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<h2>Depend</h2>

A task to manage Java class file dependencies.

<h3>Description</h3>

<p>
The depend task works by determining which classes are out of date with
respect to their source and then removing the class files of any other
classes which depend on the out-of-date classes.
</p>

<p> To determine the class dependencies, the depend task analyzes the class 
files of all class files passed to it. Depend does not parse your source code in 
any way but relies upon the class references encoded into the class files by the 
compiler. This is generally faster than parsing the Java source.</p>

<p>
To learn more about how this information is obtained from the class files, 
please refer to <a href="http://docs.oracle.com/javase/specs/">the Java
Virtual Machine Specification</a>
</p>

<p> Since a class' dependencies only change when the class itself changes, the 
depend task is able to cache dependency information. Only those class files 
which have changed will have their dependency information re-analysed. Note that 
if you change a class' dependencies by changing the source, it will be 
recompiled anyway. You can examine the dependency files created to understand 
the  dependencies of your classes. Please do not rely, however, on the format of 
the information, as it may change in a later release. </p>

<p> Once depend discovers all of the class dependencies, it &quot;inverts&quot; 
this relation to determine, for each class, which other classes are dependent 
upon it. This &quot;affects&quot; list is used to discover which classes are 
invalidated by the out of date class. The class files of the invalidated 
classes are removed, triggering the compilation of the affected classes. </p>

<p> The depend task supports an attribute, &quot;closure&quot; which controls 
whether depend will only consider direct class-class relationships or whether it 
will also consider transitive, indirect relationships. For example, say there 
are three classes, A, which depends on B, which in-turn depend on C. Now say 
that class C is out of date. Without closure, only class B would be removed by 
depend. With closure set, class A would also be removed. Normally direct 
relationships are sufficient - it is unusual for a class to depend on another 
without having a direct relationship. With closure set, you will notice that 
depend typically removes far more class files. </p>

<p>The classpath attribute for <code>&lt;depend&gt;</code> is optional. If it is present, 
depend will check class dependencies against classes and jars on this classpath.
Any classes which depend on an element from this classpath and which are older 
than that element will be deleted. A typical example where you would use this 
facility would be where you are building a utility jar and want to make sure 
classes which are out of date with respect to this jar are rebuilt. You should
<b>not</b> include jars in this classpath which you do not expect to change, 
such as the JDK runtime jar or third party jars, since doing so will just slow 
down the dependency check. This means that if you do use a classpath for the 
depend task it may be different from the classpath necessary to actually 
compile your code.</p>

<h3>Performance</h3> 

<p> The performance of the depend task is dependent on a 
number of factors such as class relationship complexity and how many class files 
are out of date. The decision about whether it is cheaper to just recompile all 
classes or to use the depend task will depend on the size of your project and 
how interrelated your classes are. </p>


<h3>Limitations</h3>

<p> There are some source dependencies which depend will not detect. </p>

<ul>
<li>If the Java compiler optimizes away a class relationship, 
    there can be a source dependency without a class dependency. </li>
    
<li>Non public classes cause two problems. Firstly depend cannot relate
    the class file to a source file. In the future this may be addressed
    using the source file attribute in the classfile. Secondly, neither 
    depend nor the compiler tasks can detect when a non public class is
    missing. Inner classes are handled by the depend task.</li>
</ul>

The most obvious example of these limitations is that the task can't tell
which classes to recompile when a constant primitive data type exported 
by other classes is changed. For example, a change in the definition of
something like
<pre>
public final class Constants {
  public final static boolean DEBUG=false;
}
</pre> will not be picked up by other classes.

<h3>Parameters</h3>
<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">srcDir</td>
    <td valign="top">This is the directory where the source exists. depend
will examine this to determine which classes are out of date. If you use multiple
source directories you can pass this attribute a path of source directories.</td>
    <td valign="top" align="center">Yes</td>
  </tr>
  <tr>
    <td valign="top">destDir</td>
    <td valign="top">This is the root directory of the class files which
will be analysed. If this is not present, the srcdir is used.</td>
    <td valign="top" align="center">No</td>
  </tr>
  <tr>
    <td valign="top">cache</td>
    <td valign="top">This is a directory in which depend can store and
retrieve dependency information. If this is not present, depend will not
use a cache </td>
    <td valign="top" align="center">No</td>
  </tr>
  <tr>
    <td valign="top">closure</td>
    <td valign="top">This attribute controls whether depend only removes
classes which directly depend on out of date classes. If this is set to true,
depend will traverse the class dependency graph deleting all affected
classes. Defaults to false</td>
    <td valign="top" align="center">No</td>
  </tr>
  <tr>
    <td valign="top">dump</td>
    <td valign="top">If true the dependency information will be written to the debug level log
                     </td>
    <td valign="top" align="center">No</td>
  </tr>
  <tr>
    <td valign="top">classpath</td>
    <td valign="top">The classpath containing jars and classes for which <code>&lt;depend&gt;</code> should also
                     check dependencies</td>
    <td valign="top" align="center">No</td>
  </tr>
  <tr>
    <td valign="top">warnOnRmiStubs</td>
    <td valign="top">Flag to disable warnings about files that look like rmic generated stub/skeleton
    classes, and which have no .java source. Useful when doing rmi development. </td>
    <td valign="top" align="center">No, default=true</td>
  </tr>
</table>

<h3>Parameters specified as nested elements</h3>
<p>The <code>depend</code> task's <code>classpath</code> attribute is a 
<a href="../using.html#path">PATH-like structure</a> and can also be set
via a nested <code>&lt;classpath&gt;</code> element.</p>

<p>Additionally,
this task forms an implicit
<a href="../Types/fileset.html">FileSet</a>
and supports most attributes of
<code>&lt;fileset&gt;</code> (<code>dir</code> becomes <code>srcdir</code>),
as well as the nested <code>&lt;include&gt;</code>,
<code>&lt;exclude&gt;</code>, and <code>&lt;patternset&gt;</code> elements.

<h3>Examples</h3>
<pre>&lt;depend srcdir=&quot;${java.dir}&quot;
        destdir=&quot;${build.classes}&quot;
        cache=&quot;depcache&quot;
        closure=&quot;yes&quot;/&gt;</pre>

<p>removes any classes in the <code>${build.classes}</code> directory
that depend on out-of-date classes. Classes are considered out-of-date with 
respect to the source in the <code>${java.dir}</code> directory, using the same
mechanism as the <code>&lt;javac&gt;</code> task. In this example, the
<code>&lt;depend&gt;</code> task caches its dependency 
information in the <code>depcache</code> directory. </p>

<pre>
&lt;depend srcdir=&quot;${java.dir}&quot; destdir=&quot;${build.classes}&quot;
        cache=&quot;depcache&quot; closure=&quot;yes&quot;&gt;
  &lt;include name=&quot;**/*.java&quot;/&gt;
  &lt;excludesfile name=&quot;${java.dir}/build_excludes&quot;/&gt;
&lt;/depend&gt;
</pre>
<p>does the same as the previous example, but explicitly includes all
<code>.java</code> files, except those that match the list given
in <code>${java.dir}/build_excludes</code>.</p>


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