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Introducing Spring AOP

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Introducing Spring AOP CHAPTER 5 Introducing Spring AOP Besides Dependency Injection (DI), another core feature that the Spring Framework offers is support for aspect-oriented programming (AOP. AOP is often referred to as a tool for implementing crosscutting concerns. The term crosscutting concerns refers to logic in an application that cannot be decomposed from the rest of the application and may result in code duplication and tight coupling. By using AOP for modularizing individual pieces of logic, known as concerns, you can apply these to many parts of an application without duplicating the code or creating hard dependencies. Logging and security are typical examples of crosscutting concerns that are present in many applications. Consider an application that logs the start and end of every method for debugging purposes. You will probably refactor the logging code into a special class, but you still have to call methods on that class twice per method in your application in order to perform the logging. Using AOP, you can simply specify that you want the methods on your logging class to be invoked before and after each method call in your application. It is important to understand that AOP complements object-oriented programming (OOP), rather than competing with it. OOP is very good at solving a wide variety of problems that we, as programmers, encounter. However, if you look at the logging example again, it is obvious to see where OOP is lacking when it comes to implementing crosscutting logic on a large scale. Using AOP on its own to develop an entire application is practically impossible, given that AOP functions on top of OOP. Likewise, although it is certainly possible to develop entire applications by using OOP, you can work smarter by employing AOP to solve certain problems that involve crosscutting logic. This chapter covers the following topics: • AOP basics: Before discussing Spring’s AOP implementation, we cover the basics of AOP as a technology. Most of the concepts covered in this section are not specific to Spring and can be found in any AOP implementation. If you are already familiar with another AOP implementation, feel free to skip this section. • Types of AOP: There are two distinct types of AOP: static and dynamic. In static AOP, like that provided by AspectJ’s (http://eclipse.org/aspectj/) compile-time weaving mechanisms, the crosscutting logic is applied to your code at compile time, and you cannot change it without modifying the code and recompiling. With dynamic AOP, such as Spring AOP, crosscutting logic is applied dynamically at runtime. This allows you to make changes to the AOP configuration without having to recompile the application. These types of AOP are complementary, and, when used together, they form a powerful combination that you can use in your applications. • Spring AOP architecture: Spring AOP is only a subset of the full AOP feature set found in other implementations such as AspectJ. In this section, we take a high-level look at which features are present in Spring, how they are implemented, and why some features are excluded from the Spring implementation. 161 CHAPTER 5 ■ INTRODUCING SPRING AOP • Proxies\ in Spring AOP: Proxies are a huge part of how Spring AOP works, and you must understand them to get the most out of Spring AOP. In this section, we look at the two kinds of proxy: the JDK dynamic proxy and the CGLIB proxy. In particular, we look at the different scenarios in which Spring uses each proxy, the performance of the two proxy types, and some simple guidelines to follow in your application to get the most from Spring AOP. • Using Spring AOP: In this section, we present some practical examples of AOP usage. We start off with a simple “Hello World” example to ease you into Spring’s AOP code, and we continue with a detailed description of the AOP features that are available in Spring, complete with examples. • Advanced use of pointcuts: We explore the ComposablePointcut and ControlFlowPointcut classes, introductions, and the appropriate techniques you should employ when using pointcuts in your application. • AOP framework services: The Spring Framework fully supports configuring AOP transparently and declaratively. We look at three ways (the ProxyFactoryBean class, the aop namespace, and @AspectJ-style annotations) to inject declaratively defined AOP proxies into your application objects as collaborators, thus making your application completely unaware that it is working with advised objects. • Integrating AspectJ: AspectJ is a fully featured AOP implementation. The main difference between AspectJ and Spring AOP is that AspectJ applies advice to target objects via weaving (either compile-time or load-time weaving), while Spring AOP is based on proxies. The feature set of AspectJ is much greater than that of Spring AOP, but it is much more complicated to use than Spring. AspectJ is a good solution when you find that Spring AOP lacks a feature you need. AOP Concepts As with most technologies, AOP comes with its own specific set of concepts and terms, and it’s important to understand what they mean. The following are the core concepts of AOP: • Joinpoints: A joinpoint is a well-defined point during the execution of your application. Typical examples of joinpoints include a call to a method, the method invocation itself, class initialization, and object instantiation. Joinpoints are a core concept of AOP and define the points in your application at which you can insert additional logic using AOP. • Advice: The code that is executed at a particular joinpoint is the advice, defined by a method in your class. There are many types of advice, such as before, which executes before the joinpoint, and after, which executes after it. • Pointcuts: A pointcut is a collection of joinpoints that you use to define when advice should be executed. By creating pointcuts, you gain fine-grained control over how you apply advice to the components in your application. As mentioned previously, a typical joinpoint is a method invocation, or the collection of all method invocations in a particular class. Often you can compose pointcuts in complex relationships to further constrain when advice is executed. • Aspects: An aspect is the combination of advice and pointcuts encapsulated in a class. This combination results in a definition of the logic that should be included in the application and where it should execute. 162 CHAPTER 5 ■ INTRODUCING SPRING AOP • Weaving: This is the process of inserting aspects into the application code at the appropriate point. For compile-time AOP solutions, this weaving is generally done at build time. Likewise, for runtime AOP solutions, the weaving process is executed dynamically at runtime. AspectJ supports another weaving mechanism called load-time weaving (LTW), in which it intercepts the underlying JVM class loader and provides weaving to the bytecode when it is being loaded by the class loader. • Target: An object whose execution flow is modified by an AOP process is referred to as the target object. Often you see the target object referred to as the advised object. • Introduction: This is the process by which you can modify the structure of an object by introducing additional methods or fields to it. You can use introduction AOP to make any object implement a specific interface without needing the object’s class to implement that interface explicitly. Don’t worry if you find these concepts confusing; this will all become clear when you see some examples. Also, be aware that you are shielded from many of these concepts in Spring AOP, and some are not relevant because of Spring’s choice of implementation. We will discuss each of these features in the context of Spring as we progress through the chapter. Types of AOP As we mentioned earlier, there are two distinct types of AOP: static and dynamic. The difference between them is really the point at which the weaving process occurs and how this process is achieved. Using Static AOP In static AOP, the weaving process forms another step in the build process for an application. In Java terms, you achieve the weaving process in a static AOP implementation by modifying the actual bytecode of your application, changing and extending the application code as necessary. This is a well-performing way of achieving the weaving process because the end result is just Java bytecode, and you do not perform any special tricks at runtime to determine when advice should be executed. The drawback of this mechanism is that any modifications you make to the aspects, even if you simply want to add another joinpoint, require you to recompile the entire application. AspectJ’s compile-time weaving is an excellent example of a static AOP implementation. Using Dynamic AOP Dynamic AOP implementations, such as Spring AOP, differ from static AOP implementations in that the weaving process is performed dynamically at runtime. How this is achieved is implementation-dependent, but as you will see, Spring’s approach is to create proxies for all advised objects, allowing for advice to be invoked as required. The drawback of dynamic AOP is that, typically, it does not perform as well as static AOP, but the performance is steadily increasing. The major benefit of dynamic AOP implementations is the ease with which you can modify the entire aspect set of an application without needing to recompile the main application code. Choosing an AOP Type Choosing whether to use static or dynamic AOP is quite a hard decision. Both have their own benefits, and you are not restricted to using only one type. In general, static AOP implementations have been around longer and tend to have more feature-rich implementations, with a greater number of available joinpoints.
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