Google 开源的这个库，性能快到让程序员飞起来！

Google开源的一个依赖注入类库Guice，相比于Spring IoC 来说更小更快。Elasticsearch大量使用了Guice，本文简单的介绍下Guice的基本概念和使用方式。

学习目标

• 概述：了解Guice是什么，有什么特点；

• 快速开始：通过实例了解Guice；

• 核心概念：了解Guice涉及的核心概念，如绑定（Binding）、范围（Scope）和注入（Injection）；

• 最佳实践：官方推荐的最佳实践；

Guice概述

• Guice是Google开源的依赖注入类库，通过Guice减少了对工厂方法和new的使用，使得代码更易交付、测试和重用；

• Guice可以帮助我们更好地设计API，它是个轻量级非侵入式的类库；

• Guice对开发友好，当有异常发生时能提供更多有用的信息用于分析；

快速开始

假设一个在线预订Pizza的网站，其有一个计费服务接口：

public interface BillingService {  
 /**   
  * 通过信用卡支付。无论支付成功与否都需要记录交易信息。  
  *    
  * @return 交易回执。支付成功时返回成功信息，否则记录失败原因。 
  */    
  Receipt chargeOrder(PizzaOrder order, CreditCard creditCard);  
}

使用new的方式获取信用卡支付处理器和数据库交易日志记录器：

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());    
    }   
  } 
}

使用new的问题是使得代码耦合，不易维护和测试。比如在UT里不可能直接用真实的信用卡支付，需要Mock一个CreditCardProcessor。相比于new，更容易想到的改进是使用工厂方法，但是工厂方法在测试中仍存在问题（因为通常使用全局变量来保存实例，如果在用例中未重置可能会影响其他用例）。更好的方式是通过构造方法注入依赖：

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());    
    }   
  } 
}

对于真实的网站应用可以注入真正的业务处理服务类：

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

而在测试用例中可以注入Mock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());  
  } 
}

那通过Guice怎么实现依赖注入呢？首先我们需要告诉Guice如果找到接口对应的实现类，这个可以通过模块来实现：

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); 
  }  
}

这里的模块只需要实现Module接口或继承自AbstractModule，然后在configure方法中设置绑定（后面会继续介绍）即可。然后只需在原有的构造方法中增加@Inject注解即可注入：

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，再看看main方法中是如何调用的：

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

绑定

连接绑定

连接绑定是最常用的绑定方式，它将一个类型和它的实现进行映射。下面的例子中将TransactionLog接口映射到它的实现类DatabaseTransactionLog。

public class BillingModule extends AbstractModule { 
  @Override  
  protected void configure() { 
    bind(TransactionLog.class).to(DatabaseTransactionLog.class); 
  }  
}

连接绑定还支持链式，比如下面的例子最终将TransactionLog接口映射到实现类MySqlDatabaseTransactionLog。

public class BillingModule extends AbstractModule { 
  @Override  
  protected void configure() { 
    bind(TransactionLog.class).to(DatabaseTransactionLog.class); 
    bind(DatabaseTransactionLog.class).to(MySqlDatabaseTransactionLog.class);    
  }  
}

注解绑定

通过一个类型可能存在多个实现，比如在信用卡支付处理器中存在PayPal的支付和Google支付，这样通过连接绑定就搞不定。这时我们可以通过注解绑定来实现：

@BindingAnnotation    
@Target({ FIELD, PARAMETER, METHOD })    
@Retention(RUNTIME)  
public @interface PayPal {}  
public class RealBillingService implements BillingService {    
  @Inject    
  public RealBillingService(@PayPal CreditCardProcessor processor,   
      TransactionLog transactionLog) {   
    ...  
  } 
}   
// 当注入的方法参数存在@PayPal注解时注入PayPalCreditCardProcessor实现 
bind(CreditCardProcessor.class).annotatedWith(PayPal.class).to(PayPalCreditCardProcessor.class);

可以看到在模块的绑定时用annotatedWith方法指定具体的注解来进行绑定，这种方式有一个问题就是我们必须增加自定义的注解来绑定，基于此Guice内置了一个@Named注解满足该场景：

public class RealBillingService implements BillingService {    
  @Inject    
  public RealBillingService(@Named("Checkout") CreditCardProcessor processor,   
      TransactionLog transactionLog) {   
    ...  
  }  
}    
// 当注入的方法参数存在@Named注解且值为Checkout时注入CheckoutCreditCardProcessor实现  
bind(CreditCardProcessor.class).annotatedWith(Names.named("Checkout")).to(CheckoutCreditCardProcessor.class);

实例绑定

将一个类型绑定到一个具体的实例而非实现类，这个通过是在无依赖的对象（比如值对象）中使用。如果toInstance包含复杂的逻辑会导致启动速度，此时应该通过@Provides方法绑定。

bind(String.class).annotatedWith(Names.named("JDBC URL")).toInstance("jdbc:mysql://localhost/pizza");  
bind(Integer.class).annotatedWith(Names.named("login timeout seconds")).toInstance(10);

@Provides方法绑定

模块中定义的、带有@Provides注解的、方法返回值即为绑定映射的类型。

public class BillingModule extends AbstractModule {  
  @Override  
  protected void configure() { 
    ...  
  }  
  @Provides  
  TransactionLog provideTransactionLog() {   
    DatabaseTransactionLog transactionLog = new DatabaseTransactionLog();  
    transactionLog.setJdbcUrl("jdbc:mysql://localhost/pizza");  
    transactionLog.setThreadPoolSize(30);  
    return transactionLog;    
  } 
  @Provides @PayPal 
  CreditCardProcessor providePayPalCreditCardProcessor(@Named("PayPal API key") String apiKey) {    
    PayPalCreditCardProcessor processor = new PayPalCreditCardProcessor();    
    processor.setApiKey(apiKey);    
    return processor; 
  } 
}

Provider绑定

如果使用@Provides方法绑定逻辑越来越复杂时就可以通过Provider绑定（一个实现了Provider接口的实现类）来实现。

public interface Provider<T> {   
  T get();   
}    
public class DatabaseTransactionLogProvider implements Provider<TransactionLog> {    
  private final Connection connection;   
  @Inject    
  public DatabaseTransactionLogProvider(Connection connection) { 
    this.connection = connection;  
  }  
  public TransactionLog get() { 
    DatabaseTransactionLog transactionLog = new DatabaseTransactionLog(); 
    transactionLog.setConnection(connection);   
    return transactionLog;    
  } 
}   
public class BillingModule extends AbstractModule { 
  @Override 
  protected void configure() {    
    bind(TransactionLog.class).toProvider(DatabaseTransactionLogProvider.class);    
  } 
}

无目标绑定

当我们想提供对一个具体的类给注入器时就可以采用无目标绑定。

bind(MyConcreteClass.class);    
bind(AnotherConcreteClass.class).in(Singleton.class);

构造器绑定

3.0新增的绑定，适用于第三方提供的类或者是有多个构造器参与依赖注入。通过@Provides方法可以显式调用构造器，但是这种方式有一个限制：无法给这些实例应用AOP。

public class BillingModule extends AbstractModule { 
  @Override  
  protected void configure() { 
    try {  
      bind(TransactionLog.class).toConstructor(DatabaseTransactionLog.class.getConstructor(DatabaseConnection.class));   
    } catch (NoSuchMethodException e) {    
      addError(e);   
    }    
  }  
}

范围

默认情况下，Guice每次都会返回一个新的实例，这个可以通过范围（Scope）来配置。常见的范围有单例（@Singleton）、会话（@SessionScoped）和请求（@RequestScoped），另外还可以通过自定义的范围来扩展。

范围的注解可以应该在实现类、@Provides方法中，或在绑定的时候指定（优先级最高）：

@Singleton  
public class InMemoryTransactionLog implements TransactionLog {    
  /* everything here should be threadsafe! */  
}    
// scopes apply to the binding source, not the binding target 
bind(TransactionLog.class).to(InMemoryTransactionLog.class).in(Singleton.class);  
@Provides @Singleton 
TransactionLog provideTransactionLog() { 
    ...  
}

另外，Guice还有一种特殊的单例模式叫饥饿单例（相对于懒加载单例来说）：

// Eager singletons reveal initialization problems sooner,   
// and ensure end-users get a consistent, snappy experience.   
bind(TransactionLog.class).to(InMemoryTransactionLog.class).asEagerSingleton();

注入

依赖注入的要求就是将行为和依赖分离，它建议将依赖注入而非通过工厂类的方法去查找。注入的方式通常有构造器注入、方法注入、属性注入等。

// 构造器注入  
public class RealBillingService implements BillingService { 
  private final CreditCardProcessor processorProvider;   
  private final TransactionLog transactionLogProvider;   
  @Inject    
  public RealBillingService(CreditCardProcessor processorProvider,   
      TransactionLog transactionLogProvider) {   
    this.processorProvider = processorProvider;    
    this.transactionLogProvider = transactionLogProvider;  
  } 
}   
// 方法注入  
public class PayPalCreditCardProcessor implements CreditCardProcessor {    
  private static final String DEFAULT_API_KEY = "development-use-only";   
  private String apiKey = DEFAULT_API_KEY;   
  @Inject   
  public void setApiKey(@Named("PayPal API key") String apiKey) { 
    this.apiKey = apiKey; 
  } 
}   
// 属性注入  
public class DatabaseTransactionLogProvider implements Provider<TransactionLog> {    
  @Inject Connection connection;    
  public TransactionLog get() { 
    return new DatabaseTransactionLog(connection);  
  } 
}   
// 可选注入：当找不到映射时不报错   
public class PayPalCreditCardProcessor implements CreditCardProcessor {    
  private static final String SANDBOX_API_KEY = "development-use-only";   
  private String apiKey = SANDBOX_API_KEY;   
  @Inject(optional=true)  
  public void setApiKey(@Named("PayPal API key") String apiKey) { 
    this.apiKey = apiKey; 
  } 
}

辅助注入

辅助注入（Assisted Inject）属于Guice扩展的一部分，它通过@Assisted注解自动生成工厂来加强非注入参数的使用。

// RealPayment中有两个参数startDate和amount无法直接注入   
public class RealPayment implements Payment {   
  public RealPayment(    
        CreditService creditService,  // from the Injector    
        AuthService authService,  // from the Injector 
        Date startDate, // from the instance‘s creator  
        Money amount); // from the instance‘s creator  
  }   
  ...    
}   
// 一种方式是增加一个工厂来构造    
public interface PaymentFactory {    
  public Payment create(Date startDate, Money amount);   
}   
public class RealPaymentFactory implements PaymentFactory {   
  private final Provider<CreditService> creditServiceProvider;    
  private final Provider<AuthService> authServiceProvider;    
  @Inject   
  public RealPaymentFactory(Provider<CreditService> creditServiceProvider,    
      Provider<AuthService> authServiceProvider) {    
    this.creditServiceProvider = creditServiceProvider;   
    this.authServiceProvider = authServiceProvider;   
  } 
  public Payment create(Date startDate, Money amount) {  
    return new RealPayment(creditServiceProvider.get(), 
      authServiceProvider.get(), startDate, amount);    
  } 
}   
bind(PaymentFactory.class).to(RealPaymentFactory.class);    
// 通过@Assisted注解可以减少RealPaymentFactory   
public class RealPayment implements Payment {  
  @Inject   
  public RealPayment(   
        CreditService creditService,    
        AuthService authService,    
        @Assisted Date startDate,    
        @Assisted Money amount);    
  } 
  ...   
}   
// Guice 2.0 
//bind(PaymentFactory.class).toProvider(FactoryProvider.newFactory(PaymentFactory.class, RealPayment.class)); 
// Guice 3.0  
install(new FactoryModuleBuilder().implement(Payment.class, RealPayment.class).build(PaymentFactory.class));

最佳实践

• 最小化可变性：尽可能注入的是不可变对象；

• 只注入直接依赖：不用注入一个实例来获取真正需要的实例，增加复杂性且不易测试；

• 避免循环依赖

• 避免静态状态：静态状态和可测试性就是天敌；

• 采用@Nullable：Guice默认情况下禁止注入null对象；

• 模块的处理必须要快并且无副作用

• 在Providers绑定中当心IO问题：因为Provider不检查异常、不支持超时、不支持重试；

• 不用在模块中处理分支逻辑

• 尽可能不要暴露构造器