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                      <meta name="author" value="Peter Donald">
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      <title>Apache Myrmidon - On Librarys in Ant2</title>
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          <td valign="bottom"><div align="right"><b><font size="+3" color="#525D76">Apache Myrmidon</font></b></div></td>
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                <p><strong>Myrmidon</strong></p>
    <ul>
          <li>      <a href="./index.html">Introduction</a>
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          <li>      <a href="./getinvolved.html">Get Involved</a>
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          <li>      <a href="./user.html">Building Myrmidon</a>
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          <li>      <a href="./todo.html">Todo List</a>
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      <p><strong>User Guide</strong></p>
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          <li>      <a href="./buildfile.html">Build file</a>
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          <li>      <a href="./vfs.html">Virtual File System</a>
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          <li>      <a href="./ant1compat.html">Ant1 Compatibility Layer</a>
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          <li>      <a href="./differences.html">Differences to Ant1</a>
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      <p><strong>Extending Ant</strong></p>
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          <li>      <a href="./task.html">Task Writers HOWTO</a>
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          <li>      <a href="./classloader.html">ClassLoader HOWTO</a>
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          <li>      <a href="./librarys.html">Library HOWTO</a>
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      <p><strong>Container Design</strong></p>
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        <a name="Library Management"><strong>Library Management</strong></a>
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      <blockquote>
                        <p>Long ago there was identified the need for librarys that contain
tasks and other elements present in the build file. This document
attempts to describe the mechanism via which these libraries will be
defined and used in Ant2. The librarys (also referred to as
deployments) will be termed antlibs.</p>
                                <p>Ant librarys can be packaged and signed into a ANt Type Library
format (.atl) using the standard Java Archive tools. (For details on
the .jar file format see the
<a href="http://java.sun.com/j2se/1.3/docs/guide/jar/index.html">
Jar Specification</a>.</p>
                                <p>When packaged into such a form the META-INF/ directory contains
ant specific descriptors in addition to the standard Jar manifest
and other descriptor files. The archive will also contain the
<code>.class</code> files for all the tasks and other types the
library defines. It may also contain additional resources that can
be referenced in the build file (an example being DTDs).</p>
                                <p>The library may also need access to other librarys or resources
to perform its job. For instance, if the task loaded an XML document
and then processed said document using the <em>Trax API</em> then
the Ant library needs to have access to the <em>Trax API</em> and an
implementation of the API. The Antlib mechanism thus uses the standard
"Optional Package" Specification to declare dependencies on other
libraries.</p>
                                <p>The libraries will usually be installed in standard locations that
make it possible for the Ant container to automatically locate and scan
the libraries. It will also be possible for the users to specify
additional search locations or even the specific location of ant
libraries.</p>
                                <p>The following sections will describe each of these different facets
in greater detail.</p>
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        <a name="Descriptors"><strong>Descriptors</strong></a>
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      <blockquote>
                        <p>FIXME: Import this part from other doco</p>
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        <a name="Class and Resource Files"><strong>Class and Resource Files</strong></a>
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      <blockquote>
                        <p>The class and resources files should be stored as in standard jars. The
root directory being the base via which code and resources are loaded. So
the <code>.class</code> file for the Java class <code>com.biz.tasks.Mytask</code>
would be stored in <code>/com/biz/tasks/Mytask.class</code></p>
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        <a name="Dependencies"><strong>Dependencies</strong></a>
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      <blockquote>
                        <p>It is often the case that a library will need external resources. The
example given above described dependence on an external XML library. The
ant library thus needs a mechanism via which to declare dependencies on
external libraries.</p>
                                <p>Ant2 uses the "Optional Package" mechanism. Prior to JDK1.3, an "Optional
Package" was known as an <em>Extension</em>. The specification for this
mechanism is available in the JDK1.3 documentation in the directory
<code>$JDK_HOME/docs/guide/extensions/versioning.html</code>. Alternatively
it is available online at
<a href="http://java.sun.com/j2se/1.3/docs/guide/extensions/versioning.html">
http://java.sun.com/j2se/1.3/docs/guide/extensions/versioning.html</a>.</p>
                                <p>This mechanism was adopted as it is an established standard. The standard
is also begining to be adopted by other specifications such as the <em>Servlet
2.3 API</em>. Thus we are likely to see an increase of jars using this mechanism
to specify dependencies.</p>
                                <p>The "Optional Package" mechanism allows jars to specify dependencies on other
jars that implement a particular specification at particular version levels. For
example you could specify a dependency on the Trax 1.1 API by adding the following
to the manifest of your jar.</p>
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Extension-List: trax
trax-Extension-Name: Java API for XML Parsing
trax-Specification-Version: 1.1
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                                <p>In some cases you may also wish to specify a dependency on a specific vendors
implementation. For instance you may need to use xalan due to it implementing a
particular extension you need. In that case you manifest may contain;</p>
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Extension-List: trax
trax-Extension-Name: Java API for XML Parsing
trax-Specification-Version: 1.1
trax-Implementation-Title: org.apache.xalan.xslt
trax-Implementation-Version: 2.1.0
trax-Implementation-Vendor: Apache Software Foundation
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                                <p>In many cases there will be no distinction between the specification and
the implementation of a library. For instance the Velocity project only has
one implementation and one specification. In which case it is sufficient to
just declare a dependency on the Velocity "Specification". A library that uses
both the Trax API and the Velocity project may look like;</p>
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Extension-List: trax velocity
velocity-Extension-Name: org.apache.velocity
velocity-Specification-Version: 1.0
trax-Extension-Name: Java API for XML Parsing
trax-Specification-Version: 1.1
trax-Implementation-Title: org.apache.xalan.xslt
trax-Implementation-Version: 2.1.0
trax-Implementation-Vendor: Apache Software Foundation
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                                <p>To make other jars available to Ant librarys as "Optional Packages"
or Extensions then you need to add a few lines to the manifest of the
other jar. The minimal manifest is the following;</p>
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Extension-Name: org.realityforge.dve
Specification-Vendor: Peter Donald
Specification-Version: 1.0
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                                <p>It is important to note that looking for dependencies is recursive. For example,
if the ant library depends upon jar A and and A depends on B then both A and B will
need to be loaded by the container.</p>
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        <a name="Implementation Notes"><strong>Implementation Notes</strong></a>
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      <blockquote>
                        <p>So far there has been no mention of implementation strategies for
managing ClassLoaders and other details about where the "Optional Packages"
are stored. This section will outline such details but they could change
in the future. The above specification will still be accurate but the approach
to implementing specification will be different.</p>
                                <p>In the current architecture all of the "Optional Packages" are assumed to
be stored in the <code>$ANT_HOME/ext</code> directory. The runtime will scan
this directory for jars and add all the "optional Packages" found into a
registry. This registry will be used by the library loading mechanism to locate
all the "Optional Packages". The user is able to specify an alternative directory
or add a new directory to search on the commandline.</p>
                                <p>When the container attempts to load an ant library it will also try to load
any needed dependencies. First it will check the parent ClassLoaders to see if any
of them contain the required dependencies. If not then it will search the
"Optional Packages" registry. If the dependency is not found then a error will be
signaled. If the dependency is found in the "Optional Packages" registry then it is
loaded by the same ClassLoader that is used to load the Ant library.</p>
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