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.
Ant2 uses the "Optional Package" mechanism. Prior to JDK1.3, an "Optional
Package" was known as an
Extension. The specification for this
mechanism is available in the JDK1.3 documentation in the directory
$JDK_HOME/docs/guide/extensions/versioning.html. Alternatively
it is available online at
http://java.sun.com/j2se/1.3/docs/guide/extensions/versioning.html.
This mechanism was adopted as it is an established standard. The standard
is also begining to be adopted by other specifications such as the
Servlet
2.3 API. Thus we are likely to see an increase of jars using this mechanism
to specify dependencies.
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.
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Extension-List: trax
trax-Extension-Name: Java API for XML Parsing
trax-Specification-Version: 1.1
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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;
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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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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;
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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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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;
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Extension-Name: org.realityforge.dve
Specification-Vendor: Peter Donald
Specification-Version: 1.0
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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.
