Google Guice + Dependency Injection ( Example)

SOURCE: https://code.google.com/p/google-guice/wiki/Motivation?tm=6
normal method, dependency injection, dependency injection with GUICE

*****************************************************************************

--Direct constructor calls

public class RealBillingService implements BillingService {
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());
}
}
}

----------------------------------------------------
--Factories
public class CreditCardProcessorFactory {

private static CreditCardProcessor instance;

public static void setInstance(CreditCardProcessor creditCardProcessor) {
instance = creditCardProcessor;
}

public static CreditCardProcessor getInstance() {
if (instance == null) {
return new SquareCreditCardProcessor();
}
return instance;
}
}

--client code:
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());
}
}
}

--------------------------------------------------------------
-- unit test

public class RealBillingServiceTest extends TestCase {

private final PizzaOrder order = new PizzaOrder(100);
private final CreditCard creditCard = new CreditCard("1234", 11, 2010);

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, receipt.getAmountOfCharge());
assertEquals(creditCard, creditCardProcessor.getCardOfOnlyCharge());
assertEquals(100, creditCardProcessor.getAmountOfOnlyCharge());
assertTrue(transactionLog.wasSuccessLogged());
}
}

************************DEPENDENCY INJECTION*****************************************************
--DEPENDENCY INJECTION:

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());
}
}
}

--TEST
public class RealBillingServiceTest extends TestCase {

private final PizzaOrder order = new PizzaOrder(100);
private final CreditCard creditCard = new CreditCard("1234", 11, 2010);

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, receipt.getAmountOfCharge());
assertEquals(creditCard, creditCardProcessor.getCardOfOnlyCharge());
assertEquals(100, creditCardProcessor.getAmountOfOnlyCharge());
assertTrue(transactionLog.wasSuccessLogged());
}
}

public static void main(String[] args) {
CreditCardProcessor processor = new PaypalCreditCardProcessor();
TransactionLog transactionLog = new DatabaseTransactionLog();
BillingService billingService
= new RealBillingService(creditCardProcessor, transactionLog);
...
}

*****************************DEPENDENCY INEJCTION WITH GUICE************************************************

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);
}
}

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());
}
}
}

public static void main(String[] args) {
Injector injector = Guice.createInjector(new BillingModule());
BillingService billingService = injector.getInstance(BillingService.class);
...
}