使用Guice框架的動機
在應用中組裝各個封裝好的類，有時候是一件很乏味的事情。有幾種辦法可以把資料層、業務層、表現層的程式碼整合在一起。下面通過一個線上披薩下訂單的業務來對比這幾種實現方法。
// 定義下訂單介面
public interface BillingService {
/**
* Attempts to charge the order to the credit card. Both successful and
* failed transactions will be recorded.
*
* @return a receipt of the transaction. If the charge was successful, the
* receipt will be successful. Otherwise, the receipt will contain a
* decline note describing why the charge failed.
*/
Receipt chargeOrder(PizzaOrder order, CreditCard creditCard);
}
複製程式碼我們需要為下訂單實現類寫單元測試，這裡為了避免真正的支付，我們需要定義一個FakeCreditCardProcessor類
直接在構造方法中呼叫依賴的類
下面示例程式碼中，直接在構造方法中
new信用卡處理類CreditCardProcessor跟事務日誌處理類TransactionLog
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());
}
}
}
複製程式碼上面程式碼的主要問題是沒有進行模組化，也不好測試。不然，你測試的時候，真的要從你的信用卡扣費了。而且，也很難測試支付失敗，或者支付閘道器服務不可用的情況。
使用工廠方法
工廠方法解耦了呼叫類跟實現類之間的耦合。一般工廠方法使用靜態的set跟get方法來設定跟獲取實現類。如下面的示例程式碼：
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;
}
}
複製程式碼在下面的示例程式碼中，我們用getInstance來代替new來獲取相關物件。
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());
}
}
}
複製程式碼用工廠方法，我們可以對支付的流程進行單元測試。
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 processor = new FakeCreditCardProcessor();
@Override public void setUp() {
TransactionLogFactory.setInstance(transactionLog);
CreditCardProcessorFactory.setInstance(processor);
}
@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, processor.getCardOfOnlyCharge());
assertEquals(100, processor.getAmountOfOnlyCharge());
assertTrue(transactionLog.wasSuccessLogged());
}
}
複製程式碼上面程式碼看起來也很笨拙。因為使用全域性變數來儲存模擬支付類FakeCreditCardProcessor的例項，需要在teardown對於全域性變數進行釋放。如果teardown執行失敗，而且後面的測試也用到了這個變數，會對後面的測試造成影響。同樣，由於全域性變數的汙染，也無法進行並行測試。
最嚴重的問題是，所有的依賴都隱藏碼在程式碼中了。比如，在CreditCardFraudTracker類中增加了一個依賴，所有的單元測試都要跑一遍，來看一下哪個測試方法沒有通過。
我們也很難知道一個工廠方法是否初始化，除非哪天被呼叫到了。
雖然QA跟充分的驗收測試能解決這些問題，但是我們肯定有更好的辦法來處理這個問題。
依賴注入
跟工廠模式一樣，依賴注入也是一種設計模式。依賴注入的核心原則是：把使用依賴跟查詢依賴分離。就像下面的例子，RealBillingService並不負責查詢TransactionLog 和 CreditCardProcessor，而是由使用者傳入到對應的建構函式中。
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());
}
}
}
複製程式碼這樣我們就不需要使用工廠，如下面的程式碼，我們可以把setUp跟tearDown去掉。
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 processor = new FakeCreditCardProcessor();
public void testSuccessfulCharge() {
RealBillingService billingService
= new RealBillingService(processor, transactionLog);
Receipt receipt = billingService.chargeOrder(order, creditCard);
assertTrue(receipt.hasSuccessfulCharge());
assertEquals(100, receipt.getAmountOfCharge());
assertEquals(creditCard, processor.getCardOfOnlyCharge());
assertEquals(100, processor.getAmountOfOnlyCharge());
assertTrue(transactionLog.wasSuccessLogged());
}
}
複製程式碼如果我們在RealBillingService增加依賴，編譯器會提醒我們哪個測試方法需要被修復了。
但是現在BillingService的使用者需要知道它的依賴，並在構造方法中傳入這些依賴，通常在一個入口類傳入。
public static void main(String[] args) {
CreditCardProcessor processor = new PaypalCreditCardProcessor();
TransactionLog transactionLog = new DatabaseTransactionLog();
BillingService billingService
= new RealBillingService(processor, transactionLog);
...
}
複製程式碼使用Guice來實現依賴注入
依賴注入讓設計模式讓程式碼可以模組化、可測試化，Guice讓依賴注入的程式碼更容易書寫。
在上面的例子中使用Guice的話，我們首先要告訴Guice怎麼通過介面找到它的實現類。我們通過一個實現了Guice Module介面的配置類來完成這個工作。
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);
}
}
複製程式碼我們通過新增@Inject到RealBillingService的構造方法，讓Guice能夠找到它的依賴。
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());
}
}
}
複製程式碼最後，可以使用Injector去幫我們找到任何一個繫結過的類的例項。
public static void main(String[] args) {
Injector injector = Guice.createInjector(new BillingModule());
BillingService billingService = injector.getInstance(BillingService.class);
...
}
複製程式碼