Google Guice之牛刀小试

Google Guice由google推出的一开源软件，是超轻量级的，下一代的，为Java 5及后续版本设计的依赖注入容器，其功能类似于如日中天的Spring。

下面我们就来了解一下Guice，在此之前，先看一个官方例子：在应用程序中，要把所有的东西装配起来是一件很乏味的事件，这要涉及到连接数据，服务，表现层类等方面，这是一个比萨饼订购网站的计费代码例子用于这些方面的对比。

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);
}

BillingService的实现类，我们会用单元测试进行测试，剩下的我们需要一个FakeCreditCardProcessor来避免其直接与CreditCard打交道，这是面向对象中封装的表现。

第一种实现方式：直接调用构造方法

public class RealBillingService implements BillingService {
  public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {
    CreditCardProcessor processor = new PaypalCreditCardProcessor();//构造方法创建CreditCardProcessor
    TransactionLog transactionLog = new DatabaseTransactionLog();//构造方法创建TransactionLog对象

    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());
    }
  }
}

这样的代码缺乏模块性与可测试性，因为这在编译期就直接依赖了CreditCardProcessor实现类，耦合性太强。

第二种实现方式：使用工厂模式：

使用一个工厂类可以使客户端与实现解耦，一个简单工厂使用一静态方法来获取或设置接口实现，下面是一样版：

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;
  }
}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;
  }
}

在客户端代码中，只需要使用工厂类把new关键字替换就行了：

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());
    }
  }
}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());
    }
  }
}

在使用了工厂模式后的单元测试：在使用了工厂模式后的单元测试：

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());
  }
}

这样代码还是有点笨拙，一个全局变量保存了实现实例，这样我们要非常小心该变量的赋值与值释放，如果tailDown方法失败了，全局变量仍然有效，这可能就会给其它的测试带来问题，这样还不能并行运行多个测试用例。最大的问题在于，随着应用的扩大，有新的依赖的时候就会出现越来越多的工厂类，使应用效率下降。

第三种方式：依赖注入

像工厂模式一样，依赖注入也是一种设计模式，其主要原则是将行为与依赖分离开来，在上面的例子中RealBillingService不负责TransactionLog与CreditCardProcessor对象的创建，换之的是这两个对象在RealBillingService的构造方法参数中传递进来。

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());
    }
  }
}

这样，我们不需要任何的工厂类，还可以移除setUp与tearDown方法来简化单元测试：

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());
  }
}

现在不幸的是，BillingService的客户端需要创建它的依赖，现在最好是有一框架来自动创建这些依赖，不然我们就要手动地去创建这些循环依赖。

现在到Guice出场的时候，使用Guice进行依赖注入。

依赖注入模式可以让代码更具模块性，更易于测试，而且Guice使其易于编写。在上面的计费例子中，我们第一步要告诉Guice怎么映射接口与实现类，这是通过Guice的Module进行配置的，它可以是任何一个实现了Module接口的Java类。

public class BillingModule extends AbstractModule {
  @Override 
  protected void configure() {
    bind(TransactionLog.class).to(DatabaseTransactionLog.class);//将接口与实现进行映射绑定
    bind(CreditCardProcessor.class).to(PaypalCreditCardProcessor.class);
    bind(BillingService.class).to(RealBillingService.class);
  }
}

当进行依赖注入的时候，对象在它们的构造参数中接收依赖。要创建一个对象，必须先创建出它的依赖，但是要创建每一个依赖，就要创建依赖的每一个依赖，如此往复。所以当你创建一个对象的时候真正要创建的是一张对象图。手动创建一张对象图是费时费力的，趋于错误的，而且使测试变得困难。好在Guice可以为我们创建这张对象图，而我们要做的就是进行配置告诉它如果去准确地创建这张对象图。

在RealBillingService的构造方法中添加@Inject注解，Guice会检查添加了注解的构造方法，并为每一个参数查找值。添加@Inject注解就是在进行配置式作，告诉Guice如果创建对象图，当然@Inject注解不仅可以放置于构造方法上，也可以放置于setter方法与字段上。

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());
    }
  }
}

最后，我们将这些整合在一起如下，Injector类用于获取任何绑定类的实例：

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

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