Apache Ant

Apache Ant is a software tool for automating software build processes. It is similar to Make but is implemented using the Java language, requires the Java platform, and is best suited to building Java projects.

The most immediately noticeable difference between Ant and Make is that Ant uses XML to describe the build process and its dependencies, whereas Make uses Makefile format. By default the XML file is named build.xml.

Ant is an Apache project. It is open source software, and is released under the Apache Software License.

History

Ant ("Another Neat Tool"^[1]) was conceived by James Duncan Davidson while turning a product from Sun into open source. That product, Sun's reference JSP/Servlet engine, later became Apache Tomcat. A proprietary version of make was used to build it on the Solaris Operating Environment, but in the open source world there was no way of controlling which platform was used to build Tomcat. Ant was created as a simple platform-independent tool to build Tomcat from directives in an XML "build file". Ant (version 1.1) was officially released as a stand-alone product on July 19, 2000.

Several proposals for an Ant version 2 have been made, such as AntEater by James Duncan Davidson, Myrmidon by Peter Donald and Mutant by Conor MacNeill, none of which were able to find large acceptance with the developer community.^[2]

Today, Ant is the build tool used by most Java development projects.^[3] For example, most open source developers include build.xml files with their distribution.

Because Ant made it trivial to integrate JUnit tests with the build process, Ant has made it easy for willing developers to adopt test-driven development, and even Extreme Programming.

Other Java-based build tools include Maven and JavaMake.

Sample build.xml file

Below is listed a sample build.xml file for a simple Java "Hello, world" application. It defines four targets - clean, clobber, compile and jar, each of which has an associated description. The jar target lists the compile target as a dependency. This tells Ant that before it can start the jar target it must first complete the compile target.

<?xml version="1.0"?>
<project name="Hello" default="compile">
    <target name="clean" description="remove intermediate files">
        <delete dir="classes"/>
    </target>
    <target name="clobber" depends="clean" description="remove all artifact files">
        <delete file="hello.jar"/>
    </target>
    <target name="compile" description="compile the Java source code to class files">
        <mkdir dir="classes"/>
        <javac srcdir="." destdir="classes"/>
    </target>
    <target name="jar" depends="compile" description="create a Jar file for the application">
        <jar destfile="hello.jar">
            <fileset dir="classes" includes="**/*.class"/>
            <manifest>
                <attribute name="Main-Class" value="HelloProgram"/>
            </manifest>
        </jar>
    </target>
</project>

Within each target are the actions that Ant must take to build that target; these are performed using built-in tasks. For example, to build the compile target Ant must first create a directory called classes (Ant will only do so if it does not already exist) and then invoke the Java compiler. Therefore, the tasks used are mkdir and javac. These perform a similar task to the command-line utilities of the same name.

Another task used in this example is named jar:

 <jar destfile="hello.jar">

This ant task has the same name as the common java command-line utility, JAR, but is really a call to the ant program's built-in jar/zip file support. This detail is not relevant to most end users, who just get the JAR they wanted, with the files they asked for.

Many Ant tasks delegate their work to external programs, either native or Java. They use Ant's own <exec> and <java> tasks to set up the command lines, and handle all the details of mapping from information in the build file to the program's arguments -and interpreting the return value. Users can see which tasks do this (e.g. <cvs>, <signjar>, <chmod>, <rpm>), by trying to execute the task on a system without the underlying program on the path, or without a full Java Development Kit (JDK) installed.

Extensions

WOProject-Ant^[4] is just one of many examples of a task extension written for Ant. These extensions are put to use by copying their jar files into ant's lib directory. Once this is done, these extension tasks can be invoked directly in the typical build.xml file. The WOProject extensions allow WebObjects developers to use ant in building their frameworks and applications, instead of using Apple's Xcode suite.

Antcontrib^[5] provides a collection of tasks such as conditional statements and operations on properties as well as other useful tasks.^[6]

Other task extensions exist for Perforce, .Net, EJB, and filesystem manipulations, just to name a few.^[7]

Portability

One of the primary aims of Ant was to solve make's portability problem. In a Makefile the actions required to create a target are specified as shell commands which are specific to the platform Make runs on. Different platforms require different shell commands. Ant solves this problem by providing a large amount of built-in functionality that is supposed to behave the same on all platforms.

For example, in the sample build.xml file above the clean target deletes the classes directory and everything in it. In a Makefile this would typically be done with the command:

rm -rf classes/

rm is a Unix-specific command unavailable in some other environments. Microsoft Windows, for example, would use:

rmdir /S /Q classes

In an Ant build file the same thing would be accomplished using a built-in command:

 <delete dir="classes"/>

A common difference between platforms is the symbol used to delimit elements of file system directory path components. Unix uses a forward slash (/) to delimit components whereas Windows uses a backslash (\). Ant build files let authors choose their favorite convention: forward slash or backslash for directories; semicolon or colon for path separators. It converts each to the symbol appropriate to the platform it executes on.

Limitations

• Ant build files are written in XML. For unfamiliar users, both XML itself and the complex structure (hierarchical, partly ordered, and pervasively cross-linked) of Ant documents can be a barrier to learning. A GUI called Antidote was available for a time, but never gained a following and has been retired from the Apache project. Moreover, the language of Ant is quite verbose, and the build files of large or complex projects become unmanageably large. Good design and modularization of build files can improve readability but not necessarily reduce size. Other build tools like Maven use more concise scripts at the expense of generality and flexibility.
• Many of the older tasks—the core ones that are used every day, such as <javac>, <exec> and <java>—use default values for options that are not consistent with more recent versions of the tasks. Changing those defaults would break existing Ant scripts.
• When expanding properties in a string or text element, undefined properties are not raised as an error, but left as an unexpanded reference (e.g. ${unassigned.property}).
• Ant has limited fault handling rules, and no persistence of state, so it cannot be used as a workflow tool for any workflow other than classic build and test processes.
• The Ant target model does not treat artifacts as targets. In most build tools a target is an artifact created by the build—a program, library, intermediate object file, PDF documentation, etc.—and rules specify the dependencies between targets and the tasks to run to build a target when it is out of date. In Ant a target is a group of tasks rather than an artifact. This means that Ant is sometimes unable to determine the relationship between an artifact and the task sequence to build the artifact and this logic must be implemented by the programmer using Ant's control structures.
• Once a global property is defined it cannot be changed by any of the core tasks. Antcontrib provides a variable task to work around this problem. And AntXtras provides a variable property type that complements readonly properties.
• Lazy property evaluation is not supported. For instance, when working within a Antcontrib <for> loop, a property cannot be re-evaluated for a sub-value which may be part of the iteration. (Again you can workaround this depending on the third-party extension you're using; AntXtras flow-control tasksets do provide for cursor redefinition for loops.)
• In makefiles, any rule to create one file type from another file type can be written inline within the makefile. For example, you may transform a document into some other format by using rules to execute another tool. Creating a similar task in Ant is more complex: a separate task must be written in Java and included with the Ant build file in order to handle the same type of functionality. However, this separation can enhance the readability of the Ant script by hiding some of the details of how a task is executed on different platforms.

There exists a myriad of third-party Ant extensions (called antlibs) that provide much of the missing functionality. Also the Eclipse IDE can build and execute Ant scripts while the NetBeans IDE uses Ant for its internal build system. As both these IDEs are very popular development platforms, they can simplify Ant use significantly (as a bonus Ant scripts generated by NetBeans can be used outside of that IDE as standalone scripts).

See also

• Build Automation
• Maven, a project management and build automation tool primarily for Java
• Apache Jelly, a tool for turning XML into executable code
• Nant, Ant-like tool targeted at the .NET environment rather than Java
• Ivy, a dependency manager which integrates tightly with Ant, subproject of Ant
• List of build automation software

References

Bibliography

External links

• Official website of Apache Ant
• Apache Ant manual (tasks, types)
• Apache Ant wiki
• WinAnt - Windows installer for Apache Ant
• Introduction to Ant (slide show)
• Linguine Maps visualization library will automatically produce easy to read diagrams from Ant build files.
• antro - a profiler for Ant scripts
• Wiki Book on learning Apache Ant
• Ant tutorial
• Ant Automation, a good handy example of automation with Ant.
• A simple Windows GUI for running Ant.