来自| 开发者技术前线 编辑 | 可可
作者:GinoBeFunny
来源:https://url.cn/5cb6Lkw
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
END
原文地址:https://www.cnblogs.com/jpfss/p/12096764.html