Umetanje zavisnosti - Dependency Injection

Prof. dr Igor Dejanović (igord at uns ac rs)

Kreirano 2023-01-16 Mon 18:19, pritisni ESC za mapu, m za meni, Ctrl+Shift+F za pretragu

Sadržaj

1. Motivacija
2. Dependency Injection
3. Google Guice
4. Injector
5. Flask injector
6. Reference

1. Motivacija

1.1. Motivacija

Objekti iole složenijih aplikacija formiraju složene grafove zavisnosti.
Kako objekat “dobija” reference na zavisne objekte?

1.2. Motivacija

1.3. Klasičan pristup dobavljanja referenci

 public class RealBillingService implements BillingService {

   @Override
   public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {
       CreditCardProcessor processor = new PaypalCreditCardProcessor();
       TransactionLog transactionLog = new DatabaseTransactionLog();

       try {
         ChargeResult result = processor.charge(creditCard, order.getAmount());
         transactionLog.logChargeResult(result);

         return result.wasSuccessful()
             ? Receipt.forSuccessfulCharge(order.getAmount())
             : Receipt.forDeclinedCharge(result.getDeclineMessage());
        } catch (UnreachableException e) {
         transactionLog.logConnectException(e);
         return Receipt.forSystemFailure(e.getMessage());
       }
   }
 }

1.4. Upotreba Singleton/Factory obrasca

Objekat se sam brine o dobavljanju referenci ali to čini posredstvom globalne deljene reference.

 public class RealBillingService implements BillingService {

   public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {
     CreditCardProcessor processor = CreditCardProcessorFactory.getInstance();
     TransactionLog transactionLog = TransactionLogFactory.getInstance();

     try {
       ChargeResult result = processor.charge(creditCard, order.getAmount());
       transactionLog.logChargeResult(result);

       return result.wasSuccessful() ?
             Receipt.forSuccessfulCharge(order.getAmount()) :
             Receipt.forDeclinedCharge(result.getDeclineMessage());
     } catch (UnreachableException e) {
       transactionLog.logConnectException(e);
       return Receipt.forSystemFailure(e.getMessage());
     }
   }
 }

1.5. Singleton/Factory - testiranje

 public class RealBillingServiceTest extends TestCase {

   private final PizzaOrder order = new PizzaOrder(100);
   private final CreditCard creditCard = new CreditCard(5000);
   private final InMemoryTransactionLog transactionLog = new InMemoryTransactionLog();
   private final FakeCreditCardProcessor creditCardProcessor = new FakeCreditCardProcessor();

   @Override
   public void setUp() {
     TransactionLogFactory.setInstance(transactionLog);
     CreditCardProcessorFactory.setInstance(creditCardProcessor);
   }

   @Override
   public void tearDown() {
     TransactionLogFactory.setInstance(null);
     CreditCardProcessorFactory.setInstance(null);
   }

   public void testSuccessfulCharge() {
     RealBillingService billingService = new RealBillingService();
     Receipt receipt = billingService.chargeOrder(order, creditCard);

     assertTrue(receipt.hasSuccessfulCharge());
     assertEquals(100.0, receipt.getAmount(), 0.001);
     assertEquals(creditCard, creditCardProcessor.getCardOfOnlyCharge());
     assertEquals(100.0, creditCardProcessor.getAmountOfOnlyCharge(), 0.001);
     assertTrue(transactionLog.wasSuccessLogged());
   }
 }

1.6. Upotreba Singleton/Factory obrasca - problemi

Deljena referenca - moramo posebno da pazimo da je postavljamo na prave vrednosti.
Nemoguće paralelizovati testove.

2. Dependency Injection

2.1. Umetanje zavisnosti - Dependency Injection

Izmeštanje nadležnosti za dobavljanje referenci van objekta - neko drugi će se brinuti da “umetne” reference pre njihove upotrebe.
Prednosti:
- Kod se pojednostavljuje. Zavisnost između klasa je bazirana na apstraktnim interfejsima što pozitivno utiče na održavanje (maintability), ponovnu iskoristljivost (reusability) i podelu posla i nadležnosti.
- Objekat će do trenutka poziva njegovih servisnih metoda već biti na odgovarajući način inicijalizovan. Smanjuje se tzv. boilerplate kod.
- Testiranje je daleko jednostavnije. Kreiranje “lažnih” objekata (mockup) je moguće i jednostavno se izvodi. Moguća paralelizacija testova.

2.2. Mehanizmi umetanja zavisnosti

Putem parametara konstruktora.
Putem mutator metoda (setters).
Putem implementiranog interfejsa.

2.3. Injekcija putem parametara konstruktora

 Client(Service service) {
     this.service = service;
 }

2.4. Injekcija putem setter metoda

 public void setService(Service service) {
     this.service = service;
 }

2.5. Injekcija putem interfejsa

 public interface ServiceSetter {
     public void setService(Service service);
 }
 public class client implements ServiceSetter {

     private Service service;

     @Override
     public void setService(Service service) {
         this.service = service;
     }
 }

2.6. Upotreba DI

 public class RealBillingService implements BillingService {
   private final CreditCardProcessor processor;
   private final TransactionLog transactionLog;

   public RealBillingService(CreditCardProcessor processor,
       TransactionLog transactionLog) {
     this.processor = processor;
     this.transactionLog = transactionLog;
   }

   public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {
     try {
       ChargeResult result = processor.charge(creditCard, order.getAmount());
       transactionLog.logChargeResult(result);

       return result.wasSuccessful() ?
            Receipt.forSuccessfulCharge(order.getAmount()) :
            Receipt.forDeclinedCharge(result.getDeclineMessage());
     } catch (UnreachableException e) {
       transactionLog.logConnectException(e);
       return Receipt.forSystemFailure(e.getMessage());
     }
   }
 }

2.7. Upotreba DI - testiranje

 public class RealBillingServiceTest extends TestCase {

   private final PizzaOrder order = new PizzaOrder(100);
   private final CreditCard creditCard = new CreditCard(5000);

   private final InMemoryTransactionLog transactionLog =
                                              new InMemoryTransactionLog();
   private final FakeCreditCardProcessor creditCardProcessor =
                                              new FakeCreditCardProcessor();

   public void testSuccessfulCharge() {
     RealBillingService billingService = new RealBillingService(
         creditCardProcessor, transactionLog);
     Receipt receipt = billingService.chargeOrder(order, creditCard);

     assertTrue(receipt.hasSuccessfulCharge());
     assertEquals(100.0, receipt.getAmount(), 0.001);
     assertEquals(creditCard, creditCardProcessor.getCardOfOnlyCharge());
     assertEquals(100.0, creditCardProcessor.getAmountOfOnlyCharge(), 0.001);
     assertTrue(transactionLog.wasSuccessLogged());
   }
 }

2.8. DI kontejneri

DI se može implementirati i bez posebnog alata/okvira.
DI kontejneri omogućavaju nametanje određenih konvencija za primenu ovog obrasca.
Korišćenje DI kontejera donosi određene prednosti:
- Upotreba najbolje prakse
- Standardizacija

2.9. DI kontejneri za Javu

Google Guice
PicoContainer
Spring
…

2.10. Standardizacija za programski jezik Java

JSR-330¹
Definiše skup standardnih Java anotacija za DI:
- Provider<T> - Provides instances of T
- Inject - Identifies injectable constructors, methods, and fields.
- Named - String-based qualifier.
- Qualifier - Identifies qualifier annotations.
- Scope - Identifies scope annotations.
- Singleton - Identifies a type that the injector only instantiates once.

3. Google Guice

3.1. Google Guice

Lightweight okvir za DI u Javi.
Razvijen od strane Google-a.
Konfiguracija bazirana na Java anotacijama.

3.2. Injekcija putem konstruktora

 public class RealBillingService implements BillingService {
   private final CreditCardProcessor processor;
   private final TransactionLog transactionLog;

   @Inject
   public RealBillingService(CreditCardProcessor processor,
       TransactionLog transactionLog) {
     this.processor = processor;
     this.transactionLog = transactionLog;
   }

   public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {
     try {
       ChargeResult result = processor.charge(creditCard, order.getAmount());
       transactionLog.logChargeResult(result);

       return result.wasSuccessful()
           ? Receipt.forSuccessfulCharge(order.getAmount())
           : Receipt.forDeclinedCharge(result.getDeclineMessage());
      } catch (UnreachableException e) {
       transactionLog.logConnectException(e);
       return Receipt.forSystemFailure(e.getMessage());
     }
   }
 }

3.3. Konfiguracija za povezivanje - binding/wiring

 public class BillingModule extends AbstractModule {
   @Override
   protected void configure() {
     bind(TransactionLog.class).to(DatabaseTransactionLog.class);
     bind(CreditCardProcessor.class).to(PaypalCreditCardProcessor.class);
     bind(BillingService.class).to(RealBillingService.class);
   }
 }

3.4. Upotreba kontejnera

 public static void main(String[] args) {
   Injector injector = Guice.createInjector(new BillingModule());
   BillingService billingService = injector.getInstance(BillingService.class);
   Receipt result = billingService.chargeOrder(new PizzaOrder(100),
                                               new CreditCard(500));
   System.out.println(result.hasSuccessfulCharge());
 }

3.5. Linked Bindings

 public class BillingModule extends AbstractModule {
   @Override
   protected void configure() {
     bind(TransactionLog.class).to(DatabaseTransactionLog.class);
     bind(DatabaseTransactionLog.class).to(MySqlDatabaseTransactionLog.class);
   }
 }

3.6. Custom Bindings Annotations

 package example.pizza;
 import com.google.inject.BindingAnnotation;
 import java.lang.annotation.Target;
 import java.lang.annotation.Retention;
 import static java.lang.annotation.RetentionPolicy.RUNTIME;
 import static java.lang.annotation.ElementType.PARAMETER;
 import static java.lang.annotation.ElementType.FIELD;
 import static java.lang.annotation.ElementType.METHOD;

 @BindingAnnotation @Target({ FIELD, PARAMETER, METHOD }) @Retention(RUNTIME)
 public @interface PayPal {}
 ...
 public class RealBillingService implements BillingService {

   @Inject
   public RealBillingService(@PayPal CreditCardProcessor processor,
       TransactionLog transactionLog) {
     ...
   }
 ...
     bind(CreditCardProcessor.class)
         .annotatedWith(PayPal.class)
         .to(PayPalCreditCardProcessor.class);

3.7. @Named Binding Annotation

 public class RealBillingService implements BillingService {

   @Inject
   public RealBillingService(@Named("Checkout") CreditCardProcessor processor,
       TransactionLog transactionLog) {
     ...
   }
  ...
  ...
     bind(CreditCardProcessor.class)
         .annotatedWith(Names.named("Checkout"))
         .to(CheckoutCreditCardProcessor.class);

3.8. Instance Bindings

 bind(String.class)
     .annotatedWith(Names.named("JDBC URL"))
     .toInstance("jdbc:mysql://localhost/pizza");
 bind(Integer.class)
     .annotatedWith(Names.named("login timeout seconds"))
     .toInstance(10);

3.9. @Provides Methods

 public class BillingModule extends AbstractModule {
   @Override
   protected void configure() {
     ...
   }

   @Provides
   TransactionLog provideTransactionLog() {
     DatabaseTransactionLog transactionLog = new DatabaseTransactionLog();
     transactionLog.setJdbcUrl("jdbc:mysql://localhost/pizza");
     transactionLog.setThreadPoolSize(30);
     return transactionLog;
   }
 }
 ...
   @Provides @PayPal
   CreditCardProcessor providePayPalCreditCardProcessor(
       @Named("PayPal API key") String apiKey) {
     PayPalCreditCardProcessor processor = new PayPalCreditCardProcessor();
     processor.setApiKey(apiKey);
     return processor;
   }

3.10. Provider Bindings

 public class DatabaseTransactionLogProvider 
       implements Provider<TransactionLog> {
   private final Connection connection;

   @Inject
   public DatabaseTransactionLogProvider(Connection connection) {
     this.connection = connection;
   }

   public TransactionLog get() {
     DatabaseTransactionLog transactionLog = new DatabaseTransactionLog();
     transactionLog.setConnection(connection);
     return transactionLog;
   }
 }
 ...
 public class BillingModule extends AbstractModule {
   @Override
   protected void configure() {
     bind(TransactionLog.class)
         .toProvider(DatabaseTransactionLogProvider.class);
   }
 }

3.11. Scopes

 @Singleton
 public class InMemoryTransactionLog implements TransactionLog {
   /* everything here should be threadsafe! */
 }
 ...
 bind(TransactionLog.class)
   .to(InMemoryTransactionLog.class).in(Singleton.class);
 ...
 @Provides @Singleton
 TransactionLog provideTransactionLog() {
   ...
 }
 ...
 bind(Bar.class).to(Applebees.class).in(Singleton.class);
 bind(Grill.class).to(Applebees.class).in(Singleton.class);

4. Injector

4.1. Injector

https://github.com/alecthomas/injector
Python biblioteka za DI modelovana prema Google Guice ali sa Pythonic API.

4.2. Jednostavan primer

 >>> from injector import Injector, inject
 >>> class Inner(object):
 ...     def __init__(self):
 ...         self.forty_two = 42
 ...
 >>> class Outer(object):
 ...     @inject
 ...     def __init__(self, inner: Inner):
 ...         self.inner = inner
 ...
 >>> injector = Injector()
 >>> outer = injector.get(Outer)
 >>> outer.inner.forty_two
 42

4.3. Složeniji primer

 from injector import Key
 Name = Key('name')
 Description = Key('description')

 from injector import inject, provider, Module

 class User(object):
     @inject
     def __init__(self, name: Name, description: Description):
         self.name = name
         self.description = description


 class UserModule(Module):
     def configure(self, binder):
        binder.bind(User)


 class UserAttributeModule(Module):
     def configure(self, binder):
         binder.bind(Name, to='Sherlock')

     @provider
     def describe(self, name: Name) -> Description:
         return '%s is a man of astounding insight' % name

4.4. Složeniji primer

 from injector import Injector
 injector = Injector([UserModule(), UserAttributeModule()])

ili

 injector = Injector([UserModule, UserAttributeModule])

Upotreba:

 >>> injector.get(Name)
 'Sherlock'
 >>> injector.get(Description)
 'Sherlock is a man of astounding insight'

 >>> user = injector.get(User)
 >>> isinstance(user, User)
 True
 >>> user.name
 'Sherlock'
 >>> user.description
 'Sherlock is a man of astounding insight'

5. Flask injector

5.1. Flask injector

Veza između injector biblioteke i Flask okvira za razvoj.

5.2. Primer upotrebe

 import sqlite3
 from flask import Flask, Config
 from flask.views import View
 from flask_injector import FlaskInjector
 from injector import inject

 app = Flask(__name__)

 @app.route("/bar")
 def bar():
     return render("bar.html")

 @app.route("/foo")
 @inject(db=sqlite3.Connection)
 def foo(db):
     users = db.execute('SELECT * FROM users').all()
     return render("foo.html")
    
 def configure(binder):
     binder.bind(
         sqlite3.Connection,
         to=sqlite3.Connection(':memory:'),
         scope=request,
     )

 FlaskInjector(app=app, modules=[configure])

 app.run()

6. Reference

Dependency Injection on Wikipedia
Martin Fowler, Inversion of Control Containers and the Dependency Injection pattern, January 2004.
Google Guice Wiki
Injector dokumentacija
Flask Injector projekat