Re: [merlin] getting started

[Stephen McConnell]

Jakob Praher wrote:

hi merlin insiders,

I am just getting my hands dirty on writing a simple merlin block.

Here are some questions:

-> can any block act as root block? 

Any block you care to define or that can be resolved from a target jar 
file, e.g. The following comand loads the <block/> defintion contained 
in block.xml as the root block within the kernel.

 $ merlin block.xml

Another example - the following command loads a jar file and establishes 
a block by locating a block.xml inside the jar at the 
/block-inf/block.xml location:

 $ merlin my-demo.jar

-> if not, what are the minimum requirements...

A minimal block:

<block>
 <implementation>
   <component name="fred"class="MyClass" activation="true"/>
 </implementation>
</block>
-> afaik is that the default block is the one you pass to the merlin-cli
Merlin.class - but:
in fact I don't understand why StandardBlockLoader doesn't throw, when
you don't have a child "engine" under "implementation" in your
block.xml.... .
 

The <engine/> tag simly defines a classloader (in fact the preferred tag 
name is <classloader/>).  Merlin creates a root classloader which is 
used as the parent for the root block.

<snip>

        Configuration classLoaderConfig = implementation.getChild(
"classloader", false );
       if( classLoaderConfig == null )
       {
           classLoaderConfig = implementation.getChild( "engine" );
       }
       /******* comment ::: ***********/
	/* for my block the classLoaderConfig must be null, but this 
          classLoaderConfig is used in createEngine, without null
          checks .. */

The following method returns null (because of the false argument) if a 
<classloader/> element is not found.

  implementation.getChild(
    "classloader", false );
In the case we check for the old fashioned usage of </engine> but because of the method used, the returned configuration will either be (a) the actual engine configuration framgement, or (b) a empty configuration. I.e. this code gurantees that you get a valid classloader defintion.


       EngineClassLoader engine = 
         createEngine( classLoaderConfig, path );

</snip>

-> After startup of a very simplistic block - just one component and a
block meta data, I get a strange Exception with a stack trace like:
<snip>
java.lang.IllegalArgumentException:
commons-cli,excalibur-i18n-Extension-Name
       at java.util.jar.Attributes$Name.<init>(Attributes.java:434)
       at java.util.jar.Attributes.getValue(Attributes.java:97)
This is becuase you have built merlin partially with a non-patched 
version of Maven 9.  Before you applied you patch to the jar plugin you 
seem to have already created some jar files - and the manifest 
declaration in those jar files are corrupted (mevan is delimited 
extension dependecies with a "," instead of " ".  You should delete the 
maven/repository/avalon and maven/repository/merlin directories and your 
merlin installation and re-install.

the funny thing is that I actually don't declared these extension -
where are their declared ? (looked at the kernel*.xinfo but couldn't
find them ).
Maven declared that when you were playing around with the unpactch 
version (check you manifest declarations for extension dependecies - you 
should be able to spot the error).

-> What is the override configuration good for - the -cofnig file passt
to merlin - I assume that it is another block.xml file ? 

Nope. Its a special configuration file containing a set of targets.

<targets>
 <target path="/hello">
   <configuration>
     <source>External configuration target.</source>
   </configuration>
 </target>
</targets>
Take a look at the tutorial in: 
merlin/merlin-platform/tutorials/configuration/override/.

-> stupid one: is there somewhere a more complex block example (perhaps
in the sandbox-cvs - perhaps I overlooked it ;-) )
The merlin-platform/tutorials covers a lot of ground.  There is also the 
james example in merlin-platform/examles/james/conf/block.xml 
(demonstrates multiple nest includes of block - i.e. block.xml includes 
james.xml, james.xml is a block that includes a bunch of cornerstone blocks.

Cheers, Steve.

--

Stephen J. McConnell
mailto:[EMAIL PROTECTED]
http://www.osm.net
Sent via James running under Merlin as an NT service.
http://avalon.apache.org/sandbox/merlin


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