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An example of an EJB bean being a POJO: Enterprise JavaBeans (EJB), Java Persistence API (JPA) (including Hibernate) CDI (Contexts and Dependency Injection for the Java EE platform) The following shows a fully functional EJB bean, demonstrating how EJB3 leverages the POJO model:
(Dependency injection is an example of the separate, specific idea of "inverting control over the implementations of dependencies" popularised by Java frameworks.) [4] Inversion of control is sometimes referred to as the "Hollywood Principle: Don't call us, we'll call you".
In object-oriented design, the dependency inversion principle is a specific methodology for loosely coupled software modules.When following this principle, the conventional dependency relationships established from high-level, policy-setting modules to low-level, dependency modules are reversed, thus rendering high-level modules independent of the low-level module implementation details.
Software dependencies can either be explicit or implicit. Examples of explicit dependencies includes: Include statements, such as #include in C/C++, using in C# and import in Java. Dependencies stated in the build system (e.g. dependency tags in Maven configuration). Examples of implicit dependencies includes: [3]
With interface injection, dependencies are completely ignorant of their clients, yet still send and receive references to new clients. In this way, the dependencies become injectors. The key is that the injecting method is provided through an interface. An assembler is still needed to introduce the client and its dependencies.
The dependency cannot be substituted, or its "signature" changed, without requiring a change to the dependent class. Loose coupling occurs when the dependent class contains a pointer only to an interface, which can then be implemented by one or many concrete classes. This is known as dependency inversion. The dependent class's dependency is to ...
This PHP example shows interface implementations instead of subclassing (however, the same can be achieved through subclassing). The factory method can also be defined as public and called directly by the client code (in contrast to the previous Java example).
In Java, the multiton pattern can be implemented using an enumerated type, with the values of the type corresponding to the instances. In the case of an enumerated type with a single value, this gives the singleton pattern. In C#, we can also use enums, as the following example shows: