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# guice

Dependency Injection by Google for Java

## Before DI

Need to wire everything together

```java
public class RealBillingService implements BillingService {
  public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {
    CreditCardProcessor processor = new PaypalCreditCardProcessor();
    TransactionLog transactionLog = new DatabaseTransactionLog();
```

This relies on actual implementations of creditcardprocesser making testing difficult as well as switching implementations

### Factories?

```java
public class CreditCardProcessorFactory {

  private static CreditCardProcessor instance;

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

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

    return instance;
  }
}
```

```java
//so
new PaypalCreditCardProcessor();
//becomes
CreditCardProcessorFactory.getInstance();
```

Now can do proper tests, but need to setup and teardown and dependencies setup as needed per inner functionfunction extractMessagesFromJade(filePath, baseDirectory, callback) { fs.readFile(filePath, (err, content) => { if (err) { callback(err); } else { let messages = \[]; const jadeOptions = { compiler: yudi, filename: filePath, basedir: baseDirectory, attrs: JADE\_ATTRS, postCompile: (compiler) => { messages = compiler.strings; }, }; try { jade.compileClient(content, jadeOptions); } catch (e) { callback(e); } callback(null, { messages: cleanMessages(messages), }); } }); }

## DI

Put in constructor of object

## Guice

Map interfaces to implementations in `Module`

```java
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);
  }
}
```

add `@Inject` to constructor and directs Guice to use it by looking up values for each parameter