Software Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...
, Inversion of Control (IoC) is an abstract principle describing an aspect of some software architecture
Software architecture
The software architecture of a system is the set of structures needed to reason about the system, which comprise software elements, relations among them, and properties of both...
In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated....
Procedural programming can sometimes be used as a synonym for imperative programming , but can also refer to a programming paradigm, derived from structured programming, based upon the concept of the procedure call...
In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated....
Business logic, or domain logic, is a non-technical term generally used to describe the functional algorithms that handle information exchange between a database and a user interface.- Scope of business logic :Business logic:...
is controlled by a central piece of code, which calls reusable subroutines that perform specific functions. Using Inversion of Control this "central control" design principle is abandoned. The caller's
Caller
Caller may refer to:* Caller , a party that originates a call* Caller , a person that calls dance figures in round dances and square dances* Caller, the Catalan equivalent of Cagliari...
code deals with the program's execution order, but the business knowledge is encapsulated by the called subroutines.
In practice, Inversion of Control is a style of software construction where reusable generic code controls the execution of problem-specific code. It carries the strong connotation that the reusable code and the problem-specific code are developed independently, which often results in a single integrated application. Inversion of Control as a design guideline serves the following purposes:
There is a decoupling of the execution of a certain task from implementation.
Every system can focus on what it is designed for.
The systems make no assumptions about what other systems do or should do.
In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequences of the use of a drug.Occasionally, drugs are...
on other systems.
Inversion of Control is sometimes facetiously referred to as the "Hollywood Principle
Hollywood Principle
In computer programming, the Hollywood principle is stated as "don't call us, we'll call you." It has applications in software engineering; see also implicit invocation for a related architectural principle.-Overview:...
: Don't call us, we'll call you", because implementations typically rely on callbacks.
Background
Inversion of Control is not a new term in computer science. According to Martin Fowler
Martin Fowler
-Online presentations:* at RailsConf 2006* at JAOO 2006* at QCon London 2007 * at QCon London 2008 * at ThoughtWorks Quarterly Technology Briefing, October 2008...
the etymology of the phrase dates back to 1988. In another article he also discusses the kind of pattern many advocates actually mean when talking about Inversion of Control and that Dependency Injection
Dependency injection
Dependency injection is a design pattern in object-oriented computer programming whose purpose is to improve testability of, and simplify deployment of components in very large software systems....
is a more specific term for it .
Nonetheless, there is differing opinion on whether Inversion of Control is a design pattern
Design pattern (computer science)
In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...
The software architecture of a system is the set of structures needed to reason about the system, which comprise software elements, relations among them, and properties of both...
principle, or both. In an article by Shivprasad Koirala Inversion of Control with Dependency Injection is presented as a design pattern. It is a practical example of the first five techniques mentioned (see below section). He also shows that Dependency Injection
Dependency injection
Dependency injection is a design pattern in object-oriented computer programming whose purpose is to improve testability of, and simplify deployment of components in very large software systems....
might be a design pattern, whereas Inversion of Control is implemented using Dependency Injection
Dependency injection
Dependency injection is a design pattern in object-oriented computer programming whose purpose is to improve testability of, and simplify deployment of components in very large software systems....
. In an article by Mani Malarvannan Inversion of Control is presented as a design pattern using contextualized lookup. The use of a service locator is considered using the same design pattern. In an article by Loek Bergman it is presented as an architectural principle. Dependency injection
Dependency injection
Dependency injection is a design pattern in object-oriented computer programming whose purpose is to improve testability of, and simplify deployment of components in very large software systems....
being the design pattern as its most natural implementation.
Robert Cecil Martin, known colloquially as "Uncle Bob", is an American software consultant and author. Martin has been a software professional since 1970 and an international software consultant since 1990. In 2001, he initiated the meeting of the group that created Agile software development from...
the dependency inversion principle and abstraction by layering come together. His reason to use the term "inversion" is in comparison with traditional software development methods. He describes the uncoupling of services by the abstraction of layers, when he is talking about dependency inversion. The principle is used to find out where system borders are in the design of the abstraction layers.
In object-oriented programming, the dependency inversion principle refers to a specific form of decoupling where conventional dependency relationships established from high-level, policy-setting modules to low-level, dependency modules are inverted for the purpose of rendering high-level modules...
. Dependency injection is the main method to implement Inversion of Control.
Implementation techniques
Implementation techniques are influenced by the computer language used.
In Java there are six basic techniques to implement Inversion of Control. These are:
The service locator pattern is a design pattern used in software development to encapsulate the processes involved in obtaining a service with a strong abstraction layer...
using a constructor injection
using a setter injection
using an interface injection
using a contextualized lookup
Constructor, setter, and interface injection are all aspects of Dependency injection
Dependency injection
Dependency injection is a design pattern in object-oriented computer programming whose purpose is to improve testability of, and simplify deployment of components in very large software systems....
.
In an original article by Martin Fowler, the first five different techniques are discussed. In a description about Inversion of Control types, the last one is mentioned. Often the contextualized lookup will be accomplished using a service locator.
More important than the applied technique, however, is the optimization of the purposes.A
Examples
public class ServerFacade
{
public V respondToRequest(K request) {
if (businessLayer.validateRequest(request)) {
DAO.getData(request);
return Aspect.convertData(request);
}
return null;
}
}
This basic outline in Java gives an example of code following the IoC methodology. It is important, however, that in the a lot of assumptions are made about the data returned by the data access object
Data Access Object
In computer software, a data access object is an object that provides an abstract interface to some type of database or persistence mechanism, providing some specific operations without exposing details of the database. It provides a mapping from application calls to the persistence layer...
(DAO).
Although all these assumptions might be valid at some time, they couple the implementation of the to the DAO implementation. Designing the application in the manner of Inversion of Control would hand over the control completely to the DAO object. The code would then become
public class ServerFacade
{
public V respondToRequest(K request, DAO dao) {
return dao.getData(request);
}
}
The example shows that the way the method is constructed determines, if IoC is used. It is the way that parameters are used that define IoC. This resembles the message-passing
Message passing
Message passing in computer science is a form of communication used in parallel computing, object-oriented programming, and interprocess communication. In this model, processes or objects can send and receive messages to other processes...
style that some object-oriented programming languages have been using.
SAX is an event-based sequential access parser API developed by the XML-DEV mailing list for XML documents. SAX provides a mechanism for reading data from an XML document that is an alternative to that provided by the Document Object Model...
is an example of an API that uses inversion of control throughout (after initialisation). SAX is generally more efficient than DOM
Document Object Model
The Document Object Model is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. Aspects of the DOM may be addressed and manipulated within the syntax of the programming language in use...
, but DOM is often considered more convenient to program with, because it is not necessary to deal with inversion of control.
Asynchronous I/O, or non-blocking I/O, is a form of input/output processing that permits other processing to continue before the transmission has finished....
In computer programming, a callback is a reference to executable code, or a piece of executable code, that is passed as an argument to other code. This allows a lower-level software layer to call a subroutine defined in a higher-level layer....
In computer science, a closure is a function together with a referencing environment for the non-local variables of that function. A closure allows a function to access variables outside its typical scope. Such a function is said to be "closed over" its free variables...
In computer science and programming, a continuation is an abstract representation of the control state of a computer program. A continuation reifies the program control state, i.e...
A delegate is a form of type-safe function pointer used by the .NET Framework. Delegates specify a method to call and optionally an object to call the method on. They are used, among other things, to implement callbacks and event listeners....
In object-oriented programming, the dependency inversion principle refers to a specific form of decoupling where conventional dependency relationships established from high-level, policy-setting modules to low-level, dependency modules are inverted for the purpose of rendering high-level modules...
In computer science, flow-based programming is a programming paradigm that defines applications as networks of "black box" processes, which exchange data across predefined connections by message passing, where the connections are specified externally to the processes...
Implicit invocation is a term used by some authors for a style of software architecture in which a system is structured around event handling, using a form of callback...
An interrupt handler, also known as an interrupt service routine , is a callback subroutine in microcontroller firmware, operating system or device driver whose execution is triggered by the reception of an interrupt...
Message passing in computer science is a form of communication used in parallel computing, object-oriented programming, and interprocess communication. In this model, processes or objects can send and receive messages to other processes...
The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods...
Publish–subscribe is a messaging pattern where senders of messages, called publishers, do not program the messages to be sent directly to specific receivers, called subscribers. Published messages are characterized into classes, without knowledge of what, if any, subscribers there may be...
The service locator pattern is a design pattern used in software development to encapsulate the processes involved in obtaining a service with a strong abstraction layer...
A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
In computer programming, a software framework is an abstraction in which software providing generic functionality can be selectively changed by user code, thus providing application specific software...
In computer programming, the strategy pattern is a particular software design pattern, whereby algorithms can be selected at runtime. Formally speaking, the strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable...
A user exit is a subroutine invoked by a software package for a predefined event in the execution of the package. Clients of the package can substitute their own subroutines in place of the default ones provided by the package vendor to provide customized functionality.A typical use is replacing...
In object-oriented programming and software engineering, the visitor design pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existing object structures without modifying those...
XSLT is a declarative, XML-based language used for the transformation of XML documents. The original document is not changed; rather, a new document is created based on the content of an existing one. The new document may be serialized by the processor in standard XML syntax or in another format,...
is a data-driven scripting language, meaning that the input data controls which templates (methods) are executed and in what order. Templates automatically return control to the input data upon completion or by using the command.
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