搜云库技术团队
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
