What is it

Dependency Injection and Service Locator both performs wiring of application.

wiring = connecting multiple parts together as a cohesive application.

Service = Component

Similarities
- Both are used by foreign applications.
Differences
- Component should be used locally.
- Service will be remotely accessed/used.

Registry

A well-known object that other objects can use to find common objects and services.

What is `Inversion of Control`

What is a Control.

Think control flow of a program.

Inversion of Control means providing any kind of callback (via implements or controls reaction) instead of acting on it directly

Meaning, inverting/redirecting control flow to the external handler/controller.

What is `Dependency Injection`

Dependency Injection is a specific version of IoC that is focused on removing dependency from your code.

Dependency Injection depends upon abstractions and not depend upon concretions via passing/injecting concrete implementation to the abstraction as a parameter.

Classes should depend upon interfaces or abstract classes instead of concrete classes and functions.

e.g. PersistenceManager class depends on InvoicePersistence interface instead of the classes that implement that interface.

Illustration

In below example, the TextEditor class is directly dependent upon SpellChecker because it is instantiated inside the class definition.

public class TextEditor {
  private SpellChecker checker;

  public TextEditor() {
      this.checker = new SpellChecker();
  }
}

Applying the inversion, we are creating abstraction by passing the SpellChecker in the TextEditor's constructor signature.

Not initializing the dependency in the class definition.

public class TextEditor {
  private IocSpellChecker checker;

  public TextEditor(IocSpellChecker checker) {
      this.checker = checker;
  }
}

Passing the actual implementation of SpellChecker would look like this:

SpellChecker sc = new SpellChecker(); // dependency
TextEditor textEditor = new TextEditor(sc);

This allows TextEditor class to choose which SpellChecker implementation to use because the dependency is injected.

Injection Patterns

also see "service-locator" page

Constructor Injection

// injects the finder
class MovieLister...
  public MovieLister(MovieFinder finder) {
    this.finder = finder;       
  }

// finder
class ColonMovieFinder...
  public ColonMovieFinder(String filename) {
    this.filename = filename;
  }

// injection container
private MutablePicoContainer configureContainer() {
  MutablePicoContainer pico = new DefaultPicoContainer();
  Parameter[] finderParams =  {new ConstantParameter("movies1.txt")};
  
  // register both containers to global
  pico.registerComponentImplementation(MovieFinder.class, ColonMovieFinder.class, finderParams);
  pico.registerComponentImplementation(MovieLister.class);
  
  return pico;
}

// test
public void testWithPico() {
  // mock container
  MutablePicoContainer pico = configureContainer();
  
  // calling the lister
  MovieLister lister = (MovieLister) pico.getComponentInstance(MovieLister.class);
  
  // calling the method on the service
  Movie[] movies = lister.moviesDirectedBy("Sergio Leone");

  // test assertion
  assertEquals("Once Upon a Time in the West", movies[0].getTitle());
}

Setter Injection

// lister to accept injection by defining a setter method
class MovieLister...
  private MovieFinder finder;
  public void setFinder(MovieFinder finder) {
    this.finder = finder;
  }

// setter for filename
class ColonMovieFinder...
  public void setFilename(String filename) {
    this.filename = filename;
  }

// configuration
<beans>
  <bean id="MovieLister" class="spring.MovieLister">
    <property name="finder">
      <ref local="MovieFinder"/>
    </property>
  </bean>
  <bean id="MovieFinder" class="spring.ColonMovieFinder">
    <property name="filename">
      <value>movies1.txt</value>
    </property>
  </bean>
</beans>

// test
public void testWithSpring() throws Exception {
  ApplicationContext ctx = new FileSystemXmlApplicationContext("spring.xml");
  MovieLister lister = (MovieLister) ctx.getBean("MovieLister");
  Movie[] movies = lister.moviesDirectedBy("Sergio Leone");
  assertEquals("Once Upon a Time in the West", movies[0].getTitle());
}

Interface Injection

// finder interface
public interface InjectFinder {
  void injectFinder(MovieFinder finder);
}

// must be implemented by class that wants to use a finder (lister)
class MovieLister implements InjectFinder {
  public void injectFinder(MovieFinder finder) {
    this.finder = finder;
  }
}

// injecting filename to finder implementation
public interface InjectFinderFilename {
  void injectFilename (String filename);
}

class ColonMovieFinder implements MovieFinder, InjectFinderFilename...
  public void injectFilename(String filename) {
    this.filename = filename;
  }

// configuration
class Tester...
  private Container container;
  
  private void configureContainer() {
    container = new Container();
    registerComponents();
    registerInjectors();
    container.start();
  }

// container registration
class Tester...
  private void registerComponents() {
    container.registerComponent("MovieLister", MovieLister.class);
    container.registerComponent("MovieFinder", ColonMovieFinder.class);
  }

// injector registration
class Tester...
  private void registerInjectors() {
    container.registerInjector(InjectFinder.class, container.lookup("MovieFinder"));
    container.registerInjector(InjectFinderFilename.class, new FinderFilenameInjector());
  }
  public interface Injector {
    public void inject(Object target);
  }

// component implementing the injector itself (should we do this?)
class ColonMovieFinder implements Injector...
  public void inject(Object target) {
    ((InjectFinder) target).injectFinder(this);
  }

class Tester...
  public static class FinderFilenameInjector implements Injector {
    public void inject(Object target) {
      ((InjectFinderFilename)target).injectFilename("movies1.txt");
    }
  }

// corresponding test
class Tester…
  public void testIface() {
    configureContainer();
    MovieLister lister = (MovieLister)container.lookup("MovieLister");
    Movie[] movies = lister.moviesDirectedBy("Sergio Leone");
    assertEquals("Once Upon a Time in the West", movies[0].getTitle());
  }

Dependency Injection