Aspects An aspect is another term for a cross-cutting concern.
In aspect orientation, the aspects provide a mechanism by which a cross-cutting concern can be specified in a modular way. To fully harness the power of aspects, we need to have some basic concepts in place to allow us to specify and apply aspects in a generic manner. These constructs are advice, join points, and pointcuts. Advice The code that is executed when an aspect is invoked is called advice. Advice contains its own set of rules as to when it is to be invoked in relation to the join point that has been triggered.
Chapter 13 deals directly with recipes for different forms of advice and shows some of the more advanced features of advice that are available within AspectJ, such as precedence between multiple advices. Join Points Join points are simply specific points within the application that may or may not invoke some advice.
The specific set of available join points is dependent on the tools being used and the programming language of the application under development. They encapsulate the decision-making logic that is evaluated to decide if a particular piece of advice should be invoked when a join point is encountered.
The concept of a pointcut is crucial to the aspect-oriented approach because it provides an abstract mechanism by which to specify an interest in a selection of join points without having to tie to the specifics of what join points are in a particular application. How to define and use pointcuts is shown in the recipes found in Chapter 4 though Chapter Putting It All Together Figure shows the relationships between join points, aspects, pointcuts, advice, and your application classes.
Figure The relationships between apects, pointcuts, and advice 1. Chapter 2. Installing AspectJ Recipe 2. Developing a Simple Aspect Recipe 2. Weaving Aspects into Jars Recipe 2. Weaving Aspects at Load Time Recipe 2. Generating Javadoc Documentation Recipe 2.
An LALR(1) grammar for the AspectJ Programming Language
Beginning with the task of getting a build environment set up and moving through the different types of builds and targets, this chapter provides a tutorial in the basics of AspectJ development. With an understanding of the fundamental concepts in aspect orientation, this chapter first covers getting your fingers dirty using AspectJ's development environment. At the core of AspectJ is its build support for weaving aspects into your Java code. This is currently done using a compiler, ajc, that interacts with the existing Java compiler, javac.
Working within the constraints of the Java compilation process ensures that your aspect-oriented programs produce standard Java classes that can be run by any Java Virtual Machine JVM. The ajc tool is used for compile-time weaving of aspects either from the command line or within an IDE or other build tool, but there is also an alternative weaving method supported in AspectJ, loadtime weaving.
As its title suggests, load-time weaving supports the weaving of aspects into your application at the time when the Java class loader is loading the application into the JVM. This is a fairly new feature of AspectJ and is briefly described in this chapter with the caveat that the current methods by which load-time weaving is achieved using AspectJ may change in the future as this facility matures.
Once you have got the hang of using the ajc command-line compiler then it's time to move on to installing and using some of the capabilities available to the AspectJ developer within the Eclipse development environment. These include how to set up an Eclipse project with an AspectJ Nature using the new project creation wizards and using Eclipse to vary the aspects that are actually built into a single project using AspectJ build configuration files.
Finally, AspectJ project housekeeping tasks such as documentation generation from AspectJ code and building using Ant are covered in this chapter demonstrating just how rich, powerful, and supported the AspectJ development tools are. Recipe 2. Solution At its simplest, AspectJ installation is a four-step process: 1. Install the downloaded JAR files by double-clicking on them. This will create an aspectj1. AspectJ-specific options Listed compiler options 1 fail abort 4. Don't worry about the 1 fail abort message here; if you get the output shown above, then the AspectJ tools have been successfully installed and located and are available for use.
Copy aspectj1. Discussion The first step in preparing to develop using aspects is to download the appropriate support tools for your environment. In the case of AspectJ, this involves downloading the AspectJ libraries and tools. Once the appropriate AspectJ development kits have been downloaded, they can be installed by double-clicking on the downloaded. After installation, you will find a directory named aspectj1. The AspectJ installer specifies some important instructions at the end of the installation process.
These instructions include setting up your search path so that the AspectJ tools are available from the command line and adding the AspectJ Java libraries to your Java classpath.
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It's worth taking care to follow those instructions to make things as easy as possible when setting up your AspectJ development environment. In fact, both of the AspectJ tools use the standard Java tools to do their job, providing enhancements to cater to the additional AspectJ syntax and structure. You have now set up the environment on your machine for development of aspect-oriented software using AspectJ.
You can also register yourself on the AspectJ users mailing list to get notifications of changes to the tools and news on how the AspectJ tools are being employed throughout the software industry. While the versions contained in these installers are not guaranteed to be the latest that are available from the individual sources at www.follow site
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See Also The next stage in getting familiar with AspectJ is to start coding your first aspect as shown in Recipe 2. Developing a Simple Aspect Problem You want to write a simple aspect. Solution First, write your business logic classes, as shown in Example Example A simple business logic Java class package com. A simple HelloWorld aspect in AspectJ package com. Run the ajc command to compile this simple application and produce the byte code. JoinPoint 1 error, 1 warning To add the aspectjrt. AspectJ 1. You have now compiled and run your first aspect-oriented application using AspectJ.
Discussion This recipe has shown you your first example of an aspect and how AspectJ extends the Java language. At first, the new syntax can appear a little strange and a good portion of this book is dedicated to examining the ways the new language constructs can be used to create your aspects. To demystify some of this syntax up front, Example briefly examines what each line of the aspect from this recipe specifies. A simple example of the new AspectJ syntax 1 package com. Lines 5 and 6 declare the logic for a single named pointcut.
The pointcut logic specifies that any join points in your application where a call is made to the void MyClass. The pointcut is named callPointcut so that it can be referred to elsewhere within the aspect's scope.
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Lines 7 through 13 declare a single advice block. The before advice simply states that it will execute before any join points that are matched by the callPointcut pointcut. When a join point is matched the advice simply outputs a couple of messages to the system to inform you that the advice has been executed. This recipe provides a good mechanism for testing a development environment to ensure that things are working as they should before performing any customization to the development tools. AspectJ aspects can be saved with the. The ajc tool compiles the file supplied, regardless of the extension.
The different extensions,. The compilation of the aspect and the Java class produces only. This is a very important feature of AspectJ; aspects are treated as objects in their own right. Because of this treatment, they can be encoded as class files; this ensures that when the application is run, the Java Runtime Environment JRE does not need to understand any additional aspect-specific file formats.
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With the inclusion of the aspectjrt. See Also Prior to using this recipe, it is necessary to get the AspectJ tools and prepare a simple command-line build environment as covered in Recipe 2. Compiling an Aspect and Multiple Java Files Problem You want to conveniently compile a selection of aspects that are to be applied to multiple Java classes. The contents of an example AspectJ build configuration. You shouldn't really worry about the interim steps that may be taking place inside the AspectJ compiler, short of a desire to get into development work on ajc itself.
It is important to consider the inputs to the build process; this is handled through the creation of the. The ajc compiler does not search the source or class path for files to compile; it must be told which files are to be involved in the compilation.
This means that all of your source that is to be compiled with aspects must be fed directly to the ajc compiler. There are three ways to supply the files to be compiled to the ajc compiler two of which are semantically equivalent : The -argfile option You can supply all the files within a.
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