I am not of the expert group, but I would like to give a partial answer.
Let us think of a kind of plugin architecture. There is a public
interface from your framework and there are other modules that are going
to provide plugins by implementing that interface. Now assume the
framework will also instantiate the plugin. This is a problem, because
normally a plugin is an implementation detail and its only public aspect
is supposed to be the interface it implements. But to create an instance
of it, you need to execute a constructor, which would then originate in
private API.
Thus the ServiceLoader API and the module logic and descriptor got
extended to support this.
bye Jochen
On 29.11.2016 14:13, Pisarev, Vitaliy wrote:
What about the intent behind how are developers supposed to use this tool?
For example, when Optional was introduced in Java 8, it was specified that it
is intended to be used as a methods return type. And people should not
use it as class member or method argument, even though the compiler will not
prevent them to.
Here is a language feature that comes with guidelines as to how it is intended
to be used.
Can you tell me how the expert group envisions usage of the ServiceLoader in
Java 9, with reference to the original question (and any other thing you can
add with regard to this).
-----Original Message-----
From: Alan Bateman [mailto:alan.bate...@oracle.com]
Sent: יום ג 29 נובמבר 2016 14:06
To: Pisarev, Vitaliy <vitaliy.pisa...@hpe.com>; jigsaw-dev@openjdk.java.net
Subject: Re: Services and Bindings - expected usage scenarios
On 29/11/2016 11:35, Pisarev, Vitaliy wrote:
Another best practices question.
I am aware that the ServiceLoader API is not new to Java. What's new
in Java 9 is that is has been put forward to the front of the stage and it is
now very easy for a service provider to register a service.
The thing is that up until now, the ServiceLoader was a relatively low level
component, primarily used by infrastructure components that loaded plugins that
implemented various java specs.
It is no surprise than that the ServiceLoader is something that very few ever
heard of.
Is it the intent of project Jigsaw that the service loader becomes a central
and ubiquitous mechanism in the day to day work of plain old java developers?
As far as I am concerned, it is a very useful tool even when I do not
have a plugin architecture and just want a module to expose a certain service
via an interface without having to jump throw hooks in order to provide the
consumer a way to instantiate it..
Which leads me to the next question: is the returned instance a singleton or a
prototype?
Services are a first class part of the module system but are a somewhat
advanced topic. The ServiceLoader API is part of the story but arguably just a
small aspect when compared to the architectural and design issues involved in
moving to services, loose coupling, and improved SoC.
Provider factories can implement singletons where needed, the details are in
the javadoc [1].
-Alan
[1] http://download.java.net/java/jigsaw/docs/api/index.html
