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196 lines
8.1 KiB
HTML
196 lines
8.1 KiB
HTML
<html>
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<meta http-equiv="Content-Language" content="en-us">
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<title>Depend Task</title>
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</head>
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<body>
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<h2>Depend</h2>
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A task to manage Java class file dependencies.
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<h3>Description</h3>
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<p>
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The depend task works by determining which classes are out of date with
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respect to their source and then removing the class files of any other
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classes which depend on the out-of-date classes.
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</p>
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<p> To determine the class dependencies, the depend task analyses the class
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files of all class files passed to it. Depend does not parse your source code in
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any way but relies upon the class references encoded into the class files by the
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compiler. This is generally faster than parsing the Java source.</p>
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<p>
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To learn more about how this information is obtained from the class files,
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please refer to <a href="http://java.sun.com/docs/books/vmspec/">the Java
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Virtual Machine Specification</a>
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</p>
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<p> Since a class' dependencies only change when the class itself changes, the
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depend task is able to cache dependency information. Only those class files
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which have changed will have their dependency information re-analysed. Note that
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if you change a class' dependencies by changing the source, it will be
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recompiled anyway. You can examine the dependency files created to understand
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the dependencies of your classes. Please do not rely, however, on the format of
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the information, as it may change in a later release. </p>
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<p> Once depend discovers all of the class dependencies, it "inverts"
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this relation to determine, for each class, which other classes are dependent
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upon it. This "affects" list is used to discover which classes are
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invalidated by the out of date class. The class files of the invalidated
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classes are removed, triggering the compilation of the affected classes. </p>
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<p> The depend task supports an attribute, "closure" which controls
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whether depend will only consider direct class-class relationships or whether it
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will also consider transitive, indirect relationships. For example, say there
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are three classes, A, which depends on B, which in-turn depend on C. Now say
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that class C is out of date. Without closure, only class B would be removed by
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depend. With closure set, class A would also be removed. Normally direct
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relationships are sufficient - it is unusual for a class to depend on another
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without having a direct relationship. With closure set, you will notice that
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depend typically removes far more class files. </p>
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<p>The classpath attribute for <code><depend></code> is optional. If it is present,
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depend will check class dependencies against classes and jars on this classpath.
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Any classes which depend on an element from this classpath and which are older
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than that element will be deleted. A typical example where you would use this
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facility would be where you are building a utility jar and want to make sure
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classes which are out of date with respect to this jar are rebuilt. You should
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<b>not</b> include jars in this classpath which you do not expect to change,
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such as the JDK runtime jar or third party jars, since doing so will just slow
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down the dependency check. This means that if you do use a classpath for the
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depend task it may be different from the classpath necessary to actually
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compile your code.</p>
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<h3>Performance</h3>
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<p> The performance of the depend task is dependent on a
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number of factors such as class relationship complexity and how many class files
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are out of date. The decision about whether it is cheaper to just recompile all
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classes or to use the depend task will depend on the size of your project and
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how interrelated your classes are. </p>
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<h3>Limitations</h3>
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<p> There are some source dependencies which depend will not detect. </p>
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<ul>
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<li>If the Java compiler optimizes away a class relationship,
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there can be a source dependency without a class dependency. </li>
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<li>Non public classes cause two problems. Firstly depend cannot relate
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the class file to a source file. In the future this may be addressed
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using the source file attribute in the classfile. Secondly, neither
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depend nor the compiler tasks can detect when a non public class is
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missing. Inner classes are handled by the depend task.</li>
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</ul>
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The most obvious example of these limitations is that the task can't tell
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which classes to recompile when a constant primitive data type exported
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by other classes is changed. For example, a change in the definition of
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something like
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<pre>
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public final class Constants {
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public final static boolean DEBUG=false;
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}
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</pre> will not be picked up by other classes.
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<h3>Parameters</h3>
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<table border="1" cellpadding="2" cellspacing="0">
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<tr>
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<td valign="top"><b>Attribute</b></td>
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<td valign="top"><b>Description</b></td>
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<td align="center" valign="top"><b>Required</b></td>
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</tr>
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<tr>
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<td valign="top">srcDir</td>
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<td valign="top">This is the directory where the source exists. depend
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will examine this to determine which classes are out of date. If you use multiple
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source directories you can pass this attribute a path of source directories.</td>
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<td valign="top" align="center">Yes</td>
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</tr>
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<tr>
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<td valign="top">destDir</td>
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<td valign="top">This is the root directory of the class files which
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will be analysed. If this is not present, the srcdir is used.</td>
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<td valign="top" align="center">No</td>
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</tr>
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<tr>
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<td valign="top">cache</td>
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<td valign="top">This is a directory in which depend can store and
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retrieve dependency information. If this is not present, depend will not
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use a cache </td>
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<td valign="top" align="center">No</td>
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</tr>
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<tr>
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<td valign="top">closure</td>
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<td valign="top">This attribute controls whether depend only removes
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classes which directly depend on out of date classes. If this is set to true,
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depend will traverse the class dependency graph deleting all affected
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classes. Defaults to false</td>
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<td valign="top" align="center">No</td>
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</tr>
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<tr>
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<td valign="top">dump</td>
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<td valign="top">If true the dependency information will be written to the debug level log
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</td>
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<td valign="top" align="center">No</td>
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</tr>
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<tr>
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<td valign="top">classpath</td>
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<td valign="top">The classpath containg jars and classes for which <code><depend></code> should also
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check dependencies</td>
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<td valign="top" align="center">No</td>
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</tr>
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</table>
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<h3>Parameters specified as nested elements</h3>
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<p>The <code>depend</code> task's <code>classpath</code> attribute is a
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<a href="../using.html#path">PATH-like structure</a> and can also be set
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via a nested <code><classpath></code> element.</p>
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<p>Additionally,
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this task forms an implicit
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<a href="../CoreTypes/fileset.html">FileSet</a>
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and supports all attributes of
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<code><fileset></code> (<code>dir</code> becomes <code>srcdir</code>),
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as well as the nested <code><include></code>,
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<code><exclude></code>, and <code><patternset></code> elements.
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<h3>Examples</h3>
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<pre><depend srcdir="${java.dir}"
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destdir="${build.classes}"
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cache="depcache"
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closure="yes"/></pre>
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<p>removes any classes in the <code>${build.classes}</code> directory
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that depend on out-of-date classes. Classes are considered out-of-date with
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respect to the source in the <code>${java.dir}</code> directory, using the same
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mechanism as the <code><javac></code> task. In this example, the
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<code><depend></code> task caches its dependency
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information in the <code>depcache</code> directory. </p>
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<pre>
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<depend srcdir="${java.dir}" destdir="${build.classes}"
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cache="depcache" closure="yes">
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<include name="**/*.java"/>
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<excludesfile name="${java.dir}/build_excludes"/>
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</depend>
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</pre>
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<p>does the same as the previous example, but explicitly includes all
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<code>.java</code> files, except those that match the list given
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in <code>${java.dir}/build_excludes</code>.</p>
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<hr>
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<p align="center">Copyright © 2001-2002 Apache Software Foundation.
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All rights Reserved.</p>
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</body>
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</html>
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