STEP FOUR: IMPOSING STRUCTURE in Java

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STEP FOUR: IMPOSING STRUCTURE
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<target name="compile" depends="init" > <target name="archive" depends="compile" > <target name="clean" depends="init">
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If a target directly depends on more than one predecessor target, then you should list both dependencies in the dependency attribute, for example depends="compile,test". In our example build, the archive task does depend upon both init and compile, but we do not bother to state the dependency upon init because the compile target depends upon it. If Ant must execute init before compile, and archive depends upon compile then Ant must run init before archive. Put formally, dependencies are transitive. They are not however reflexive: the compile target does not know or care about the archive target. Another useful fact is that the order of targets inside the build file is not important: Ant reads in the whole file before it builds the dependency tree and executes targets. There is no need to worry about forward references. If you look at the dependency tree of targets in the current example, it looks like figure 2.3. Before Ant executes any target, all the predecessor targets must already have been executed. If these predecessors depend on targets themselves, the execution order will also consider those and produce an order that satisfies all dependencies. If two targets in this execution order share a common dependency, then that predecessor will only execute once. Experienced makefile editors will recognize that Ant targets resemble Make s pseudotargets targets in a makefile that you refer to by name in the dependencies of other makefile targets. Usually in Make, you name the source files that a target depends on, and the build tool itself works out what to do to create the target file from the source files. In Ant, you name stages of work as targets, and the tasks inside each target work out for themselves what their dependencies are.
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Figure 2.3 Once you add dependencies, the graph of targets gets more complex. Here clean depends upon init; archive depends on compile directly and init indirectly. All of a target s dependencies will be executed ahead of the target itself.
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Running the new build file Now that there are multiple targets in the build file, we need a way of specifying which to run. You can simply list one or more targets on the command line, so all of the following are valid, as are other combinations:
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ant ant init ant clean
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GETTING STARTED WITH ANT
ant compile ant archive ant clean archive
Calling Ant with no target is the same as calling the default target named in the project. In this example, it is the archive target:
init: [mkdir] Created dir: C:\AntBook\secondbuild\build [mkdir] Created dir: C:\AntBook\secondbuild\dist compile: [javac] Compiling 1 source file to C:\AntBook\secondbuild\build archive: [jar] Building jar: C:\AntBook\secondbuild\dist\project.jar BUILD SUCCESSFUL Total time: 2 seconds
This demonstrates that Ant has determined execution order of tasks. When you invoke a target with dependencies, all their dependencies execute first. As both the compile and archive targets depend upon the init target, Ant must call init before it executes either of those targets. It orders the targets so that first the directories get created, then the source compiled, and finally the JAR archive built. 2.5.7 Rerunning the build What happens when the build is run a second time Let s try it and see:
init: compile: archive: BUILD SUCCESSFUL Total time: 1 second
We go through all the targets, but none of the tasks say that they are doing any work. Here s why: all of these tasks check their dependencies, <mkdir> does not create directories that already exist, <javac> compares source and class file timestamps, and the <jar> task compares the time of all files to be added to the archive with the time of the file itself. Only if a source file is newer than the generated archive file does the task rebuild the JAR file. If you add the -verbose flag to the command line you will get more detail on what did or, in this case, did not take place.
>ant -verbose Apache Ant version 1.5alpha compiled on February 1 2002 Buildfile: build.xml Detected Java version: 1.3 in: D:\Java\jdk13\jre Detected OS: Windows 2000 parsing buildfile C:\AntBook\secondbuild\build.xml with
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