Unfortunately, now the clients of BillingService need to lookup its dependencies. We can fix
some of these by applying the pattern again! Classes that depend on it can accept a
BillingService in their constructor. For top-level classes, it's useful to have a framework.
Otherwise you'll need to construct dependencies recursively when you need to use a service:
public static void main(String[] args) {
CreditCardProcessor processor = new PaypalCreditCardProcessor();
TransactionLog transactionLog = new DatabaseTransactionLog();
BillingService billingService
= new RealBillingService(creditCardProcessor, transactionLog);
...
}
Dependency Injection with Guice
All implementation instances are created by Injector and configured in Modules or by
Annotations.
The dependency injection pattern leads to code that's modular and testable, and Guice makes it
easy to write. To use Guice in our billing example, we first need to tell it how to map our
interfaces to their implementations. This configuration is done in a Guice module, which is any
Java class that implements the Module interface:
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);
}
}
We add @Inject to RealBillingService's constructor, which directs Guice to use it. Guice
will inspect the annotated constructor, and lookup values for each of parameter.
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);
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