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5. Aspect Oriented Programming with Spring

5.1. AOP Concepts

让我们从定义一些核心 AOP 概念和术语开始。 这些术语不是 Spring 特定的。 不幸的是，AOP 术语并不是特别直观。 但是，如果 Spring 使用它自己的术语，那就更令人困惑了。

Let us begin by defining some central AOP concepts and terminology. These terms are not Spring-specific. Unfortunately, AOP terminology is not particularly intuitive. However, it would be even more confusing if Spring used its own terminology.

  • 面向方面编程 (AOP) 通过提供另一种思考程序结构的方式来补充面向对象编程 (OOP)。 OOP 中模块化的关键单元是类，而 AOP 中模块化的单元是方面。 方面支持跨多种类型和对象的关注点（例如事务管理）的模块化。 （这种关注点在 AOP 文献中通常被称为“横切”关注点。）

​ Aspect-oriented Programming (AOP) complements Object-oriented Programming (OOP) by providing another way of thinking about program structure. The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect. Aspects enable the modularization of concerns (such as transaction management) that cut across multiple types and objects. (Such concerns are often termed “crosscutting” concerns in AOP literature.)

  • 方面/切面：跨多个类的关注点的模块化。事务管理是企业 Java 应用程序中横切关注点的一个很好的例子。在 Spring AOP 中，方面是通过使用常规类（基于模式的方法）或使用 @Aspect 注释（@AspectJ 样式）注释的常规类来实现的。

​ Aspect: A modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in enterprise Java applications. In Spring AOP, aspects are implemented by using regular classes (the schema-based approach) or regular classes annotated with the @Aspect annotation (the @AspectJ style).

  • 连接点：程序执行过程中的一个点，例如方法的执行或异常的处理。在 Spring AOP 中，一个连接点总是代表一个方法执行。

​ Join point: A point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.

  • 建议：方面在特定连接点采取的行动。不同类型的建议包括“周围”、“之前”和“之后”建议。 （通知类型将在后面讨论。）许多 AOP 框架，包括 Spring，将通知建模为拦截器，并在连接点周围维护一个拦截器链。

​ Advice: Action taken by an aspect at a particular join point. Different types of advice include “around”, “before” and “after” advice. (Advice types are discussed later.) Many AOP frameworks, including Spring, model an advice as an interceptor and maintain a chain of interceptors around the join point.

  • 切入点：匹配连接点的谓词。 Advice 与切入点表达式相关联，并在与切入点匹配的任何连接点处运行（例如，执行具有特定名称的方法）。切入点表达式匹配的连接点的概念是 AOP 的核心，Spring 默认使用 AspectJ 切入点表达式语言。

​ Pointcut: A predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name). The concept of join points as matched by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut expression language by default.

  • Introduction：代表一个类型声明额外的方法或字段。 Spring AOP 允许您向任何建议的对象引入新接口（和相应的实现）。例如，您可以使用介绍使 bean 实现 IsModified 接口，以简化缓存。 （介绍在 AspectJ 社区中称为类型间声明。）

​ Introduction: Declaring additional methods or fields on behalf of a type. Spring AOP lets you introduce new interfaces (and a corresponding implementation) to any advised object. For example, you could use an introduction to make a bean implement an IsModified interface, to simplify caching. (An introduction is known as an inter-type declaration in the AspectJ community.)

  • 目标对象：一个或多个方面建议的对象。也称为“建议对象”。由于 Spring AOP 是使用运行时代理实现的，因此该对象始终是代理对象。

​ Target object: An object being advised by one or more aspects. Also referred to as the “advised object”. Since Spring AOP is implemented by using runtime proxies, this object is always a proxied object.

  • AOP 代理：由 AOP 框架创建的对象，用于实现方面协定（建议方法执行等）。 在 Spring Framework 中，AOP 代理是 JDK 动态代理或 CGLIB 代理。

​ AOP proxy: An object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). In the Spring Framework, an AOP proxy is a JDK dynamic proxy or a CGLIB proxy.

  • 编织：将方面与其他应用程序类型或对象链接以创建建议对象。 这可以在编译时（例如，使用 AspectJ 编译器）、加载时或运行时完成。 Spring AOP 与其他纯 Java AOP 框架一样，在运行时执行编织。

​ Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.

5.4.1. Enabling @AspectJ Support

通过自动代理，我们的意思是，如果 Spring 确定一个 bean 由一个或多个方面通知，它会自动为该 bean 生成一个代理来拦截方法调用并确保根据需要运行通知。

By auto-proxying, we mean that, if Spring determines that a bean is advised by one or more aspects, it automatically generates a proxy for that bean to intercept method invocations and ensures that advice is run as needed.