https://martinfowler.com/articles/injection.html
One of the entertaining things about the enterprise Java world is the huge amount of activity in building alternatives to the mainstream J2EE technologies, much of it happening in open source. A lot of this is a reaction to the heavyweight complexity in the mainstream J2EE world, but much of it is also exploring alternatives and coming up with creative ideas. A common issue to deal with is how to wire together different elements: how do you fit together this web controller architecture with that database interface backing when they were built by different teams with little knowledge of each other. A number of frameworks have taken a stab at this problem, and several are branching out to provide a general capability to assemble components from different layers. These are often referred to as lightweight containers, examples include PicoContainer, and Spring.
Underlying these containers are a number of interesting design principles, things that go beyond both these specific containers and indeed the Java platform. Here I want to start exploring some of these principles. The examples I use are in Java, but like most of my writing the principles are equally applicable to other OO environments, particularly .NET.
Components and Services
The topic of wiring elements together drags me almost immediately into the knotty terminology problems that surround the terms service and component. You find long and contradictory articles on the definition of these things with ease. For my purposes here are my current uses of these overloaded terms.
I use component to mean a glob of software that's intended to be used, without change, by an application that is out of the control of the writers of the component. By 'without change' I mean that the using application doesn't change the source code of the components, although they may alter the component's behavior by extending it in ways allowed by the component writers.
A service is similar to a component in that it's used by foreign applications. The main difference is that I expect a component to be used locally (think jar file, assembly, dll, or a source import). A service will be used remotely through some remote interface, either synchronous or asynchronous (eg web service, messaging system, RPC, or socket.)
I mostly use service in this article, but much of the same logic can be applied to local components too. Indeed often you need some kind of local component framework to easily access a remote service. But writing "component or service" is tiring to read and write, and services are much more fashionable at the moment.
总结：
component是给自己的应用本地调用的
service是给第三方应用调用的
A Naive Example
To help make all of this more concrete I'll use a running example to talk about all of this. Like all of my examples it's one of those super-simple examples; small enough to be unreal, but hopefully enough for you to visualize what's going on without falling into the bog of a real example.
In this example I'm writing a component that provides a list of movies directed by a particular director. This stunningly useful function is implemented by a single method.
class MovieLister...
public Movie[] moviesDirectedBy(String arg) {
List allMovies = finder.findAll();
for (Iterator it = allMovies.iterator(); it.hasNext();) {
Movie movie = (Movie) it.next();
if (!movie.getDirector().equals(arg)) it.remove();
}
return (Movie[]) allMovies.toArray(new Movie[allMovies.size()]);
}
The implementation of this function is naive in the extreme, it asks a finder object (which we'll get to in a moment) to return every film it knows about. Then it just hunts through this list to return those directed by a particular director. This particular piece of naivety I'm not going to fix, since it's just the scaffolding for the real point of this article.
The real point of this article is this finder object, or particularly how we connect the lister object with a particular finder object. The reason why this is interesting is that I want my wonderful moviesDirectedBy method to be completely independent of how all the movies are being stored. So all the method does is refer to a finder, and all that finder does is know how to respond to the findAll method. I can bring this out by defining an interface for the finder.
public interface MovieFinder {
List findAll();
}
Now all of this is very well decoupled, but at some point I have to come up with a concrete class to actually come up with the movies. In this case I put the code for this in the constructor of my lister class.
class MovieLister...
private MovieFinder finder;
public MovieLister() {
finder = new ColonDelimitedMovieFinder("movies1.txt");
}
The name of the implementation class comes from the fact that I'm getting my list from a colon delimited text file. I'll spare you the details, after all the point is just that there's some implementation.
Now if I'm using this class for just myself, this is all fine and dandy. But what happens when my friends are overwhelmed by a desire for this wonderful functionality and would like a copy of my program? If they also store their movie listings in a colon delimited text file called "movies1.txt" then everything is wonderful. If they have a different name for their movies file, then it's easy to put the name of the file in a properties file. But what if they have a completely different form of storing their movie listing: a SQL database, an XML file, a web service, or just another format of text file? In this case we need a different class to grab that data. Now because I've defined a MovieFinder interface, this won't alter my moviesDirectedBy method. But I still need to have some way to get an instance of the right finder implementation into place.
Figure 1: The dependencies using a simple creation in the lister class
关于uml类图关系，上图中2个是dependency。还有一个是implementation
Figure 1 shows the dependencies for this situation. The MovieLister class is dependent on both the MovieFinder interface and upon the implementation. We would prefer it if it were only dependent on the interface, but then how do we make an instance to work with?
In my book P of EAA, we described this situation as a Plugin. The implementation class for the finder isn't linked into the program at compile time, since I don't know what my friends are going to use. Instead we want my lister to work with any implementation, and for that implementation to be plugged in at some later point, out of my hands. The problem is how can I make that link so that my lister class is ignorant of the implementation class, but can still talk to an instance to do its work.
Expanding this into a real system, we might have dozens of such services and components. In each case we can abstract our use of these components by talking to them through an interface (and using an adapter if the component isn't designed with an interface in mind). But if we wish to deploy this system in different ways, we need to use plugins to handle the interaction with these services so we can use different implementations in different deployments.
So the core problem is how do we assemble these plugins into an application? This is one of the main problems that this new breed of lightweight containers face, and universally they all do it using Inversion of Control.