[jboss-dev-forums] [Design of POJO Server] - Re: MetaData - replacement for MetaDataRepository

[adrian@jboss.org]

The basic idea of the MetaData implementation is to provide
overrides and additions to the annotations and objects
for a given scope.

There are two identities that are related. The identity
is defined by the scope and how you use it.

The first identity is the context you are working with
and is most likely a mutable metadata, the exception being
when the metadata source is immutable like a class file
(except via aop which is irrelevant for this discussion).

Examples contexts would be a deployment, a server, a cluster,
a subsystem (jca, ejb), a component (an ejb container), a class
or an object instance.
Other examples which I'll come to later are threads and transactions.

For each of these there is a MutableMetaData that lets you
modify just that context. This will update all the full metadata
hierarchies that it is a part of see below.

Example:

  | ScopeKey key = new ScopeKey(CommonLevels.DEPLOYMENT_NAME, "mywar.war");
  | MutableMetaData mutable = // create it
  | mutable.addAnnotation(someAnnotation);
  | mutable.addMetaData("someName", someObject);
  | repository.addMetaData(key, mutable);
  | 

The second form is the hierarchical metadata.
This is made up of the mutables but with a read only view (MetaData).
This is what will actually be used at runtime.
You select the hierarchy using a ScopeKey.

Example:

  | Scope[] scopes = new Scope[]
  | {
  |    new Scope(CommonLevels.INSTANCE_NAME, "MyServlet");
  |    new Scope(CommonLevels.CLASS_NAME, servletClass.getClass().getName());
  |    new Scope(CommonLevels.DEPLOYMENT_NAME, "mywar.war");
  |    new Scope(CommonLevels.APPLICATION_NAME, "myapp.ear");
  |    // etc.
  | }
  | ScopeKey key = new ScopeKey(scopes);
  | MetaData metaData = repository.getMetaData(key);
  | 

Finally, in the later example you will get two extra scopes
(well actually it should depend upon repository configuration).

The first is a thread local metadata object, the second
a transaction metadata object. Both are mutable using
a private api.

You can aop adding a "request scope" as well, but that
it is going to take a while to persuade to Bill and Kabir to do. :-)

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