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<link rel="stylesheet" type="text/css" href="stylesheets/style.css">
<title>Writing a Simple Buildfile</title>
</head>

<body>
<h1>Using Ant</h1>
<h2><a name="buildfile">Writing a Simple Buildfile</a></h2>
<p>Ant's buildfiles are written in XML. Each buildfile contains one project
and at least one (default) target. Targets contain task elements.
Each task element of the buildfile can have an <code>id</code> attribute and
can later be referred to by the value supplied to this. The value has
to be unique. (For additional information, see the
<a href="#tasks"> Tasks</a> section below.)</p>
<h3><a name="projects">Projects</a></h3>
<p>A <i>project</i> has three attributes:</p>
<table border="1" cellpadding="2" cellspacing="0">
  <tr>
    <td valign="top"><b>Attribute</b></td>
    <td valign="top"><b>Description</b></td>
    <td align="center" valign="top"><b>Required</b></td>
  </tr>
  <tr>
    <td valign="top">name</td>
    <td valign="top">the name of the project.</td>
    <td align="center" valign="top">No</td>
  </tr>
  <tr>
    <td valign="top">default</td>
    <td valign="top">the default target to use when no target is supplied.</td>
    <td align="center" valign="top">No; however, <b>since Ant 1.6.0</b>,
        every project includes an implicit target that contains any and
        all top-level tasks and/or types. This target will always be
        executed as part of the project's initialization, even when Ant is
        run with the <a href="running.html#options">-projecthelp</a> option.
    </td>
  </tr>
  <tr>
    <td valign="top">basedir</td>
    <td valign="top">the base directory from which all path calculations are
      done. This attribute might be overridden by setting
      the &quot;basedir&quot;
      property beforehand. When this is done, it must be omitted in the
      project tag. If neither the attribute nor the property have
      been set, the parent directory of the buildfile will be used.</td>
    <td align="center" valign="top">No</td>
  </tr>
</table>
<p>Optionally, a description for the project can be provided as a
top-level <code>&lt;description&gt;</code> element (see the <a
href="CoreTypes/description.html">description</a> type).</p>

<p>Each project defines one or more <i>targets</i>.
A target is a set of <i>tasks</i> you want
to be executed. When starting Ant, you can select which target(s) you
want to have executed. When no target is given,
the project's default is used.</p>

<h3><a name="targets">Targets</a></h3>
<p>A target can depend on other targets. You might have a target for compiling,
for example, and a target for creating a distributable. You can only build a
distributable when you have compiled first, so the distribute target
<i>depends on</i> the compile target. Ant resolves these dependencies.</p>
<p>It should be noted, however, that Ant's <code>depends</code> attribute
only specifies the <i>order</i> in which targets should be executed - it
does not affect whether the target that specifies the dependency(s) gets
executed if the dependent target(s) did not (need to) run.
</p>
<p>Ant tries to execute the targets in the <code>depends</code>
attribute in the order
they appear (from left to right). Keep in mind that it is possible that a target
can get executed earlier when an earlier target depends on it:</p>
<blockquote>
<pre>&lt;target name=&quot;A&quot;/&gt;
&lt;target name=&quot;B&quot; depends=&quot;A&quot;/&gt;
&lt;target name=&quot;C&quot; depends=&quot;B&quot;/&gt;
&lt;target name=&quot;D&quot; depends=&quot;C,B,A&quot;/&gt;</pre>
</blockquote>
<p>Suppose we want to execute target D. From its
<code>depends</code> attribute, you
might think that first target C, then B and then A is executed.
Wrong! C depends on B, and B depends on A, so first A is executed, then B, then C, and finally D.</p>
<p>In a chain of dependencies stretching back from a given target such
as D above, each target gets executed only once, even when more than
one target depends on it. Thus, executing the D target will first
result in C being called, which in turn will first call B, which in
turn will first call A. After A, then B, then C have executed,
execution returns to the dependency list of D, which will <u>not</u>
call B and A, since they were already called in process of dependency
resolution for C and B respectively as dependencies of D. Had no such
dependencies been discovered in processing C and B, B and A would
have been executed after C in processing D's dependency list.</p>
<p>A target also has the ability to perform its execution if (or
unless) a property has been set. This allows, for example, better
control on the building process depending on the state of the system
(java version, OS, command-line property defines, etc.).  To make a target
<i>sense</i> this property, you should add the <code>if</code> (or
<code>unless</code>) attribute with the name of the property that the target
should react to. <strong>Note:</strong> Ant will only check whether
the property has been set, the value doesn't matter.  A property set
to the empty string is still an existing property.  For example:</p>
<blockquote>
  <pre>&lt;target name=&quot;build-module-A&quot; if=&quot;module-A-present&quot;/&gt;</pre>
  <pre>&lt;target name=&quot;build-own-fake-module-A&quot; unless=&quot;module-A-present&quot;/&gt;</pre>
</blockquote>
<p>In the first example, if the <code>module-A-present</code>
property is set (to any value, e.g. <i>false</i>), the target will be run. In the second
example, if the <code>module-A-present</code> property is set
(again, to any value), the target will not be run.
</p>
<p>Only one propertyname can be specified in the if/unless clause. If you want to check
multiple conditions, you can use a dependend target for computing the result for the check:</p>
<blockquote><pre>
&lt;target name="myTarget" depends="myTarget.check" if="myTarget.run"&gt;
    &lt;echo&gt;Files foo.txt and bar.txt are present.&lt;/echo&gt;
&lt/target&gt;

&lt;target name="myTarget.check"&gt;
    &lt;condition property="myTarget.run"&gt;
        &lt;and&gt;
            &lt;available file="foo.txt"/&gt;
            &lt;available file="bar.txt"/&gt;
        &lt;/and&gt;
    &lt;/condition&gt;
&lt/target&gt;
</pre></blockquote>
<p>If no <code>if</code> and no <code>unless</code> attribute is present,
the target will always be executed.</p>

<p>
<b>Important:</b> the <code>if</code> and <code>unless</code> attributes only
enable or disable the target to which they are attached. They do not control
whether or not targets that a conditional target depends upon get executed.
In fact, they do not even get evaluated until the target is about to be executed,
and all its predecessors have already run.

<p>The optional <code>description</code> attribute can be used to provide a one-line description of this target, which is printed by the
<nobr><code>-projecthelp</code></nobr> command-line option. Targets
without such a description are deemed internal and will not be listed,
unless either the <nobr><code>-verbose</code></nobr> or
<nobr><code>-debug</code></nobr> option is used.
</p>
<p>It is a good practice to place your <a
href="CoreTasks/tstamp.html">tstamp</a> tasks in a so-called
<i>initialization</i> target, on which
all other targets depend. Make sure that target is always the first one in
the depends list of the other targets. In this manual, most initialization targets
have the name <code>&quot;init&quot;</code>.</p>
<p>If the depends attribute and the if/unless attribute are set, the depends attribute is
executed first.</p>
<p>A target has the following attributes:</p>
<table border="1" cellpadding="2" cellspacing="0">
  <tr>
    <td valign="top"><b>Attribute</b></td>
    <td valign="top"><b>Description</b></td>
    <td align="center" valign="top"><b>Required</b></td>
  </tr>
  <tr>
    <td valign="top">name</td>
    <td valign="top">the name of the target.</td>
    <td align="center" valign="top">Yes</td>
  </tr>
  <tr>
    <td valign="top">depends</td>
    <td valign="top">a comma-separated list of names of targets on which this
      target depends.</td>
    <td align="center" valign="top">No</td>
  </tr>
  <tr>
    <td valign="top">if</td>
    <td valign="top">the name of the property that must be set in order for this
      target to execute.</td>
    <td align="center" valign="top">No</td>
  </tr>
  <tr>
    <td valign="top">unless</td>
    <td valign="top">the name of the property that must not be set in order
      for this target to execute.</td>
    <td align="center" valign="top">No</td>
  </tr>
  <tr>
    <td valign="top">description</td>
    <td valign="top">a short description of this target's function.</td>
    <td align="center" valign="top">No</td>
  </tr>
</table>
</p>

<p>A target name can be any alphanumeric string valid in the encoding of the XML
file. The empty string &quot;&quot; is in this set, as is
comma &quot;,&quot; and space &quot; &quot;.
Please avoid using these, as they will not be supported in future Ant versions
because of all the confusion they cause. IDE support of unusual target names,
or any target name containing spaces, varies with the IDE.</p>

<p>Targets beginning with a hyphen such as <code>&quot;-restart&quot;</code>
are valid, and can be used
to name targets that should not be called directly from the command line.</p>

<h3><a name="tasks">Tasks</a></h3>
<p>A task is a piece of code that can be executed.</p>
<p>A task can have multiple attributes (or arguments, if you prefer). The value
of an attribute might contain references to a property. These references will be
resolved before the task is executed.</p>
<p>Tasks have a common structure:</p>
<blockquote>
  <pre>&lt;<i>name</i> <i>attribute1</i>=&quot;<i>value1</i>&quot; <i>attribute2</i>=&quot;<i>value2</i>&quot; ... /&gt;</pre>
</blockquote>
<p>where <i>name</i> is the name of the task,
<i>attributeN</i> is the attribute name, and
<i>valueN</i> is the value for this attribute.</p>
<p>There is a set of <a href="coretasklist.html" target="navFrame">built-in tasks</a>, along with a
number of
<a href="optionaltasklist.html" target="navFrame"> optional tasks</a>, but it is also very
easy to <a href="develop.html#writingowntask">write your own</a>.</p>
<p>All tasks share a task name attribute. The value of
this attribute will be used in the logging messages generated by
Ant.</p>
Tasks can be assigned an <code>id</code> attribute:
<blockquote>
<pre>&lt;<i>taskname</i> id="<i>taskID</i>" ... /&gt;</pre>
</blockquote>
where <i>taskname</i> is the name of the task, and <i>taskID</i> is
a unique identifier for this task.
You can refer to the
corresponding task object in scripts or other tasks via this name.
For example, in scripts you could do:
<blockquote>
<pre>
&lt;script ... &gt;
  task1.setFoo("bar");
&lt;/script&gt;
</pre>
</blockquote>
to set the <code>foo</code> attribute of this particular task instance.
In another task (written in Java), you can access the instance via
<code>project.getReference("task1")</code>.
<p>
Note<sup>1</sup>: If &quot;task1&quot; has not been run yet, then
it has not been configured (ie., no attributes have been set), and if it is
going to be configured later, anything you've done to the instance may
be overwritten.
</p>
<p>
Note<sup>2</sup>: Future versions of Ant will most likely <i>not</i>
be backward-compatible with this behaviour, since there will likely be no
task instances at all, only proxies.
</p>

<h3><a name="properties">Properties</a></h3>
<p>A project can have a set of properties. These might be set in the buildfile
by the <a href="CoreTasks/property.html">property</a> task, or might be set outside Ant. A
property has a name and a value; the name is case-sensitive. Properties may be used in the value of
task attributes. This is done by placing the property name between
&quot;<code>${</code>&quot; and &quot;<code>}</code>&quot; in the
attribute value. For example,
if there is a &quot;builddir&quot; property with the value
&quot;build&quot;, then this could be used in an attribute like this:
<code>${builddir}/classes</code>.
This is resolved at run-time as <code>build/classes</code>.</p>
<p>In the event you should need to include this construct literally
(i.e. without property substitutions), simply "escape" the '$' character
by doubling it. To continue the previous example:
<pre>  &lt;echo&gt;$${builddir}=${builddir}&lt;/echo&gt;</pre>
will echo this message:
<pre>  ${builddir}=build/classes</pre></p>
<p>In order to maintain backward compatibility with older Ant releases,
a single '$' character encountered apart from a property-like construct
(including a matched pair of french braces) will be interpreted literally;
that is, as '$'.  The "correct" way to specify this literal character,
however, is by using the escaping mechanism unconditionally, so that "$$"
is obtained by specifying "$$$$".  Mixing the two approaches yields
unpredictable results, as "$$$" results in "$$".</p>

<h3><a name="built-in-props">Built-in Properties</a></h3>
<p>Ant provides access to all system properties as if they had been
defined using a <code>&lt;property&gt;</code> task.
For example, <code>${os.name}</code> expands to the
name of the operating system.</p>
<p>For a list of system properties see
<a href="http://java.sun.com/j2se/1.3/docs/api/java/lang/System.html#getProperties()">the Javadoc of System.getProperties</a>.
</p>
<p>In addition, Ant has some built-in properties:</p>
<pre>
basedir             the absolute path of the project's basedir (as set
                    with the basedir attribute of <a href="#projects">&lt;project&gt;)</a>.
ant.file            the absolute path of the buildfile.
ant.version         the version of Ant
ant.project.name    the name of the project that is currently executing;
                    it is set in the name attribute of &lt;project&gt;.
ant.java.version    the JVM version Ant detected; currently it can hold
                    the values &quot;1.2&quot;, &quot;1.3&quot;, &quot;1.4&quot; and &quot;1.5&quot;.
</pre>
<p>There is also another property, but this is set by the launcher script and therefore
maybe not set inside IDEs:</p>
<pre>
ant.home            home directory of Ant
</pre>

<a name="example"><h3>Example Buildfile</h3></a>
<pre>
&lt;project name=&quot;MyProject&quot; default=&quot;dist&quot; basedir=&quot;.&quot;&gt;
    &lt;description&gt;
        simple example build file
    &lt;/description&gt;
  &lt;!-- set global properties for this build --&gt;
  &lt;property name=&quot;src&quot; location=&quot;src&quot;/&gt;
  &lt;property name=&quot;build&quot; location=&quot;build&quot;/&gt;
  &lt;property name=&quot;dist&quot;  location=&quot;dist&quot;/&gt;

  &lt;target name=&quot;init&quot;&gt;
    &lt;!-- Create the time stamp --&gt;
    &lt;tstamp/&gt;
    &lt;!-- Create the build directory structure used by compile --&gt;
    &lt;mkdir dir=&quot;${build}&quot;/&gt;
  &lt;/target&gt;

  &lt;target name=&quot;compile&quot; depends=&quot;init&quot;
        description=&quot;compile the source &quot; &gt;
    &lt;!-- Compile the java code from ${src} into ${build} --&gt;
    &lt;javac srcdir=&quot;${src}&quot; destdir=&quot;${build}&quot;/&gt;
  &lt;/target&gt;

  &lt;target name=&quot;dist&quot; depends=&quot;compile&quot;
        description=&quot;generate the distribution&quot; &gt;
    &lt;!-- Create the distribution directory --&gt;
    &lt;mkdir dir=&quot;${dist}/lib&quot;/&gt;

    &lt;!-- Put everything in ${build} into the MyProject-${DSTAMP}.jar file --&gt;
    &lt;jar jarfile=&quot;${dist}/lib/MyProject-${DSTAMP}.jar&quot; basedir=&quot;${build}&quot;/&gt;
  &lt;/target&gt;

  &lt;target name=&quot;clean&quot;
        description=&quot;clean up&quot; &gt;
    &lt;!-- Delete the ${build} and ${dist} directory trees --&gt;
    &lt;delete dir=&quot;${build}&quot;/&gt;
    &lt;delete dir=&quot;${dist}&quot;/&gt;
  &lt;/target&gt;
&lt;/project&gt;
</pre>

<p>Notice that we are declaring properties outside any target.  As of
Ant 1.6 all tasks can be declared outside targets (earlier version
only allowed <tt>&lt;property&gt;</tt>,<tt>&lt;typedef&gt;</tt> and
<tt>&lt;taskdef&gt;</tt>).  When you do this they are evaluated before
any targets are executed.  Some tasks will generate build failures if
they are used outside of targets as they may cause infinite loops
otherwise (<code>&lt;antcall&gt;</code> for example).</p>

<p>
We have given some targets descriptions; this causes the <tt>projecthelp</tt>
invocation option to list them as public targets with the descriptions; the
other target is internal and not listed.
<p>
Finally, for this target to work the source in the <tt>src</tt> subdirectory
should be stored in a directory tree which matches the package names. Check the
<tt>&lt;javac&gt;</tt> task for details.

<a name="filters"><h3>Token Filters</h3></a>
<p>A project can have a set of tokens that might be automatically expanded if
found when a file is copied, when the filtering-copy behavior is selected in the
tasks that support this. These might be set in the buildfile
by the <a href="CoreTasks/filter.html">filter</a> task.</p>
<p>Since this can potentially be a very harmful behavior,
the tokens in the files <b>must</b>
be of the form <code>@</code><i>token</i><code>@</code>, where
<i>token</i> is the token name that is set
in the <code>&lt;filter&gt;</code> task. This token syntax matches the syntax of other build systems
that perform such filtering and remains sufficiently orthogonal to most
programming and scripting languages, as well as with documentation systems.</p>
<p>Note: If a token with the format <code>@</code><i>token</i><code>@</code>
is found in a file, but no
filter is associated with that token, no changes take place;
therefore, no escaping
method is available - but as long as you choose appropriate names for your
tokens, this should not cause problems.</p>
<p><b>Warning:</b> If you copy binary files with filtering turned on, you can corrupt the
files. This feature should be used with text files <em>only</em>.</p>

<h3><a name="path">Path-like Structures</a></h3>
<p>You can specify <code>PATH</code>- and <code>CLASSPATH</code>-type
references using both
&quot;<code>:</code>&quot; and &quot;<code>;</code>&quot; as separator
characters. Ant will
convert the separator to the correct character of the current operating
system.</p>
<p>Wherever path-like values need to be specified, a nested element can
be used. This takes the general form of:</p>
<pre>
    &lt;classpath&gt;
      &lt;pathelement path=&quot;${classpath}&quot;/&gt;
      &lt;pathelement location=&quot;lib/helper.jar&quot;/&gt;
    &lt;/classpath&gt;
</pre>
<p>The <code>location</code> attribute specifies a single file or
directory relative to the project's base directory (or an absolute
filename), while the <code>path</code> attribute accepts colon-
or semicolon-separated lists of locations. The <code>path</code>
attribute is intended to be used with predefined paths - in any other
case, multiple elements with <code>location</code> attributes should be
preferred.</p>
<p>As a shortcut, the <code>&lt;classpath&gt;</code> tag
supports <code>path</code> and
<code>location</code> attributes of its own, so:</p>
<pre>
    &lt;classpath&gt;
      &lt;pathelement path=&quot;${classpath}&quot;/&gt;
    &lt;/classpath&gt;
</pre>
<p>can be abbreviated to:</p>
<pre>
    &lt;classpath path=&quot;${classpath}&quot;/&gt;
</pre>
<p>In addition, one or more
<a href="CoreTypes/resources.html#collection">Resource Collection</a>s
can be specified as nested elements (these must consist of
<a href="CoreTypes/resources.html#file">file</a>-type resources only).
Additionally, it should be noted that although resource collections are
processed in the order encountered, certain resource collection types
such as <a href="CoreTypes/fileset.html">fileset</a>,
<a href="CoreTypes/dirset.html">dirset</a> and
<a href="CoreTypes/resources.html#files">files</a>
are undefined in terms of order.</p>
<pre>
    &lt;classpath&gt;
      &lt;pathelement path=&quot;${classpath}&quot;/&gt;
      &lt;fileset dir=&quot;lib&quot;&gt;
        &lt;include name=&quot;**/*.jar&quot;/&gt;
      &lt;/fileset&gt;
      &lt;pathelement location=&quot;classes&quot;/&gt;
      &lt;dirset dir=&quot;${build.dir}&quot;&gt;
        &lt;include name=&quot;apps/**/classes&quot;/&gt;
        &lt;exclude name=&quot;apps/**/*Test*&quot;/&gt;
      &lt;/dirset&gt;
      &lt;filelist refid=&quot;third-party_jars&quot;/&gt;
    &lt;/classpath&gt;
</pre>
<p>This builds a path that holds the value of <code>${classpath}</code>,
followed by all jar files in the <code>lib</code> directory,
the <code>classes</code> directory, all directories named
<code>classes</code> under the <code>apps</code> subdirectory of
<code>${build.dir}</code>, except those
that have the text <code>Test</code> in their name, and
the files specified in the referenced FileList.</p>
<p>If you want to use the same path-like structure for several tasks,
you can define them with a <code>&lt;path&gt;</code> element at the
same level as <i>target</i>s, and reference them via their
<i>id</i> attribute--see <a href="#references">References</a> for an
example.</p>
<p>A path-like structure can include a reference to another path-like
structure (a path being itself a resource collection)
via nested <code>&lt;path&gt;</code> elements:</p>
<pre>
    &lt;path id=&quot;base.path&quot;&gt;
      &lt;pathelement path=&quot;${classpath}&quot;/&gt;
      &lt;fileset dir=&quot;lib&quot;&gt;
        &lt;include name=&quot;**/*.jar&quot;/&gt;
      &lt;/fileset&gt;
      &lt;pathelement location=&quot;classes&quot;/&gt;
    &lt;/path&gt;

    &lt;path id=&quot;tests.path&quot;&gt;
      &lt;path refid=&quot;base.path&quot;/&gt;
      &lt;pathelement location=&quot;testclasses&quot;/&gt;
    &lt;/path&gt;
</pre>
 The shortcuts previously mentioned for <code>&lt;classpath&gt;</code> are also valid for <code>&lt;path&gt;</code>.For example:
<pre>
    &lt;path id=&quot;base.path&quot;&gt;
      &lt;pathelement path=&quot;${classpath}&quot;/&gt;
    &lt;/path&gt;
</pre>
can be written as:
<pre>
    &lt;path id=&quot;base.path&quot; path=&quot;${classpath}&quot;/&gt;
</pre>

  <h4><a name="pathshortcut">Path Shortcut</a></h4>
  <p>
    In Ant 1.6 a shortcut for converting paths to OS specific strings
    in properties has been added. One can use the expression 
    ${toString:<em>pathreference</em>} to convert a path element
    reference to a string that can be used for a path argument.
    For example:
  </p>
<pre>
  &lt;path id="lib.path.ref"&gt;
    &lt;fileset dir="lib" includes="*.jar"/&gt;
  &lt;/path&gt;
  &lt;javac srcdir="src" destdir="classes"&gt;
    &lt;compilerarg arg="-Xbootstrap/p:${toString:lib.path.ref}"/&gt;
  &lt;/javac&gt;
</pre>


<h3><a name="arg">Command-line Arguments</a></h3>
<p>Several tasks take arguments that will be passed to another
process on the command line. To make it easier to specify arguments
that contain space characters, nested <code>arg</code> elements can be used.</p>
<table border="1" cellpadding="2" cellspacing="0">
<tr>
  <td width="12%" valign="top"><b>Attribute</b></td>
  <td width="78%" valign="top"><b>Description</b></td>
  <td width="10%" valign="top"><b>Required</b></td>
</tr>
  <tr>
    <td valign="top">value</td>
    <td valign="top">a single command-line argument; can contain space
      characters.</td>
    <td align="center" rowspan="5">Exactly one of these.</td>
  </tr>
  <tr>
    <td valign="top">file</td>
    <td valign="top">The name of a file as a single command-line
      argument; will be replaced with the absolute filename of the file.</td>
  </tr>
  <tr>
    <td valign="top">path</td>
    <td valign="top">A string that will be treated as a path-like
      string as a single command-line argument; you can use <code>;</code>
      or <code>:</code> as
      path separators and Ant will convert it to the platform's local
      conventions.</td>
  </tr>
  <tr>
    <td valign="top">pathref</td>
    <td valign="top"><a href="#references">Reference</a> to a path
      defined elsewhere.  Ant will convert it to the platform's local
      conventions.</td>
  </tr>
  <tr>
    <td valign="top">line</td>
    <td valign="top">a space-delimited list of command-line arguments.</td>
  </tr>
</table>

<p>It is highly recommended to avoid the <code>line</code> version
when possible.  Ant will try to split the command line in a way
similar to what a (Unix) shell would do, but may create something that
is very different from what you expect under some circumstances.</p>

<h4>Examples</h4>
<blockquote><pre>
  &lt;arg value=&quot;-l -a&quot;/&gt;
</pre></blockquote>
<p>is a single command-line argument containing a space character,
<i>not</i> separate commands "-l" and "-a".</p>
<blockquote><pre>
  &lt;arg line=&quot;-l -a&quot;/&gt;
</pre></blockquote>
<p>This is a command line with two separate arguments, "-l" and "-a".</p>
<blockquote><pre>
  &lt;arg path=&quot;/dir;/dir2:\dir3&quot;/&gt;
</pre></blockquote>
<p>is a single command-line argument with the value
<code>\dir;\dir2;\dir3</code> on DOS-based systems and
<code>/dir:/dir2:/dir3</code> on Unix-like systems.</p>

<h3><a name="references">References</a></h3>

<p>Any project element can be assigned an identifier using its
<code>id</code> attribute. In most cases the element can subsequently
be referenced by specifying the <code>refid</code> attribute on an
element of the same type.  This can be useful if you are going to
replicate the same snippet of XML over and over again--using a
<code>&lt;classpath&gt;</code> structure more than once, for example.</p>
<p>The following example:</p>
<blockquote><pre>
&lt;project ... &gt;
  &lt;target ... &gt;
    &lt;rmic ...&gt;
      &lt;classpath&gt;
        &lt;pathelement location=&quot;lib/&quot;/&gt;
        &lt;pathelement path=&quot;${java.class.path}/&quot;/&gt;
        &lt;pathelement path=&quot;${additional.path}&quot;/&gt;
      &lt;/classpath&gt;
    &lt;/rmic&gt;
  &lt;/target&gt;

  &lt;target ... &gt;
    &lt;javac ...&gt;
      &lt;classpath&gt;
        &lt;pathelement location=&quot;lib/&quot;/&gt;
        &lt;pathelement path=&quot;${java.class.path}/&quot;/&gt;
        &lt;pathelement path=&quot;${additional.path}&quot;/&gt;
      &lt;/classpath&gt;
    &lt;/javac&gt;
  &lt;/target&gt;
&lt;/project&gt;
</pre></blockquote>
<p>could be rewritten as:</p>
<blockquote><pre>
&lt;project ... &gt;
  &lt;path id=&quot;project.class.path&quot;&gt;
    &lt;pathelement location=&quot;lib/&quot;/&gt;
    &lt;pathelement path=&quot;${java.class.path}/&quot;/&gt;
    &lt;pathelement path=&quot;${additional.path}&quot;/&gt;
  &lt;/path&gt;

  &lt;target ... &gt;
    &lt;rmic ...&gt;
      &lt;classpath refid=&quot;project.class.path&quot;/&gt;
    &lt;/rmic&gt;
  &lt;/target&gt;

  &lt;target ... &gt;
    &lt;javac ...&gt;
      &lt;classpath refid=&quot;project.class.path&quot;/&gt;
    &lt;/javac&gt;
  &lt;/target&gt;
&lt;/project&gt;
</pre></blockquote>
<p>All tasks that use nested elements for 
<a href="CoreTypes/patternset.html">PatternSet</a>s, 
<a href="CoreTypes/fileset.html">FileSet</a>s, 
<a href="CoreTypes/zipfileset.html">ZipFileSet</a>s or 
<a href="#path">path-like structures</a> accept references to these structures 
as shown in the examples. Using <code>refid</code> on a task will ordinarily 
have the same effect (referencing a task already declared), but the user 
should be aware that the interpretation of this attribute is dependent on the 
implementation of the element upon which it is specified. Some tasks (the 
<a href="CoreTasks/property.html">property</a> task is a handy example) 
deliberately assign a different meaning to <code>refid</code>.</p>


<h3><a name="external-tasks">Use of external tasks</a></h3> 
Ant supports a plugin mechanism for using third party tasks. For using them you 
have to do two steps: 
<ol>
  <li>place their implementation somewhere where Ant can find them</li>
  <li>declare them.</li>
</ol> 
Don't add anything to the CLASSPATH environment variable - this is often the  
reason for very obscure errors. Use Ant's own <a href="install.html#optionalTasks">mechanisms</a> 
for adding libraries: 
<ul>
  <li>via command line argument <code>-lib</code></li>
  <li>adding to <code>${user.home}/.ant/lib</code></li>
  <li>adding to <code>${ant.home}/lib</code></li>
</ul>
For the declaration there are several ways:
<ul>
  <li>declare a single task per using instruction using  
      <code>&lt;<a href="CoreTasks/taskdef.html">taskdef</a> name=&quot;taskname&quot; 
      classname=&quot;ImplementationClass&quot;/&gt;</code>
      <br>
      <code>&lt;taskdef name=&quot;for&quot; classname=&quot;net.sf.antcontrib.logic.For&quot; /&gt; 
      &lt;for ... /&gt;</code>
  </li>
  <li>declare a bundle of tasks using a properties-file holding these 
      taskname-ImplementationClass-pairs and <code>&lt;taskdef&gt;</code>
      <br>
      <code>&lt;taskdef resource=&quot;net/sf/antcontrib/antcontrib.properties&quot; /&gt; 
      &lt;for ... /&gt;</code>
  </li>
  <li>declare a bundle of tasks using a <a href="CoreTypes/antlib.html">xml-file</a> holding these  
      taskname-ImplementationClass-pairs and <code>&lt;taskdef&gt;</code>
      <br>
      <code>&lt;taskdef resource=&quot;net/sf/antcontrib/antlib.xml&quot; /&gt; 
      &lt;for ... /&gt;</code>
  </li>
  <li>declare a bundle of tasks using a xml-file named antlib.xml, XML-namespace and 
      <a href="CoreTypes/antlib.html#antlibnamespace"><code>antlib:</code> protocoll handler</a> 
      <br>
      <code>&lt;project xmlns:ac=&quot;antlib:net.sf.antconrib&quot;/&gt; 
      &lt;ac:for ... /&gt;</code>
  </li>
</ul>

If you need a special function, you should
<ol>
  <li>have a look at this manual, because Ant provides lot of tasks</li>
  <li>have a look at the external task page in the <a href="../external.html">manual</a>
      (or better <a href="http://ant.apache.org/external.html">online</a>)</li>
  <li>have a look at the external task <a href="http://wiki.apache.org/ant/AntExternalTaskdefs">wiki 
      page</a></li>
  <li>ask on the <a href="http://ant.apache.org/mail.html#User%20List">Ant user</a> list</li>
  <li><a href="tutorial-writing-tasks.html">implement </a>(and share) your own</li>
</ol>

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