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Google 开源的依赖注入库,比 Spring 更小更快!

号外:限时领取:2020,全网最新、最全的技术知识体系

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

1、概述:了解Guice是什么,有什么特点;
2、快速开始:通过实例了解Guice;
3、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);
4、最佳实践:官方推荐的最佳实践;

Guice概述

1、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;
2、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;
3、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。整编:微信公众号,搜云库技术团队,ID:souyunku

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。整编:微信公众号,搜云库技术团队,ID:souyunku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。整编:微信公众号,搜云库技术团队,ID:souyunku

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

1、最小化可变性:尽可能注入的是不可变对象;
2、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;
3、避免循环依赖
4、避免静态状态:静态状态和可测试性就是天敌;
5、采用@Nullable:Guice默认情况下禁止注入null对象;
6、模块的处理必须要快并且无副作用
7、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;
8、不用在模块中处理分支逻辑
9、尽可能不要暴露构造器

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