Question

How to use external libraries in Apache Ant, explain with example?

Answer 1

Somebody told us not to use System-statements. For output, we should use a Logging API—customizable to a high degree (including switching off during usual life (= not development) execution). We use Log4J for that, because

• it is not part of the JDK and we want to show how to use external libs
• it's highly configurable
• it's from Apache ;-)

We store our external libraries in a new directory lib. Log4J can be downloaded [1] from Logging's Homepage. Create the lib directory and extract the log4j-1.2.17.jar into that directory. After that we have to modify our Java source file to use that library and our buildfile so that this library could be accessed during compilation and run.

Working with Log4J is documented inside its manual. Here we use the MyApp-example from the Short Manual [2]. First we have to modify the java source to use the logging framework:

package oata;

import org.apache.log4j.Logger;
import org.apache.log4j.BasicConfigurator;

public class HelloWorld {
    static Logger logger = Logger.getLogger(HelloWorld.class);

    public static void main(String[] args) {
        BasicConfigurator.configure();
        logger.info("Hello World");          // the old SysO-statement
    }
}

Most of the modifications are "framework overhead" which has to be done once. The blue line is our "old System-out" statement.

Don't try to run ant—you will only get lot of compiler errors. Log4J is not on the classpath so we have to do a little work here. But do not change the CLASSPATH environment variable! This is only for this project and maybe you would break other environments (this is one of the most famous mistakes when working with Ant). We introduce Log4J (or to be more precise: all libraries (jar-files) which are somewhere under .\lib) into our buildfile:

<project name="HelloWorld" basedir="." default="main">
    ...
    <property name="lib.dir"     value="lib"/>

    <path id="classpath">
        <fileset dir="${lib.dir}" includes="**/*.jar"/>
    </path>

    ...

    <target name="compile">
        <mkdir dir="${classes.dir}"/>
        <javac srcdir="${src.dir}" destdir="${classes.dir}" classpathref="classpath"/>
    </target>

    <target name="run" depends="jar">
        <java fork="true" classname="${main-class}">
            <classpath>
                <path refid="classpath"/>
                <path location="${jar.dir}/${ant.project.name}.jar"/>
            </classpath>
        </java>
    </target>

    ...

</project>

In this example we start our application not via its Main-Class manifest-attribute, because we could not provide a jar-name and a classpath. So add our class in the red line to the already defined path and start as usual. Running ant would give (after the usual compile stuff):

[java] 0 [main] INFO oata.HelloWorld  - Hello World

What's that?

• [java] Ant task running at the moment
• 0 sorry, don't know—some Log4J stuff
• [main] the running thread from our application
• INFO log level of that statement
• oata.HelloWorld source of that statement
• - separator
• Hello World the message

For another layout ... have a look inside Log4J's documentation about using other PatternLayouts.