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Introducing AspectJ
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This chapter covers
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AspectJ language overview AspectJ Hello, world! The join point model The aspect construct
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AspectJ: a bird s eye view
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AspectJ is a general-purpose, aspect-oriented extension to the Java programming language. Given that AspectJ is an extension to Java, every valid Java program is also a valid AspectJ program. An AspectJ compiler produces class files that conform to the Java byte-code specification, allowing any compliant Java virtual machine (VM) to execute those class files. By using Java as the base language, AspectJ passes on all the benefits of Java and makes it easy for Java programmers to understand the AspectJ language. AspectJ consists of two parts: the language specification and the language implementation. The language specification part defines the language in which you write the code; with AspectJ, you implement the core concerns using the Java programming language, and you use the extensions provided by AspectJ to implement the weaving of crosscutting concerns. The language implementation part provides tools for compiling, debugging, and integrating with popular integrated development environments (IDEs). In this chapter, we introduce you to the core concepts that will get you started with AspectJ. The next chapter will delve more deeply into the syntax of AspectJ. Together, the two chapters should give you enough information to start writing simple code in order to see the benefits that AspectJ offers. 4 will introduce more advanced concepts. These three chapters also serve as reference material for part 2 of this book.
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2.1 AspectJ: a bird s eye view
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In chapter 1, we introduced the AOP concept of weaving the crosscutting concerns into the core logic using weaving rules. Weaving rules specify what action to perform when certain points in the execution of the program are encountered. In the AspectJ implementation of AOP, the AspectJ compiler uses the modules containing the weaving rules, which address the crosscutting concerns, to add new behavior into the modules that address the core concerns all without making any modifications to the core modules source code; the weaving occurs only in the byte code that the compiler produces. 2.1.1 Crosscutting in AspectJ In AspectJ, the implementation of the weaving rules by the compiler is called crosscutting; the weaving rules cut across multiple modules in a systematic way in order to modularize the crosscutting concerns. AspectJ defines two types of crosscutting: static crosscutting and dynamic crosscutting.
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Introducing AspectJ
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Dynamic crosscutting Dynamic crosscutting is the weaving of new behavior into the execution of a program. Most of the crosscutting that happens in AspectJ is dynamic. Dynamic crosscutting augments or even replaces the core program execution flow in a way that cuts across modules, thus modifying the system behavior. For example, if you want to specify that a certain action be executed before the execution of certain methods or exception handlers in a set of classes, you can just specify the weaving points and the action to take upon reaching those points in a separate module. Static crosscutting Static crosscutting is the weaving of modifications into the static structure the classes, interfaces, and aspects of the system. By itself, it does not modify the execution behavior of the system. The most common function of static crosscutting is to support the implementation of dynamic crosscutting. For instance, you may want to add new data and methods to classes and interfaces in order to define class-specific states and behaviors that can be used in dynamic crosscutting actions. Another use of static crosscutting is to declare compile-time warnings and errors across multiple modules. 2.1.2 Crosscutting elements AspectJ uses extensions to the Java programming language to specify the weaving rules for the dynamic and static crosscutting. The extensions are designed in such a way that a Java programmer should feel at home while using them. The AspectJ extensions use the following constructs to specify the weaving rules programmatically; they are the building blocks that form the modules that express the crosscutting concern s implementation. While we introduce them in this section, each construct will be discussed in depth in the following sections of this and the next chapter. Join point A join point is an identifiable point in the execution of a program. It could be a call to a method or an assignment to a member of an object. In AspectJ, everything revolves around join points, since they are the places where the crosscutting actions are woven in. Let s look at some join points in this code snippet:
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