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A Simple Aspect
Listing 1 shows a simple Java class. They don’t get much simpler than this! This is based on the "Hello World" example included in the AspectJ product. The folder for this is InstallDir\AspectJava\doc\examples\ltw.
Listing 1 A Simple "Hello World" Java Class
public class HelloWorld {
public static void main (String[] args) {
System.out.println("Hello World!");
}
}
You might not think there’s much of interest in Listing 1. Listing 2 provides our very first aspect. This is contained in the file Tracing.aj in the same folder as HelloWorld.java.
Listing 2 A Tracing Aspect
public aspect Tracing {
private pointcut mainMethod () :
execution(public static void main(String[]));
before () : mainMethod() {
System.out.println("> Before mainMethod()" + thisJoinPoint);
}
after () : mainMethod() {
System.out.println("< After mainMethod()" + thisJoinPoint);
}
}
Don’t worry about the details of Listing 2—we’ll see its purpose in a moment. Let’s build these files, modify the CLASSPATH, and then run the program to learn what it all means. Listing 3 shows these three steps, all executed (as mentioned earlier) from within the ltw folder in the AspectJava\doc\examples folder.
Listing 3 Building and Executing an Aspect
ajc -outjar hello.jar HelloWorld.java ajc -outjar tracing.jar -outxml Tracing.aj set classpath=%classpath%;.;.\hello.jar;.\tracing.jar; aj HelloWorld > Before mainMethod()execution(void HelloWorld.main(String[])) Hello World! < After mainMethod()execution(void HelloWorld.main(String[]))
The last three lines of Listing 3 display the magic of AOP. We have intercepted the normal execution of the HelloWorld Java class at two different entry points:
- Just before main()—and then main() executes
- Just after main()
Take a quick look at Listing 2 and compare it with Listing 3 to find the interception. What you’re looking at in Listing 3 is a process called load-time weaving.