </target>

    <target name="test" depends="compile-tests" description="Run unit tests and code coverage reporting">
        <taskdef uri="antlib:org.jacoco.ant" resource="org/jacoco/ant/antlib.xml" classpathref="build.path"/>

        <jacoco:coverage destfile="${build.dir}/jacoco.exec" xmlns:jacoco="antlib:org.jacoco.ant">
            <junit haltonfailure="yes" fork="true">
                <classpath>
                    <path refid="test.path"/>
                    <pathelement path="${classes.dir}"/>
                    <pathelement path="${test.classes.dir}"/>
                </classpath>
                <formatter type="plain" usefile="false" />
                <formatter type="xml"/>
                <batchtest fork="yes" todir="${reports.dir}/junit">
                    <fileset dir="${test.src.dir}">
                        <include name="**/*Test*.java"/>
                        <exclude name="**/AllTests.java"/>
                    </fileset>
                </batchtest>
            </junit>
        </jacoco:coverage>    
    </target>

    <target name="sonar" depends="test" description="Upload metrics to Sonar">
        <taskdef uri="antlib:org.sonar.ant" resource="org/sonar/ant/antlib.xml" classpathref="build.path"/>

        <ivy:cachepath pathid="sonar.libraries" conf="compile"/> 

        <sonar:sonar xmlns:sonar="antlib:org.sonar.ant"/>
    </target>

    <target name="clean" description="Cleanup build files">
        <delete dir="${build.dir}"/>
    </target>

    <target name="clean-all" depends="clean" description="Additionally purge ivy cache">
        <ivy:cleancache/>
    </target>

</project>

build.properties

# Build properties
build.dir=build

src.dir=src/main/java
test.src.dir=src/test/java

classes.dir=${build.dir}/classes
test.classes.dir=${build.dir}/test-classes

reports.dir=${build.dir}/reports

# Sonar properties
sonar.projectKey=org.demo:demo
sonar.projectName=Demo project
sonar.projectVersion=1.0
sonar.projectDescription=This is my demo Sonar project

sonar.host.url=http://localhost:9000

sonar.jdbc.url=jdbc:h2:tcp://localhost:9092/sonar
sonar.jdbc.driverClassName=org.h2.Driver
sonar.jdbc.username=sonar
sonar.jdbc.password=sonar

sonar.working.directory=${build.dir}/sonar

sonar.language=java
sonar.sources=${src.dir}
sonar.binaries=${classes.dir}
sonar.tests=${test.src.dir}

sonar.dynamicAnalysis=reuseReports
sonar.surefire.reportsPath=${reports.dir}/junit
sonar.java.coveragePlugin=jacoco
sonar.jacoco.reportPath=${build.dir}/jacoco.exec