This link has been bookmarked by 430 people . It was first bookmarked on 05 Jul 2006, by Jeff Schilling.
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The key difference is that with a Service Locator every user of a service has a dependency to the locator. The locator can hide dependencies to other implementations, but you do need to see the locator.
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Using dependency injection can help make it easier to see what the component dependencies are. With dependency injector you can just look at the injection mechanism, such as the constructor, and see the dependencies
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A lot of this depends on the nature of the user of the service. If you are building an application with various classes that use a service, then a dependency from the application classes to the locator isn't a big deal.
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Interface injection is more invasive since you have to write a lot of interfaces to get things all sorted out.
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Another advantage with constructor initialization is that it allows you to clearly hide any fields that are immutable by simply not providing a setter. I think this is important
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But with any situation there are exceptions. If you have a lot of constructor parameters things can look messy, particularly in languages without keyword parameters.
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17 Oct 15
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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.
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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.)
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The question is: "what aspect of control are they inverting?" When I first ran into inversion of control, it was in the main control of a user interface. Early user interfaces were controlled by the application program. You would have a sequence of commands like "Enter name", "enter address"; your program would drive the prompts and pick up a response to each one. With graphical (or even screen based) UIs the UI framework would contain this main loop and your program instead provided event handlers for the various fields on the screen. The main control of the program was inverted, moved away from you to the framework.
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It tends to be hard to understand and leads to problems when you are trying to debug. So on the whole I prefer to avoid it unless I need it. This isn't to say it's a bad thing, just that I think it needs to justify itself over the more straightforward alternative.
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So all the method does is refer to a finder, and all that finder does is know how to respond to the
findAllmethod. -
Figure 1 shows the dependencies for this situation
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For this new breed of containers the inversion is about how they lookup a plugin implementation. In my naive example the lister looked up the finder implementation by directly instantiating it. This stops the finder from being a plugin. The approach that these containers use is to ensure that any user of a plugin follows some convention that allows a separate assembler module to inject the implementation into the lister.
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My long running default with objects is as much as possible, to create valid objects at construction time.
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The basic idea of the Dependency Injection is to have a separate object, an assembler, that populates a field in the lister class with an appropriate implementation for the finder interface, resulting in a dependency diagram along the lines of Figure 2
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Dependency Injection is to have a separate object, an assembler, that populates a field in the lister class with an appropriate implementation for the finder interface,
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The names I'm using for them are Constructor Injection, Setter Injection, and Interface Injection.
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finder = new ColonDelimitedMovieFinder("movies1.txt"); }
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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
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Inversion of control is a common characteristic of frameworks, so saying that these lightweight containers are special because they use inversion of control is like saying my car is special because it has wheels.
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In my naive example the lister looked up the finder implementation by directly instantiating it. This stops the finder from being a plugin. The approach that these containers use is to ensure that any user of a plugin follows some convention that allows a separate assembler module to inject the implementation into the lister.
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There are three main styles of dependency injection. The names I'm using for them are Constructor Injection, Setter Injection, and Interface Injection.
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To help make all of this more concrete I'll use a running example to talk about all of this. Like all of my examp
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Golam Kibriamartinfowler
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The basic idea of the Dependency Injection is to have a separate object, an assembler, that populates a field in the lister class with an appropriate implementation for the finder interface
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The key benefit of a Dependency Injector is that it removes the dependency that the
MovieListerclass has on the concreteMovieFinderimplementation. -
The basic idea behind a service locator is to have an object that knows how to get hold of all of the services that an application might need.
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Inversion of control is a common feature of frameworks, but it's something that comes at a price. It tends to be hard to understand and leads to problems when you are trying to debug. So on the whole I prefer to avoid it unless I need it. This isn't to say it's a bad thing, just that I think it needs to justify itself over the more straightforward alternative.
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With dependency injector you can just look at the injection mechanism, such as the constructor, and see the dependencies. With the service locator you have to search the source code for calls to the locator.
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Another advantage with constructor initialization is that it allows you to clearly hide any fields that are immutable by simply not providing a setter. I think this is important - if something shouldn't change then the lack of a setter communicates this very well. If you use setters for initialization, then this can become a pain.
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It's often said that configuration files shouldn't use a programing language because they need to be edited by non-programmers. But how often is this the case? Do people really expect non-programmers to alter the transaction isolation levels of a complex server-side application?
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Sachin RawatIn the Java community there's been a rush of lightweight containers that help to assemble components from different projects into a cohesive application. Underlying these containers is a common pattern to how they perform the wiring, a concept they refer under the very generic name of "Inversion of Control". In this article I dig into how this pattern works, under the more specific name of "Dependency Injection", and contrast it with the Service Locator alternative. The choice between them is less important than the principle of separating configuration from use.
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I want my wonderful
moviesDirectedBymethod to be completely independent of how all the movies are being stored -
how we connect the lister object with a particular finder object
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I want my wonderful
moviesDirectedBymethod to be completely independent of how all the movies are being stored. -
come up with a concrete class to actually come up with the movies
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need to have some way to get an instance of the right finder implementation into place.
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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.
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The main control of the program was inverted, moved away from you to the framework.
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how they lookup a plugin implementation
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some convention
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a separate assembler module to inject the implementation into the lister.
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Dependency Injection.
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an assembler
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Setter Injection
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Interface Injection
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Constructor Injection
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Constructor Injection
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which implementation class to associate with each interface,
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MovieFinder.class, ColonMovieFinder.class
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pico.getComponentInstance(MovieLister.class);
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Setter Injection
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public void setFinder(MovieFinder finder) { this.finder = finder; }
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configuration
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ApplicationContext ctx = new FileSystemXmlApplicationContext("spring.xml"); MovieLister lister = (MovieLister) ctx.getBean("MovieLister");
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Interface Injection
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defining an interface that I'll use to perform the injection through
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This interface would be defined by whoever provides the MovieFinder interface
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MovieLister implements InjectFinder...
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configuration
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registerComponents
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registerInjectors
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registerComponents
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registerInjectors
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Injection isn't the only way to break this dependency, another is to use a service locator.
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to have an object that knows how to get hold of all of the services that an application might need
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ServiceLocator
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finder = ServiceLocator.movieFinder();
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we have to configure the service locator
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ServiceLocator
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how that implementation is provided to the application class
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service locator the application class asks for it explicitly by a message to the locator
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hence the inversion of control
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With injection there is no explicit request,
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I prefer to avoid it unless I need it.
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with a Service Locator every user of a service has a dependency to the locator.
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Using dependency injection can help make it easier to see what the component dependencies are. With dependency injector you can just look at the injection mechanism, such as the constructor, and see the dependencies. With the service locator you have to search the source code for calls to the locator.
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Fabio SalzanoInversion of Control Containers and the Dependency Injection pattern
inversion control containers dependency injection patterns oop progrmmaing ioc di
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