Google Guice 3.0编程指南：接口与依赖注入实践

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"Guice 3.0编程指南是由Google提供的一个重要的IoC (Inversion of Control)容器框架，主要用于简化面向接口编程，减少对具体实现的依赖。该指南的核心理念是将软件设计聚焦在接口上，确保代码的清晰性和可测试性。接口定义了业务逻辑的流程，而Guice通过依赖注入（Dependency Injection，DI）机制来管理这些接口与其实现之间的关系，减少了手动配置和硬编码的复杂性。在编程实践中，一个典型的例子是为一个披萨订购网站编写计费服务接口。该接口`BillingService`定义了一个方法`chargeOrder`，它接受一个`PizzaOrder`对象和一个`CreditCard`对象，尝试进行支付并返回交易收据。这个接口使得代码更加模块化，每个组件明确地知道自己需要什么，而不需要关心具体的实现细节。为了测试这个接口，开发者需要编写单元测试，其中可能使用到模拟对象，如`FakeCreditCardProcessor`，以避免在测试阶段真的进行信用卡收费操作。这有助于隔离组件间的依赖，确保每个部分都能独立地进行验证，同时也方便在开发过程中快速迭代和修改。 Guice 3.0提供了一套强大的工具来管理依赖关系，包括自动装配（Automatic Injection）、生命周期管理和模块化设计。通过使用Guice，开发者可以更专注于业务逻辑，而不是底层的连接和配置工作，从而提高开发效率和代码质量。此外，由于接口和依赖关系的分离，代码的复用性更高，维护也更为简单。 Guice 3.0编程指南是构建模块化、可测试和可扩展应用的理想工具，尤其适合那些追求高内聚低耦合原则的项目。通过遵循其原则和最佳实践，开发者可以更好地利用面向接口编程和依赖注入的优势，提升软件工程的质量和灵活性。"

Unfortunately, now the clients of BillingService need to lookup its dependencies. We can fix

some of these by applying the pattern again! Classes that depend on it can accept a

BillingService in their constructor. For top-level classes, it's useful to have a framework.

Otherwise you'll need to construct dependencies recursively when you need to use a service:

public static void main(String[] args) {

CreditCardProcessor processor = new PaypalCreditCardProcessor();

TransactionLog transactionLog = new DatabaseTransactionLog();

BillingService billingService

= new RealBillingService(creditCardProcessor, transactionLog);

...

}

Dependency Injection with Guice

All implementation instances are created by Injector and configured in Modules or by

Annotations.

The dependency injection pattern leads to code that's modular and testable, and Guice makes it

easy to write. To use Guice in our billing example, we first need to tell it how to map our

interfaces to their implementations. This configuration is done in a Guice module, which is any

Java class that implements the Module interface:

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);

}

We add @Inject to RealBillingService's constructor, which directs Guice to use it. Guice

will inspect the annotated constructor, and lookup values for each of parameter.

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);

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