Maven by Example 1.7. Comparing Maven with Ant

The authors of this book have no interest in creating a feud(不和；争执；封地) between Apache Ant and Apache Maven, but we are also cognizant（审理的；已认知的） of the fact that most organizations have to make a decision between the two standard solutions: Apache Ant and Apache Maven. In this section, we compare and contrast the tools.

Ant excels（超过；擅长） at build process, it is a build system modeled after make with targets and dependencies. Each target consists of a set of instructions which are coded in XML. There is a copy task and a javac task as well as a jar task. When you use Ant, you supply Ant with specific instructions for compiling and packaging your output. Look atthe following example of a simple build.xml file:

A Simple Ant build.xml File.

<project name="my-project" default="dist" basedir="."><description>simple example build file</description><!-- set global properties for this build --><property name="src" location="src/main/java"/><property name="build" location="target/classes"/><property name="dist"  location="target"/><target name="init"><!-- Create the time stamp --><tstamp/><!-- Create the build directory structure used by compile --><mkdir dir="${build}"/></target><target name="compile" depends="init"description="compile the source " ><!-- Compile the java code from ${src} into ${build} --><javac srcdir="${src}" destdir="${build}"/></target><target name="dist" depends="compile"description="generate the distribution" ><!-- Create the distribution directory --><mkdir dir="${dist}/lib"/><!-- Put everything in ${build} into the MyProject-${DSTAMP}.jar file --><jar jarfile="${dist}/lib/MyProject-${DSTAMP}.jar" basedir="${build}"/></target><target name="clean"description="clean up" ><!-- Delete the ${build} and ${dist} directory trees --><delete dir="${build}"/><delete dir="${dist}"/></target>
</project>

In this simple Ant example, you can see how you have to tell Ant exactly what to do. There is a compile goal which includes the javac task that compiles the source in the src/main/java directory to the target/classes directory. You have to tell Ant exactly where your source is, where you want the resulting bytecode to be stored, and how to package this all into a JAR file. While there are some recent developments that help make Ant less procedural（程序上的）, a developer’s experience with Ant is in coding a procedural language written in XML.

Contrast the previous Ant example with a Maven example. In Maven, to create a JAR file from some Java source, all you need to do is createa simple pom.xml, place your source code in${basedir}/src/main/java and then run mvn install from the command line. The example Maven pom.xml that achieves the same results as the simple Ant file listed in A Simple Ant build.xml File is shown in A Sample Maven pom.xml.

A Sample Maven pom.xml.

<project><modelVersion>4.0.0</modelVersion><groupId>org.sonatype.mavenbook</groupId><artifactId>my-project</artifactId><version>1.0-SNAPSHOT</version>
</project>

That’s all you need in your pom.xml. Running mvn install from the command line will process resources, compile source, execute unit tests, create a JAR, and install the JAR in a local repository for reuse in other projects. Without modification, you can run mvn site and then find an index.html file in target/site that contains links to JavaDoc and a few reports about your source code.

Admittedly, this is the simplest possible example project containing nothing more than some source code and producing a simple JAR. It is a project which closely follows Maven conventions and doesn’t requireany dependencies or customization. If we wanted to start customizing the behavior, our pom.xml is going to grow in size, and in the largest of projects you can see collections of very complex Maven POMs which contain a great deal of plugin customization and dependency declarations. But, even when your project’s POM files become more substantial, they hold an entirely different kind of information from the build file of a similarly sized project using Ant. Maven POMs contain declarations: "This is a JAR project", and "The source code isin src/main/java". Ant build files contain explicit instructions:"This is project", "The source is in src/main/java", "Run javac against this directory", "Put the results in target/classes","Create a JAR from the ….", etc. Where Ant had to be explicit about the process, there was something "built-in" to Maven that just knew where the source code was and how it should be processed.

The differences between Ant and Maven in this example are:

Apache Ant

Ant doesn’t have formal conventions like a common project directory structure or default behavior. You have to tell Ant exactly where to find the source and where to put the output. Informal conventions have emerged over time, but they haven’t been codified（编成法典） into the product.
Ant is procedural. You have to tell Ant exactly what to do and when to do it. You have to tell it to compile, then copy, then compress.
Ant doesn’t have a lifecycle. You have to define goals and goal dependencies. You have to attach a sequence of tasks to each goal manually.

Apache Maven

Maven has conventions. It knows where your source code is because you followed the convention. Maven’s Compiler plugin put the bytecode in target/classes, and it produces a JAR file in target.
Maven is declarative. All you had to do was create a pom.xml file and put your source in the default directory. Maven took care of the rest.
Maven has a lifecycle which was invoked when you executed mvn install. This command told Maven to execute a series of sequential lifecycle phases until it reached the install lifecycle phase. As a side-effect of this journey through the lifecycle, Maven executed a number of default plugin goals which did things like compile and create a JAR.

Maven has built-in intelligence about common project tasks in the form of Maven plugins. If you wanted to write and execute unit tests, all you would need to do is write the tests, place them in ${basedir}/src/test/java, add a test-scoped dependency on either TestNG or JUnit, and run mvn test. If you wanted to deploy a web application and not a JAR, all you would need to do is change your project type to war and put your docroot in${basedir}/src/main/webapp. Sure, you can do all of this with Ant, but you will be writing the instructions from scratch. In Ant,you would first have to figure out where the JUnit JAR file should be. Then you would have to create a classpath that includes the JUnitJAR file. Then you would tell Ant where it should look for test source code, write a goal that compiles the test source to bytecode, and execute the unit tests with JUnit.

Without supporting technologies like antlibs and Ivy (even with these supporting technologies), Ant has the feeling of a custom procedural build. An efficient set of Maven POMs in a project which adheres to Maven’s assumed conventions has surprisingly little XML compared tothe Ant alternative. Another benefit of Maven is the reliance on widely-shared Maven plugins. Everyone uses the Maven Surefire plugin for unit testing, and if someone adds support for a new unit testing framework, you can gain new capabilities in your own build by just incrementing the version of a particular Maven plugin in your project’s POM.

The decision to use Maven or Ant isn’t a binary one, and Ant still has a place in a complex build. If your current build contains some highly customized process, or if you’ve written some Ant scripts to completea specific process in a specific way that cannot be adapted to the Maven standards, you can still use these scripts with Maven. Ant is made available as a core Maven plugin. Custom Maven plugins can be implemented in Ant, and Maven projects can be configured to execute Ant scripts within the Maven project lifecycle.

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