Design Patterns. Dependency Injection. Oliver Haase

Transcription

1 Design Patterns Dependency Injection Oliver Haase 1

2 Motivation A simple, motivating example (by Martin Fowler): public interface MovieFinder { /** * returns all movies of this finder s source all movies */ List<Movie> findall(); public class ColonDelimitedMovieFinder implements MovieFinder { private final String filename; public ColonDelimitedMovieFinder(String filename) { this.filename = filename; /** * returns all movies listed in file <code>filename</code> List<Movie> findall() { 2

3 Motivation A simple, motivating example (by Martin //assuming that ColonDelimitedMovieFinder is threadsafe public class MovieLister { private final MovieFinder finder; public MovieLister() { finder = new ColonDelimitedMovieFinder( movies.txt ); public List<Movie> moviesdirectedby(string arg) { List<Movie> movies = finder.findall(); for ( Iterator<Movie> it = movies.iterator(); it.hasnext(); ) { Movie movie = it.next(); if (!movie.getdirector().equals(arg) ) { it.remove(); return movies; 3

4 Problem How to remove MovieLister s dependency on ColonDelimitedMovieFinder? Difference to DocManager example: MovieLister needs only one MovieFinder instance (service) DocManager must be able to create many Document objects at will 4

5 Idea Have the dependency (service) be injected by the client Inversion of Control Hollywood Principle ( don t call us, we ll call you ) Please note: DI is first and foremost a design pattern that can be implemented by hand. DI is, however, often equated with DI frameworks, e.g. Spring DI, Google Guice. 5

6 Types of Dependency Injection There are 3 types of dependency injection: 1. constructor injection 2. setter injection 3. interface injection 6

7 Constructor Injection - //assuming that finder instance is threadsafe public class MovieLister { private final MovieFinder finder; public MovieLister(MovieFinder finder) { this.finder = finder; public List<Movie> moviesdirectedby(string arg) { List<Movie> movies = finder.findall(); for ( Iterator<Movie> it = movies.iterator(); it.hasnext(); ) { Movie movie = it.next(); if (!movie.getdirector().equals(arg) ) { it.remove(); return movies; Usage: MovieLister movielister = new MovieLister(new ColonDelimitedMovieFinder( movies.txt )); List<Movie> movies = movielister.moviesdirectedby( Quentin Tarantino ); 7

8 Setter Injection - //assuming that finder instance is threadsafe public class MovieLister { private MovieFinder finder; public MovieLister() { public void setfinder(moviefinder finder) { this.finder = finder; public List<Movie> moviesdirectedby(string arg) { List<Movie> movies = finder.findall(); for ( Iterator<Movie> it = movies.iterator(); it.hasnext(); ) { Movie movie = it.next(); if (!movie.getdirector().equals(arg) ) { it.remove(); return movies; Usage: MovieLister movielister = new MovieLister(); movielister.setfinder(new ColonDelimitedMovieFinder( movies.txt )); List<Movie> movies = movielister.moviesdirectedby( Quentin Tarantino ); 8

9 Interface Injection - Example Provider of MovieFinder interface also defines injection interface, e.g. : public interface MovieFinderInjector { void injectmoviefinder(moviefinder finder); Each class that needs to get a MovieFinder injected has to implement injector //assuming that finder instance is threadsafe public class MovieLister implements MovieFinderInjector { private MovieFinder finder; public MovieLister() public void injectmoviefinder(moviefinder finder) { this.finder = finder; public List<Movie> moviesdirectedby(string arg) { 9

10 Setter vs. Interface Injection Only difference between setter and interface injection: Whether interface provider defines companion injection interface that implementing class must use for injection, or not. 10

11 Interface Injection - Example Usage: MovieLister movielister = new MovieLister(); movielister.injectmoviefinder(new ColonDelimitedMovieFinder( movies.txt )); List<Movie> movies = movielister.moviesdirectedby( Quentin Tarantino ); 11

12 Dependency Injection Frameworks DI Frameworks separate out instantiation configuration, i.e. bindings from abstract interfaces to concrete types. Configuration usually either in XML or in Java with annotations Wide-spread DI Frameworks: Apache Spring DI Google Guice 12

13 Guice: Constructor //assuming that finder instance is threadsafe public class MovieLister { private final MovieFinder public MovieLister(MovieFinder finder) { this.finder = finder; public List<Movie> moviesdirectedby(string arg) { List<Movie> movies = finder.findall(); for ( Iterator<Movie> it = movies.iterator(); it.hasnext(); ) { Movie movie = it.next(); if (!movie.getdirector().equals(arg) ) { it.remove(); return movies; 13

14 Guice: Constructor annotation tells Guice to create and fill in appropriate MovieFinder instance when creating MovieLister instance. Works only if bound type (e.g. ColonDelimitedMovieFinder) has zero-args non-private constructor, or uses itself constructor injection 14

15 Guice: Constructor Injection public class ColonDelimitedMovieFinder implements MovieFinder { private final String public ColonDelimitedMovieFinder(@Named("FILE NAME") String filename) { this.filename = annotation needed for instance binding,.i.e. binding of a type to an instance of that type. 15

16 Guice: Modules Bindings are defined in modules, i.e. Java classes that inherit from com.google.inject.abstractmodule and whose configure method contains the bindings: public class MovieListerModule extends AbstractModule protected void configure() { bind(moviefinder.class).to(colondelimitedmoviefinder.class); bind(string.class).annotatedwith(names.named("file NAME")).toInstance("movies.txt"); 16

17 Guice: Instantiation Instances are created by creating a Guice injector that uses a previously defined module having the injector create the application object(s) Injector injector = Guice.createInjector(new MovieListerModule()); MovieLister lister = injector.getinstance(movielister.class); 17

18 Guice: Setter //assuming that finder instance is threadsafe public class MovieLister { private MovieFinder public void setfinder(moviefinder finder) { this.finder = finder; public List<Movie> moviesdirectedby(string arg) { List<Movie> movies = finder.findall(); for ( Iterator<Movie> it = movies.iterator(); it.hasnext(); ) { Movie movie = it.next(); if (!movie.getdirector().equals(arg) ) { it.remove(); return movies; 18

19 Guice: Setter Injection public class ColonDelimitedMovieFinder implements MovieFinder { private String public void setfilename(@named("file NAME") String filename) { this.filename = public List<Movie> findall() {... MovieListerModule remains the same as before, because mappings also remain the same. 19

20 Guice: Setter Injection Object instantiation can also remain the same Guice automatically calls setter methods to inject necessary dependencies. Injector injector = Guice.createInjector(new MovieListerModule()); MovieLister lister = injector.getinstance(movielister.class); Or, objects can be instantiated as usually, and then Guice can fill in dependencies using setter methods: Injector injector = Guice.createInjector(new MovieListerModule()); MovieLister lister = new MovieLister(); injector.injectmembers(lister); 20

21 Constructor vs. Setter Injection Constructor Injection only valid and complete objects are created better chances for immutability Setter Injection can lead to unnecessarily mutable objects injection through easy-to-read methods necessary if dependencies are not available at creation time, e.g. cyclic dependencies Recommendation: Use setter injection only if necessary. 21

22 DocManager Reloaded How to apply DI pattern to DocManager example? Inject concrete DocumentFactory as a service into DocManager So... if client knows when to create objects, but doesn t know (neither care) how, then... inject client with factory that can be used to get instances as needed. 22

23 DocManager // assuming that concrete Document is threadsafe public class DocManager { private final Collection<Document> docs; private final DocumentFactory public DocManager(DocumentFactory docfactory) { this.docfactory = docfactory; docs = new ConcurrentLinkedQueue<Document>(); public void createdoc() { Document doc = docfactory.newdocument(); docs.add(doc); doc.open(); public void opendocs() { for ( Document doc : docs ) doc.open(); 23

24 DocManager Reloaded Sample Bindings: public class DocManagerModule extends AbstractModule protected void configure() { bind(documentfactory.class).to(latexdocfactory.class); Usage: Injector injector = Guice.createInjector(new DocManagerModule()); DocManager docmanager = injector.getinstance(docmanager.class); 24

25 You Want More? Read more about DI in Martin Fowler s seminal online article 25

26 Service Locator 26

27 Motivation Back to Martin Fowler s DI //assuming that ColonDelimitedMovieFinder is threadsafe public class MovieLister { private final MovieFinder finder; public MovieLister() { finder = new ColonDelimitedMovieFinder( movies.txt ); public List<Movie> moviesdirectedby(string arg) { List<Movie> movies = finder.findall(); for ( Iterator<Movie> it = movies.iterator(); it.hasnext(); ) { Movie movie = it.next(); if (!movie.getdirector().equals(arg) ) { it.remove(); return movies; 27

28 Problem & Idea How to remove MovieLister s dependency on ColonDelimitedMovieFinder? Pass a service locator into MovieLister that can be queried for all kinds of services. 28

29 Example public interface ServiceLocator { MovieFinder getmoviefinder(string //assuming that ColonDelimitedMovieFinder is threadsafe public class MovieLister { private final MovieFinder finder; private final ServiceLocator servicelocator; public MovieLister(ServiceLocator servicelocator) { finder = servicelocator.getmoviefinder( movies.txt ); public List<Movie> moviesdirectedby(string arg) {... 29

30 But... (a) there s now a dependency on the service locator... yes, but only on one object for all services. (b) how does service locator get into MovieLister? e.g. with dependency injection. 30

31 Example Using Guice dependency //assuming that ColonDelimitedMovieFinder is threadsafe public class MovieLister { private final MovieFinder finder; private final ServiceLocator public MovieLister(ServiceLocator servicelocator) { finder = servicelocator.getmoviefinder( movies.txt ); public List<Movie> moviesdirectedby(string arg) {... 31

32 Service Locator vs. Abstract Factory both can create objects of different types (services vs. products) product types belong to a product family, services unrelated with each other abstract factory creates many instances of a product type, service locator only one instance per service type 32

33 Builder 33

34 Motivating Example (J. Bloch) Suppose a class NutritionFacts that describes food items. A few specifications are mandatory, many are public class NutritionFacts { private final int servingsize; // (ml) - mandatory private final int servings; // (per container) - mandatory private final int calories; // - optional private final int fat; // (g) - optional private final int sodium; // (mg) - optional private final int carbs; // (g) - optional... 34

35 Motivating Example (J. Bloch) Question: How to create instances of NutritionFacts? Option 1: telescoping constructors 35

36 Option 1 - Telescoping C public class NutritionFacts { private final int servingsize; // (ml) - mandatory private final int servings; // (per container) - mandatory private final int calories; // - optional private final int fat; // (g) - optional private final int sodium; // (mg) - optional private final int carbs; // (g) - optional public NutritionFacts(int servingsize, int servings) { this(servingsize, servings, 0); public NutritionFacts(int servingsize, int servings, int calories) { this(servingsize, servings, calories, 0); public NutritionFacts(int servingsize, int servings, int calories, int fat) { this(servingsize, servings, calories, fat, 0); 36

37 Option 1 - Telescoping C tors public NutritionFacts(int servingsize, int servings, int calories, int fat, int sodium) { this(servingsize, servings, calories, fat, sodium, 0); public NutritionFacts(int servingsize, int servings, int calories, int fat, int sodium, int carbs) { this.servingsize = servingsize; this.servings = servings; this.calories = calories; this.fat = fat; this.sodium = sodium; this.carbs = carbs; Sample usage: NutritionFacts cocacola = new NutritionFacts(240, 8, 100, 0, 35, 27); 37

38 Second Try... Question: How to create instances of NutritionFacts? Option 2: JavaBeans Pattern 38

39 Option 2 - JavaBeans Pattern public class NutritionFacts { private int servingsize = -1; // (ml) - mandatory private int servings = -1; // (per container) - mandatory private int calories = 0; // - optional private int fat = 0; // (g) - optional private int sodium = 0; // (mg) - optional private int carbs = 0; // (g) - optional public NutritionFacts() { public void setservingsize(int servingsize) { this.servingsize = servingsize; public void setservings(int servings) { this.servings = servings; public void setcalories(int calories) { this.calories = calories; 39

40 Option 2 - JavaBeans Pattern public void setfat(int fat) { this.fat = fat; public void setsodium(int sodium) { this.sodium = sodium; public void setcarbs(int carbs) { carbs = carbs; Sample usage: NutritionFacts cocacola = new NutritionFacts(); cocacola.setservingsize(240); cocacola.setservings(8); cocacola.setcalories(100); cocacola.setsodium(35); cocacola.setcarbs(27); 40

41 Third Try... Question: How to create instances of NutritionFacts? Option 3: constructor/setter combination 41

42 Option 3 - C tor & Setters public class NutritionFacts { private final int servingsize; // (ml) - mandatory private final int servings; // (per container) - mandatory private int calories = 0; // - optional private int fat = 0; // (g) - optional private int sodium = 0; // (mg) - optional private int carbs = 0; // (g) - optional public NutritionFacts(int servingsize, int servings) { this.servingsize = servingsize; this.servings = servings; public void setcalories(int calories) { this.calories = calories; public void setfat(int fat) { this.fat = fat; 42

43 Option 3 - C tor & Setters public void setsodium(int sodium) { this.sodium = sodium; public void setcarbs(int carbs) { carbs = carbs; Sample usage: NutritionFacts cocacola = new NutritionFacts(240, 8); cocacola.setcalories(100); cocacola.setsodium(35); cocacola.setcarbs(27); 43

44 Comparison Option 1 only valid and complete objects are created preserves immutability hard to read Option 2 creation of incomplete, invalid objects loss of immutability easy to read 44

45 Comparison Option 3 only valid and complete objects are created easy to read loss of immutability There is another option that combines the best of all three options! 45

46 Option 4 - Builder Idea: Define a builder that can be fed with a combination of NutritionFacts values, and then be used to create a NutritionFacts instance. Client NutritionFactsBuilder c'tor(servingsize, servings) setcalories(int calories) setfat(int Fat) setsodium(int sodium) setcarbs(int carbs) build() NutritionFacts c'tor(nutritionfactsbuilder) return new NutritionFacts(self); 46

47 Option 4 - public class NutritionFacts { private final int servingsize; // (ml) 47 - mandatory private final int servings; // (per container) - mandatory private final int calories; // - optional private final int fat; // (g) - optional private final int sodium; // (mg) - optional private final int carbs; // (g) - optional public static class Builder { private final int servingsize; private final int servings; // optional params initialized to default values private int calories = 0; private int fat = 0; private int sodium = 0; private int carbs = 0; public Builder(int servingsize, int servings) { this.servingsize = servingsize; this.servings = servings;

48 Option 4 - Builder public void setcalories(int calories) { this.calories = calories; public void setfat(int fat) { this.fat = fat; public void setsodium(int sodium) { this.sodium = sodium; public void setcarbs(int carbs) { this.carbs = carbs; public NutritionFacts build() { return new NutritionFacts(this); 48

49 Option 4 - Builder private NutritionFacts(Builder builder) { servingsize = builder.servingsize; servings = builder.servings; calories fat sodium carbs = builder.calories; = builder.fat; = builder.sodium; = builder.carbs; Sample Usage: NutritionFacts.Builder cocacolabuilder = new NutritionFacts.Builder(240, 8); cocacolabuilder.setcalories(100); cocacolabuilder.setsodium(35); cocacolabuilder.setcarbs(27); NutritionFacts cocacola = cocacolabuilder.build(); 49

50 Builder - Structure creates Builder instance, thereby passes in all mandatory params uses set operations to set optional params calls build operation to have Product instance created Client return new Product(self); Builder c'tor(<mandatory params>) setoptionalparam1() setoptionalparam2() build() provides c'tor that expects a Builder instance and copies all values into itself Product c'tor(builder) use() provides c tor with all mandatory params initializes optional params to default values provides set operation for each optional param provides build operation that calls Product s c tor, passes in reference to itself 50

51 Pros & Cons Pro: creates only valid products easy to read preserves immutability configured builder can be used to create more than one product Con: builder object = abstract factory more verbose implementation more verbose usage than telescoping c'tors additional object needed to create product additional runtime and memory cost 51

52 Creational Patterns Discussion 52

53 Overview Abstract Factory Service Locator Prototype Builder Dependency Injection Singleton Factory Method 53

54 Similarities & Commonalities Abstract Factory Service Locator Prototype Builder Dependency Injection Singleton Factory Method : patterns that use a dedicated object to create new objects A B : A can be used to feed B into client code 54