The document presents a practical example of Aspect-Oriented Programming (AOP) using AspectJ, demonstrating the concept with code samples that include a 'retry' aspect to handle exceptions in method executions. It illustrates how to implement this AOP feature in Java classes while reducing code duplication and improving robustness. The document also provides information on necessary Maven plugin configurations for AspectJ integration.

1. /20@yegor256 1
Practical Example of AOP
with AspectJ
Yegor Bugayenko

2. /20@yegor256 2
Aspect Oriented Programming (AOP)
AspectJ in 2000
by Gregor Kiczales
in Xerox PARC

3. /20@yegor256 3
class Page {
public String html() {
// load HTML and return
}
}
class Directory {
public void delete() {
// remove all files
}
}
class File {
public long length() {
// return file size
}
}

4. /20@yegor256 4
while (true) {
try {
return load_html();
} catch (Exception ex) {
// nothing, just ignore
}
}

5. /20@yegor256 5
code duplication

6. /20@yegor256 6
class Page {
@Retry
public String html() {
// load HTML and return
}
}

7. /20@yegor256 7
@Retention(value=RUNTIME)
@Target(value=METHOD)
public @interface Retry {
}

8. /20@yegor256 8
@Aspect
class Robustness {
@Around("execution(* *(..)) && @annotation(Retry)")
public Object around(ProceedingJoinPoint point) {
while (true) {
try {
return point.proceed();
} catch (Exception ex) {
// just ignore it
}
}
}
}

9. /20@yegor256 9
class Page {
@Retry
public String html() {
// load HTML and return
}
}
@Aspect
class Robustness {
@Around(“…”)
public Object around(point) {
while (true) {
try {
return point.proceed();
} catch (Exception ex) {
// just ignore it
}
}
}
}
Page.class
Robustness.class

10. /20@yegor256 10
binary aspect weaving
javac + ajc

11. /20@yegor256 11
class Page {
private Robustness r;
public String html() {
return this.r.around(point);
}
public String html_aroundBody() {
// load HTML and return
}
}
@Aspect
class Robustness {
@Around(“…”)
public Object around(point) {
while (true) {
try {
return point.proceed();
} catch (Exception ex) {
// just ignore it
}
}
}
}
Page.class
Robustness.class

12. /20@yegor256 12
<plugin>
<groupId>com.jcabi</groupId>
<artifactId>jcabi-maven-plugin</artifactId>
<executions>
<execution>
<goals>
<goal>ajc</goal>
</goals>
</execution>
</executions>
</plugin>
jcabi-maven-plugin

13. /20@yegor256 13
<dependencies>
<dependency>
<groupId>org.aspectj</groupId>
<artifactId>aspectjrt</artifactId>
</dependency>
</dependencies>

14. /20@yegor256 14
no code duplication

15. /20@yegor256 15
procedural and clumsy

16. /20@yegor256 16
@Aspect
class Robustness {
@Around("execution(* *(..)) && @annotation(Retry)")
public Object around(ProceedingJoinPoint point) {
while (true) {
try {
return point.proceed();
} catch (Exception ex) {
// just ignore it
}
}
}
}

17. /20@yegor256 17
class RobustPage {
private final Page page;
public String html() {
while (true) {
try {
return this.page.html();
} catch (Exception ex) {
// ignore
}
}
}
}

18. /20@yegor256 18
new RobustPage(new Page());

19. /20@yegor256 19
a better Java with AOP?

20. /20@yegor256 20
@yegor256

21. /20@yegor256 21
class Page {
public String html() around retry() {
// load HTML and return
}
advice Object retry() {
while (true) {
try {
return proceed;
} catch (Exception ex) {
// ignore
}
}
}
}

22. /20@yegor256 22
class Robust<T> decorates T {
advice Object retry() {
while (true) {
try {
return proceed;
} catch (Exception ex) {
// ignore
}
}
}
}

23. /20@yegor256 23
page = new Robust<Page>(new Page()) {
@Override
public String html() around retry() {
return origin.html();
}
}