Date: 2004-04-03T05:26:54
Editor: 203.121.47.163 <>
Wiki: Apache Avalon Wiki
Page: IntroductionToEmbedding
URL: http://wiki.apache.org/avalon/IntroductionToEmbedding
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Change Log:
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@@ -1,22 +1,22 @@
This is a little introduction to embedding Merlin. The descriptions starts
off with an overview of the Avalon Repository bootstrapping system - which at
the end of the day is the core embedding technology used within the merlin
platform.
-'''''Simple Embedding Example'''''
+=== Simple Embedding Example ===
1. creating an initial context
-
+{{{
File cache = new File( "my-cache" );
InitialContext context = new DefaultInitialContext( cache );
-
+}}}
The above code defines a cache directory for the repository system to use when
loading resources needed in your embedded application - and provides the
directory as an argument when creating a new InitialContext object. The
InitialContext is you hook into the repository system and the embedding
machinery.
2. declare what you want to embed
-
+{{{
String spec =
"artifact:avalon-logging/avalon-logkit?version=1.0-SNAPSHOT"
Artifact artifact = Artifact.createArtifact( spec );
-
+}}}
An artifact is a logical reference to a jar file (or other type of resource)
that the repository can access. The avalon-repository system uses artifact
references as the key to locating meta data about embedded classloaders. The
classloader meta data is maintained as a properties file with the .meta
extension. For example the above artifact meta address translates to:
- [host]/avalon-logging/jars/avalon-logkit-impl-1.0-SNAPSHOT.jar.meta
+{{{ [host]/avalon-logging/jars/avalon-logkit-impl-1.0-SNAPSHOT.jar.meta }}}
The content of the meta file is automatically generated using the
avalon-plugin avalon:artifact goal. Two real examples are attached (the logkit
factory and the merlin factory meta).
@@ -29,37 +29,37 @@
3. create the factory
Using the initial context and the artifact you now have everything you need to
create you embedded instance.
-
+{{{
Builder builder = m_context.newBuilder( artifact );
Factory factory = builder.getFactory();
-
+}}}
Behind the scenes the avalon-repository system has gone off, pulled down the
meta data, downloaded and cached all of the classloader entries, constructed
a new classloader, and instantiated the factory.
4. parameterizing the factory
The factory object is the central entry point of the embedded application - it
is responsible for instantiation of the embedded instance based on a supplied
criteria. The initial criteria (the defaults) are established by the factory
in response to the following operation:
-
+{{{
Map criteria = factory.createDefaultCriteria();
-
+}}}
Based on the documentation about the facility your embedding you can update
the criteria using application specific keys. All of the Merlin related
criteria instances use the avalon-util Criteria as the map implementation.
This provides support for key and type validation under the put operations and
type coercion on get operations.
''For example:''
-
+{{{
String key = "avalon.logging.configuration";
File file = new File( "logging.xml" );
criteria.put( key, file );
-
+}}}
Parameterization of the criteria is typically different for each embedding
scenario. A CLI handler will for example adapt to the command line operations
and set criteria values accordingly. An application may set the criteria based
on parameters declared in a web.xml file. Typically the embedding class acts
as the adapter between the embedded context and the factory.
4. embedded instance creation
Creation of the embedded instance is now a simple one line operation:
-
+{{{
Object object = factory.create( criteria );
-
+}}}
The object that is created is the embedded application. In this example its a
logging manager that uses the logkit implementation. However, it could have
been the Merlin kernel. The only difference between the Merlin scenario and
the logging manager scenario is the initial artifact and the actions taken to
parameterize the criteria.
-'''''Examples of Embedding'''''
+=== Examples of Embedding ===
merlin/logging/logkit
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