Basically ComponentA depends on ComponentB. To enable unit-testing class ComponentAImpl, the implementation of interface ComponentB must be injected into ComponentAImpl. The Java code below shows the details of how to use a variant of Abstract Factory design pattern to do the dependency injection. |
Inversion of Control (IOC) through injection also known as Injection IOC has not been a designer or programmer friendly pattern. Many question its validity and the validity of IOC in general. IOC seems to be a contradiction to the fundamental concept of object encapsulation. Context IOC is a new approach that attempts to capture Inversion of Control as a pure design pattern to demonstrate that IOC is indeed a very powerful concept. |
It would be important to point out that in the above scenarios, the debate is about the means not the end. Either way you implement things, you still are able to leverage Dependency Injection. Note that in most situations, you have both the choices (you just might need to delve into the API documentation a bit). If the framework documentation suggests XML ? you are quite likely to be able to use Java for wiring up by just figuring out the API to do so. Also if the documentation seems to suggest that the framework you would like to use supports only constructor or setter based dependency injection, dig just a little deeper ? there's a chance you might find support for the other scenario as well. |
In the rest of the article, we'll look briefly at Avalon, and in more depth at the two most popular IoC frameworks, Spring and PicoContainer, and the types of IoC they provide. |
In this article, we will take a look at the problem in more depth and learn how others have tried to solve the problem by utilizing frameworks that implement the Inversion of Control (or IoC) pattern. First, we'll become familiar with some terms, the IoC pattern, and other patterns that have tried to implement a solution (but didn't completely succeed). Then we'll move on, to see how two of the most popular IoC frameworks are used today. These frameworks are PicoContainer and HiveMind. |
Inversion of Control (IoC), or Dependency Injection (DI), is a concept that allows for the removal of responsibilities from the service component, so it doesn't have to worry about obtaining required services or components. In this architecture, such responsibility is moved to the component of the container that is creating or managing our component. In other words, our component never creates or looks up instances of the required subcomponents, but instead it declares what types of dependencies are required for this component. This concept had been implemented in several popular containers, such as PicoContainer and Spring, and these frameworks are capable of creating the whole hierarchy of dependent components. This approach is also a key point in a EJB3 design. |
Enter Listener-Based dependency injection With listener-based dependency injection you can state this relationship explicitly. Listener injection introspects the communication channels on ManagedDirectoryPoller and identifies the addPollManager(PollManager) method as a defining messaging channel (named PollManager). Just as with JavaBeans introspection, you can then register your bean with this channel by providing an appropriate listener. |
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IoC Design Pattern Assume Class A has a relationship with Class B: it wants to use the services of Class B. The usual way to establish this relationship is to instantiate Class B inside Class A. Though this approach works, it creates tight coupling between the classes. You can't easily change Class B without modifying Class A. To eliminate the coupling, you can have a Configurator inject the instance of Class B (Object "b") to the instance of Class A (Object "a"). If you want to change the implementation of Class B later on, you simply change the Configurator object. So, the control of how Object "a" gets the reference of Object "b" is inverted. Object "a" is not responsible for getting the reference to Object "b". Instead, the Configurator is responsible for it. This is the basis for the IoC design pattern. |
Business operations consist of steps or tasks that are performed in a sequence. The sequence of tasks in a typical real-world business workflow is dynamic in nature: the sequence is normally not fixed and depends on factors that can be decided only during execution. This two-article series demonstrates the use of WS-BPEL and IoC in building configurable dynamic business workflows. Part 1 describes the overall architecture of implementing business workflows using WS-BPEL and IoC. Part 2 will demonstrate how you can use WS-BPEL to implement a production-management workflow. |
Inversion of Control (IoC) and Dependency Injection (DI) are patterns that draw a lot of attention (see Resources). They are mostly used within so-called IoC containers, which inject dependencies into a component in the form of other components. However, the patterns don't define the design of these dependency components' methods. In a classic design, value objects or data transfer objects in those methods are used as method parameters and return values when complex objects are required. |
Guice is a dependency injection (DI) framework. I've suggested for years that developers use DI, because it improves maintainability, testability, and flexibility. By watching engineers react to Guice, I've learned that the best way to convince a programmer to adopt a new technology is to make it really easy. Guice makes DI really easy, and as a result, the practice has taken off at Google. I hope to continue in the same vein in this article by making it really easy for you to learn Guice. |
Part 1 of this two-article series presented a two-layer model for analyzing dynamic business workflows and discussed how to implement the layers using IoC and Web Services Business Process Execution Language (WS-BPEL). Here in Part 2, I explain how to express a workflow's business logic using BPEL and show how to deploy IoC and BPEL to control the behavior of your business workflows. |
Krzysztof Cwalina explains the importance of Dependency Injection, how to use it, and why its important. Learn tips and techniques for supplying an external dependency to a software component. |