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程序员的量化交易之路(12)--Guice库的学习

时间:2015-06-02 17:48:13      阅读:133      评论:0      收藏:0      [点我收藏+]

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Google Guice学习
在学习Cointrader的代码时候,要用到Google的guice用于依赖注入(Dependence Injection)。所以,在这里系统的学习一遍。这里我主要是对其官方文档进行有选择性的学习翻译。

动机

写类对象之间的组合关系是非常麻烦的一件事情。这里以一个网站订披萨作为案例:
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);
}


直接用构造函数调用



public class RealBillingService implements BillingService {
  public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {
    CreditCardProcessor processor = new PaypalCreditCardProcessor();//
    TransactionLog transactionLog = new DatabaseTransactionLog();//这两个直接在构造函数里面构造,不利于模块化和单元测试


    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());
    }
  }
}

工厂方法

工厂方法能够解耦一个接口的使用者和接口的实现者之间的关系。
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;
  }
}
在使用者这边,我们就可以替换掉new语句:
public class RealBillingService implements BillingService {
  public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {
    CreditCardProcessor processor = CreditCardProcessorFactory.getInstance();
    TransactionLog transactionLog = TransactionLogFactory.getInstance();//工厂方法


    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());
    }
  }
}
这种工厂方法使得我们可以方便地写单元测试:
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 processor = new FakeCreditCardProcessor();


  @Override public void setUp() {//设置
    TransactionLogFactory.setInstance(transactionLog);
    CreditCardProcessorFactory.setInstance(processor);
  }


  @Override public void tearDown() {//重置
    TransactionLogFactory.setInstance(null);
    CreditCardProcessorFactory.setInstance(null);
  }


  public void testSuccessfulCharge() {//测试
    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());
  }
}

依赖注入

和工厂方法一样,依赖注入也是一种代码实现上的设计模式,它的原则是“separate behavior from dependency resolution”。
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());
    }
  }
}


这时候,不需要任何工厂类:
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());
  }
}


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


使用guice进行依赖注入

首先需要通过Guice的module来配置接口和实现之间的映射。
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);
  }
}


我们在RealBillingService的构造函数上添加@Inject标注,它会引导Guice去使用module。


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());
    }
  }
}


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

程序员的量化交易之路(12)--Guice库的学习

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原文地址:http://blog.csdn.net/minimicall/article/details/46332917

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