Guice是Google开源的一个依赖注入类库，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
