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Practical Example of AOP
with AspectJ
Yegor Bugayenko

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Aspect Oriented Programming (AOP)
AspectJ in 2000
by Gregor Kiczales
in Xerox PARC

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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
}
}

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while (true) {
try {
return load_html();
} catch (Exception ex) {
// nothing, just ignore
}
}

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code duplication

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

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@Retention(value=RUNTIME)
@Target(value=METHOD)
public @interface Retry {
}

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

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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

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binary aspect weaving
javac + ajc

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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

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

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<dependencies>
<dependency>
<groupId>org.aspectj</groupId>
<artifactId>aspectjrt</artifactId>
</dependency>
</dependencies>

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no code duplication

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procedural and clumsy

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

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

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new RobustPage(new Page());

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a better Java with AOP?

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@yegor256

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

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

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