cwalltx 's Library tagged osgi → View Popular

You could think of
the registry as being just like a pair of HashMaps, that allow
Eclipse to lookup either extensions or extension points by their ID
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- The registry API provides the plug-in
  with a snapshot of all the currently installed extensions for the ID
  that was queried, as an array of data structures that closely resemble
  the actual XML nodes given by the extension.
- The key thing to notice here
  is that no Java classes in the extension plug-in have yet been loaded.
  We have merely loaded some XML data; this takes far less time and
  memory than creating a ClassLoader and using it to load code
- up to the definer of the extension point to decide when the
  code in the extensions is loaded.
- The points to take away from this are:
  
  extensions are declarations
  plug-ins consult extensions to create placeholders for functionality
  the functionality behind the placeholder is realized when the extension
  point definer chooses, which enables lazy loading and smaller memory usage
  extensions need not be associated with executable Java code
- To register a service, a bundle
  must first create an object, and then it must make a call to the OSGi API
  to register the object as a service.
- ServiceTracker also makes other types of interaction with
  services easier, such as keeping track of when the service is registered
  and unregistered
- When the bundle is started, the
  runtime calls the start() method of our BundleActivator and
  supplies a BundleContext instance. Before our bundle can be started,
  it must be loaded. Therefore, until the bundle is loaded, there can be
  no services registered.
- the API for working with
  services is inherently dynamic.
- The extension model is essentially “one-to-many”,
  meaning that an extension is explicitly intended for consumption by one
  particular plug-in; i.e. the definer of the extension point. On the other
  hand services use a “many-to-many” model, meaning that a single service
  can have many consumers.
- however the responsibility for
  registering services is subsumed by a special bundle called the Service
  Component Runtime or SCR
- A bundle containing services that it wants the SCR
  to manage on its behalf contains one or more XML files, with each file
  representing a “Service Component”
- To support lazy
  registration, Declarative Services has the notion of “delayed” services. When the SCR
  registers a delayed service, it creates a proxy object to act as a
  placeholder, and registers that in the service registry. From the point of
  view of a consumer, the service is now available for use, however the
  bundle containing the real service implementation has not been started yet.
  Then when a consumer tries to actually use the service, the SCR detects
  this, and asks the OSGi runtime to fully load and activate the bundle. It
  then subtitutes the proxy for the real service. A key difference between
  extensions and Declarative Services is that, for the consumer, DS implements laziness
  transparently, i.e. the code that uses a service need not know or care that
  the service is temporarily only represented as a placeholder. In contrast,
  the consumer of an extension must create and manage the placeholder
  explicitly, and choose when to swap the placeholder for the real
  implementation.
- In the XML file for service B, we
  simply create a reference to service A – then SCR will wire up the
  dependency whenever it can be satisfied.
- dependency is
  optional or mandatory
- These are all valid questions, and the answer in each case is that it
  depends on our requirements. Therefore DS provides control in the three
  dimensions involved:
  
  Optional vs mandatory.
  Unary vs multiple.
  Static vs dynamic.
  
  Combining the first two axes gives the familar four choices of 0..1
  (optional + unary), 1..1 (mandatory + unary), 1..n (mandatory +
  multiple) and 0..n (optional + multiple). The third axis is less
  familiar. A dependency is dynamic if it can be hot-swapped, i.e. if
  service B can be supplied subtitute instances of A. Sometime this isn’t
  possible; for example A might be stateful, or B may simply not be able
  to handle substitution because of the way it was programmed. We say
  that the dependency is static – we must destroy B and create a new
  instance of it.

28 Mar 07

Spring and BEA an overview

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