Aspect Oriented Programming

Aspect Oriented Programming

Terminology

• Aspect - some feature
• Join point - point where feature can be applied
• Pointcut - predicate to designate join points
• Advice - action taken at particular point cut
• Target object - object being adviced by one or more aspects

Advice

• Before advice
• After returning advice
• After throwing advice
• After (finally) advice
• Around advice

Configuration

@Configuration
@EnableAspectJAutoProxy
public class AppConfig {

}

<aop:aspectj-autoproxy/>

Aspect

package org.xyz;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
import org.springframework.stereotype.Component;

@Component
@Aspect
public class MyAspect {
    @Before("execution(* someMethod(..))")
    public void doSomething() {
        System.out.println("Doing something before method someMethod");
    }
}

<dependency>
    <groupId>org.aspectj</groupId>
    <artifactId>aspectjweaver</artifactId>
    <version>1.8.8</version>
    <scope>compile</scope>
</dependency>

Aspect

package org.xyz;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
import org.springframework.stereotype.Component;

@Component
@Aspect
public class MyAspect {
    @Before("someMethodExecution()")
    public void doSomething() {
        System.out.println("Doing something before method someMethod");
    }

    @Pointcut("execution(* someMethod(..))")// the pointcut expression
    private void someMethodExecution() {}// the pointcut signature
}

<dependency>
    <groupId>org.aspectj</groupId>
    <artifactId>aspectjweaver</artifactId>
    <version>1.8.8</version>
    <scope>compile</scope>
</dependency>

Pointcuts

• execution - for matching methods
• within - limits to types
• this - proxy has type
• target - target has type
• args - arguments match list
• @target - target is annotated
• @args - arguments are annotated
• @within - type is annotated
• @annotation - subject has annotation
• bean - bean has name (only in Spring)
• &&, || and ! can be used

AspectJ Pointcuts

• Not supported: call, get, set, preinitialization, staticinitialization, initialization, handler, adviceexecution, withincode, cflow, cflowbelow, if, @this
• Throw IllegalArgumentException if used

Combining Pointcuts

@Pointcut("execution(public * *(..))")
private void anyPublicOperation() {}

@Pointcut("within(com.xyz.someapp.trading..*)")
private void inTrading() {}

@Pointcut("anyPublicOperation() && inTrading()")
private void tradingOperation() {}

In XML pointcut reference

package com.xyz.someapp;

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;

@Aspect
public class SystemArchitecture {
    @Pointcut("within(com.xyz.someapp.web..*)")
    public void inWebLayer() {}
}

<aop:config>
    <aop:advisor
        pointcut="com.xyz.someapp.SystemArchitecture.inWebLayer()"
        advice-ref="tx-advice"/>
</aop:config>

<tx:advice id="tx-advice">
    <tx:attributes>
        <tx:method name="*" propagation="REQUIRED"/>
    </tx:attributes>
</tx:advice>

execution

execution(modifiers-pattern? ret-type-pattern
            declaring-type-pattern?name-pattern(param-pattern)
                throws-pattern?)

Pointcuts examples

Examples

1) execution(public * *(..))

2) execution(* set*(..))

3) execution(* com.xyz.service.AccountService.*(..))

4) execution(* com.xyz.service.*.*(..))

5) execution(* com.xyz.service..*.*(..))

6) within(com.xyz.service.*)

7) within(com.xyz.service..*)

8) this(com.xyz.service.AccountService)

9) target(com.xyz.service.AccountService)

10) args(java.io.Serializable)

11) @target(org.springframework.transaction.annotation.Transactional)

12) @within(org.springframework.transaction.annotation.Transactional)

13) @annotation(org.springframework.transaction.annotation.Transactional)

14) @args(com.xyz.security.Classified)

15) bean(tradeService)

16) bean(*Service)

Before Advice

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;

@Aspect
public class BeforeExample {

    @Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
    public void doAccessCheck() {
        // ...
    }

}

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;

@Aspect
public class BeforeExample {

    @Before("execution(* com.xyz.myapp.dao.*.*(..))")
    public void doAccessCheck() {
        // ...
    }

}

After Advice

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.After;

@Aspect
public class AfterFinallyExample {

    @After("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
    public void doReleaseLock() {
        // ...
    }

}

After Returning Advice

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;

@Aspect
public class AfterReturningExample {

    @AfterReturning("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
    public void doAccessCheck() {
        // ...
    }

}

After Returning Advice and returned object

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;

@Aspect
public class AfterReturningExample {

    @AfterReturning(
        pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
        returning="retVal")
    public void doAccessCheck(Object retVal) {
        // ...
    }

}

After Throwing Advice

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;

@Aspect
public class AfterThrowingExample {

    @AfterThrowing("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
    public void doRecoveryActions() {
        // ...
    }

}

After Throwing Advice with catched exception

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;

@Aspect
public class AfterThrowingExample {

    @AfterThrowing(
        pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
        throwing="ex")
    public void doRecoveryActions(DataAccessException ex) {
        // ...
    }

}

Around Advice

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.ProceedingJoinPoint;

@Aspect
public class AroundExample {

    @Around("com.xyz.myapp.SystemArchitecture.businessService()")
    public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
        // start stopwatch
        Object retVal = pjp.proceed();
        // stop stopwatch
        return retVal;
    }

}

JoinPoint

• Can be added to any Advice as first parameter
• Has many userful methods

Passing parameters to advice

@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
public void validateAccount(Account account) {
    // ...
}

@Pointcut("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)")
private void accountDataAccessOperation(Account account) {}

@Before("accountDataAccessOperation(account)")
public void validateAccount(Account account) {
    // ...
}

Passing annotations to advice

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Auditable {
    AuditCode value();
}

@Before("com.xyz.lib.Pointcuts.anyPublicMethod() && @annotation(auditable)")
public void audit(Auditable auditable) {
    AuditCode code = auditable.value();
    // ...
}

Parameter names not available through reflection

@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)",
        argNames="bean,auditable")
public void audit(Object bean, Auditable auditable) {
    AuditCode code = auditable.value();
    // ... use code and bean
}

@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean)
                  && @annotation(auditable)",
        argNames="bean,auditable")
public void audit(JoinPoint jp, Object bean, Auditable auditable) {
    AuditCode code = auditable.value();
    // ... use code, bean, and jp
}

Advice ordering

• By default order of advice from different aspects is underfined
• We can change it using @Order
• Advices from the same aspect can't be ordered

Introductions

public interface AdditionalInterface {
    void method();
}

@Aspect
@Component
public class MyAspect {
    @DeclareParents(value = "info.owczarek..*",
        defaultImpl = AdditionalInterfaceImpl.class)
    public AdditionalInterface mixin;
}

AdditionalInterface someService = context.getBean("someService", AdditionalInterface.class);
someService.method();

Introductions used in aspects

@Aspect
public class UsageTracking {

    @DeclareParents(value="com.xzy.myapp.service.*+", defaultImpl=DefaultUsageTracked.class)
    public static UsageTracked mixin;

    @Before("com.xyz.myapp.SystemArchitecture.businessService() && this(usageTracked)")
    public void recordUsage(UsageTracked usageTracked) {
        usageTracked.incrementUseCount();
    }

}

Schema-based configuration (xml)

Declaring aspect

<aop:config>
    <aop:aspect id="myAspect" ref="aBean">
        ...
    </aop:aspect>
</aop:config>

<bean id="aBean" class="...">
    ...
</bean>

Declaring pointcut

<aop:config>

    <aop:pointcut id="businessService"
        expression="execution(* com.xyz.myapp.service.*.*(..))"/>

</aop:config>

<aop:config>

    <aop:pointcut id="businessService"
        expression="com.xyz.myapp.SystemArchitecture.businessService()"/>

</aop:config>

<aop:config>

    <aop:aspect id="myAspect" ref="aBean">

        <aop:pointcut id="businessService"
            expression="execution(* com.xyz.myapp.service.*.*(..))"/>

        ...

    </aop:aspect>

</aop:config>

Using named pointcut

<aop:config>

    <aop:aspect id="myAspect" ref="aBean">

        <aop:pointcut id="businessService"
            expression="execution(* com.xyz.myapp.service.*.*(..)) && this(service)"/>

        <aop:before pointcut-ref="businessService" method="monitor"/>

        ...

    </aop:aspect>

</aop:config>

public void monitor(Object service) {
    ...
}

Using named pointcut

<aop:config>

    <aop:aspect id="myAspect" ref="aBean">

        <aop:pointcut id="businessService"
            expression="execution(* com.xyz.myapp.service.*.*(..)) **and** this(service)"/>

        <aop:before pointcut-ref="businessService" method="monitor"/>

        ...
    </aop:aspect>
</aop:config>

• && → and
• || → or
• Pointcuts in xml can't reference one another

Advices in XML

Before advice

<aop:aspect id="beforeExample" ref="aBean">
    <aop:before
        pointcut-ref="dataAccessOperation"
        method="doAccessCheck"/>
...
</aop:aspect>

<aop:aspect id="beforeExample" ref="aBean">

    <aop:before
        pointcut="execution(* com.xyz.myapp.dao.*.*(..))"
        method="doAccessCheck"/>

    ...

</aop:aspect>

After returning advice

<aop:aspect id="afterReturningExample" ref="aBean">
    <aop:after-returning
        pointcut-ref="dataAccessOperation"
        method="doAccessCheck"/>
    ...
</aop:aspect>

<aop:aspect id="afterReturningExample" ref="aBean">

    <aop:after-returning
        pointcut-ref="dataAccessOperation"
        returning="retVal"
        method="doAccessCheck"/>

    ...
</aop:aspect>

public void doAccessCheck(Object retVal) {...

After throwing advice

<aop:aspect id="afterThrowingExample" ref="aBean">
    <aop:after-throwing
        pointcut-ref="dataAccessOperation"
        method="doRecoveryActions"/>
    ...
</aop:aspect>

<aop:aspect id="afterThrowingExample" ref="aBean">

    <aop:after-throwing
        pointcut-ref="dataAccessOperation"
        throwing="dataAccessEx"
        method="doRecoveryActions"/>
    ...
</aop:aspect>

public void doRecoveryActions(DataAccessException dataAccessEx) {...

After (finally) advice

<aop:aspect id="afterFinallyExample" ref="aBean">
    <aop:after
        pointcut-ref="dataAccessOperation"
        method="doReleaseLock"/>
    ...
</aop:aspect>

Around advice

<aop:aspect id="aroundExample" ref="aBean">
    <aop:around
        pointcut-ref="businessService"
        method="doBasicProfiling"/>
    ...
</aop:aspect>

public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
    // start stopwatch
    Object retVal = pjp.proceed();
    // stop stopwatch
    return retVal;
}

Advice parameters

<aop:before
    pointcut="com.xyz.lib.Pointcuts.anyPublicMethod() and @annotation(auditable)"
    method="audit"
    arg-names="auditable"/>

public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
    // start stopwatch
    Object retVal = pjp.proceed();
    // stop stopwatch
    return retVal;
}

Advice parameters example (1/3)

package x.y.service;

public interface FooService {

    Foo getFoo(String fooName, int age);
}

public class DefaultFooService implements FooService {

    public Foo getFoo(String name, int age) {
        return new Foo(name, age);
    }
}

Advice parameters example (2/3)

package x.y;

import org.aspectj.lang.ProceedingJoinPoint;
import org.springframework.util.StopWatch;

public class SimpleProfiler {

    public Object profile(ProceedingJoinPoint call, String name, int age) throws Throwable {
        StopWatch clock = new StopWatch("Profiling for ''" + name + "'' and ''" + age + "''");
        try {
            clock.start(call.toShortString());
            return call.proceed();
        } finally {
            clock.stop();
            System.out.println(clock.prettyPrint());
        }
    }
}

Advice parameters example (3/3)

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd">

    <!-- this is the object that will be proxied by Spring's AOP infrastructure -->
    <bean id="fooService" class="x.y.service.DefaultFooService"/>

    <!-- this is the actual advice itself -->
    <bean id="profiler" class="x.y.SimpleProfiler"/>

    <aop:config>
        <aop:aspect ref="profiler">

            <aop:pointcut id="theExecutionOfSomeFooServiceMethod"
                expression="execution(* x.y.service.FooService.getFoo(String,int))
                and args(name, age)"/>

            <aop:around pointcut-ref="theExecutionOfSomeFooServiceMethod"
                method="profile"/>

        </aop:aspect>
    </aop:config>

</beans>