Service locator pattern
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The service locator pattern is a
design pattern A design pattern is the re-usable form of a solution to a design problem. The idea was introduced by the architect Christopher Alexander and has been adapted for various other disciplines, particularly software engineering. The " Gang of Four" b ...
used in
software development Software development is the process of conceiving, specifying, designing, programming, documenting, testing, and bug fixing involved in creating and maintaining applications, frameworks, or other software components. Software development invol ...
to encapsulate the processes involved in obtaining a service with a strong abstraction layer. This pattern uses a central registry known as the "service locator", which on request returns the information necessary to perform a certain task. Proponents of the pattern say the approach simplifies component-based applications where all dependencies are cleanly listed at the beginning of the whole application design, consequently making traditional dependency injection a more complex way of connecting objects. Critics of the pattern argue that it is an
anti-pattern An anti-pattern in software engineering, project management, and business processes is a common response to a recurring problem that is usually ineffective and risks being highly counterproductive. The term, coined in 1995 by computer programmer An ...
which obscures dependencies and makes software harder to test.


Advantages

* The "service locator" can act as a simple run-time
linker Linker or linkers may refer to: Computing * Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
. This allows code to be added at run-time without re-compiling the application, and in some cases without having to even restart it. * Applications can optimize themselves at run-time by selectively adding and removing items from the service locator. For example, an application can detect that it has a better library for reading JPG images available than the default one, and alter the registry accordingly. * Large sections of a library or application can be completely separated. The only link between them becomes the registry. * An application may use multiple structured service locators purposed for particular functionality/testing. Service locator does not mandate one single static class per process. * The solution may be simpler with service locator (vs. dependency injection) in applications with well-structured component/service design. In these cases the disadvantages may actually be considered as an advantage (e.g. no need to supply various dependencies to every class and maintain dependency configurations).


Disadvantages

* The registry hides the class' dependencies, causing
run-time error Execution in computer and software engineering is the process by which a computer or virtual machine reads and acts on the instructions of a computer program. Each instruction of a program is a description of a particular action which must be car ...
s instead of compile-time errors when dependencies are missing (similar to using dependency injection). But each library is compiled, just the discovery of the concrete Class might not be found and cause an error, it's more a deployment issue than a Service Locator issue. * The registry makes code harder to
test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...
, since all tests need to interact with the same global service locator class to set the fake dependencies of a class under test. However, this is easily overcome by injecting application classes with a single service locator interface. Simulator can be implemented to simulate each interface provided by the service locator, so it's easy to swap the real implementation with a simulator.


See also

* Dependency injection *
Dependency inversion principle In object-oriented design, the dependency inversion principle is a specific methodology for loosely coupling software modules. When following this principle, the conventional dependency relationships established from high-level, policy-setting ...
*
Java Naming and Directory Interface The Java Naming and Directory Interface (JNDI) is a Java API for a directory service that allows Java software clients to discover and look up data and resources (in the form of Java objects) via a name. Like all Java APIs that interface with host ...


References


External links


Sample code





Dependencies In Disguise

Software Engineering Myths and Truisms
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