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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 大量应用了 Guice，本文简略的介绍下 Guice 的基本概念和应用形式。
学习指标
概述：理解 Guice 是什么，有什么特点；
疾速开始：通过实例理解 Guice；
外围概念：理解 Guice 波及的外围概念，如绑定（Binding）、范畴（Scope）和注入（Injection）；
最佳实际：官网举荐的最佳实际；
Guice 概述
Guice 是 Google 开源的依赖注入类库，通过 Guice 缩小了对工厂办法和 new 的应用，使得代码更易交付、测试和重用；
Guice 能够帮忙咱们更好地设计 API，它是个轻量级非侵入式的类库；
Guice 对开发敌对，当有异样产生时能提供更多有用的信息用于剖析；
疾速开始
假如一个在线预订 Pizza 的网站，其有一个计费服务接口：
public interface BillingService {
/**
* 通过信用卡领取。无论领取胜利与否都须要记录交易信息。
*
* @return 交易回执。领取胜利时返回胜利信息，否则记录失败起因。
*/
Receipt chargeOrder(PizzaOrder order, CreditCard creditCard);
}
应用 new 的形式获取信用卡领取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());
}
}
}
应用 new 的问题是使得代码耦合，不易保护和测试。比方在 UT 里不可能间接用实在的信用卡领取，须要 Mock 一个 CreditCardProcessor。相比于 new，更容易想到的改良是应用工厂办法，然而工厂办法在测试中仍存在问题（因为通常应用全局变量来保留实例，如果在用例中未重置可能会影响其余用例）。更好的形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());
}
}
}
对于实在的网站利用能够注入真正的业务解决服务类：
public static void main(String[] args) {
CreditCardProcessor processor = new PaypalCreditCardProcessor();
TransactionLog transactionLog = new DatabaseTransactionLog();
BillingService billingService
= new RealBillingService(processor, transactionLog);
…
}
而在测试用例中能够注入 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？首先咱们须要通知 Guice 如果找到接口对应的实现类，这个能够通过模块 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);
}
}
这里的模块只须要实现 Module 接口或继承自 AbstractModule，而后在 configure 办法中设置绑定（前面会持续介绍）即可。而后只需在原有的构造方法中减少 @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());
}
}
}
最初，再看看 main 办法中是如何调用的：
public static void main(String[] args) {
Injector injector = Guice.createInjector(new BillingModule());
BillingService billingService = injector.getInstance(BillingService.class);
…
}
绑定
连贯绑定
连贯绑定是最罕用的绑定形式，它将一个类型和它的实现进行映射。上面的例子中将 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);
}
}
连贯绑定还反对链式，比方上面的例子最终将 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，比方在信用卡领取处理器中存在 PayPal 的领取和 Google 领取，这样通过连贯绑定就搞不定。这时咱们能够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) {
…
}
}
// 当注入的办法参数存在 @PayPal 注解时注入 PayPalCreditCardProcessor 实现
bind(CreditCardProcessor.class).annotatedWith(PayPal.class).to(PayPalCreditCardProcessor.class);
能够看到在模块的绑定时用 annotatedWith 办法指定具体的注解来进行绑定，这种形式有一个问题就是咱们必须减少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) {
…
}
}
// 当注入的办法参数存在 @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（比方值对象）中应用。如果 toInstance 蕴含简单的逻辑会导致启动速度，此时应该通过 @Provides 办法绑定。
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 办法绑定
模块中定义的、带有 @Provides 注解的、办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
如果应用 @Provides 办法绑定逻辑越来越简单时就能够通过 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);
}
}
无指标绑定
当咱们想提供对一个具体的类给注入器时就能够采纳无指标绑定。
bind(MyConcreteClass.class);
bind(AnotherConcreteClass.class).in(Singleton.class);
结构器绑定
3.0 新增的绑定，实用于第三方提供的类或者是有多个结构器参加依赖注入。通过 @Provides 办法能够显式调用结构器，然而这种形式有一个限度：无奈给这些实例利用 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);
}
}
}
范畴
默认状况下，Guice 每次都会返回一个新的实例，这个能够通过范畴（Scope）来配置。常见的范畴有单例（@Singleton）、会话（@SessionScoped）和申请（@RequestScoped），另外还能够通过自定义的范畴来扩大。
范畴的注解能够应该在实现类、@Provides 办法中，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 还有一种非凡的单例模式叫饥饿单例（绝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拆散，它倡议将依赖注入而非通过工厂类的办法去查找。注入的形式通常有结构器注入、办法注入、属性注入等。
// 结构器注入
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;
}
}
// 办法注入
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 扩大的一部分，它通过 @Assisted 注解主动生成工厂来增强非注入参数的应用。
// RealPayment 中有两个参数 startDate 和 amount 无奈间接注入
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
}
…
}
// 一种形式是减少一个工厂来结构
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 注解能够缩小 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));
最佳实际
最小化可变性：尽可能注入的是不可变对象；
只注入间接依赖：不必注入一个实例来获取真正须要的实例，减少复杂性且不易测试；
防止循环依赖
防止动态状态：动态状态和可测试性就是天敌；
采纳 @Nullable：Guice 默认状况下禁止注入 null 对象；
模块的解决必须要快并且无副作用
在 Providers 绑定中当心 IO 问题：因为 Provider 不查看异样、不反对超时、不反对重试；
不必在模块中解决分支逻辑
尽可能不要裸露结构器
起源：zhuanlan.zhihu.com/p/24924391
