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Practical Example of AOP with AspectJ

Yegor Bugayenko

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Aspect Oriented Programming (AOP)

AspectJ in 2000by 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

/20@yegor256 21class 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(); } }