Aspect Oriented Programming

What is it?

is a programming paradigm
extends OOP
enables modularization of crosscutting concerns
is second heart of Spring Framework

Sample usages

Cross cutting layer functionality like:

security
transactional
logging
monitoring
cache

ee-cross-layer-aop

Example

A simple service method:

public Order getOrder(BigDecimal orderId) {
 return (Order) factory.openSession().get(Order.class, orderId);
}

Add permissions check:

public Order getOrder(BigDecimal orderId) {
 if (hasOrderPermission(orderId)) {
 return (Order) factory.openSession()
 .get(Order.class, orderId);
 } else {
 throw new SecurityException("Access Denied");
 }
}

Add transaction management:

public Order getOrder(BigDecimal orderId) {
 if (hasOrderPermission(orderId)) {
 Order order;
 Session session = factory.openSession();
 Transaction tx = session.beginTransaction();
try {
 order = (Order) session.get(Order.class, orderId);
 tx.commit();
} catch (RuntimeException e) {if (tx!=null) {tx.rollback();}
} finally {session.close();}
 return order;
 } else { throw new SecurityException("Access Denied");}
}

Add cache:

public Order getOrder(BigDecimal orderId) {
 if (hasOrderPermission(orderId)) {
 Order order = (Order)cache.get(orderId);
 if (order==null) {
Session session = factory.openSession();
Transaction tx = session.beginTransaction();
try {
 order = (Order) session.get(Order.class, orderId);
 tx.commit();
 cache.put(orderId, order);
} catch (RuntimeException e) {if (tx!=null) {tx.rollback();}
} finally {session.close();}
 }
 return order;
 } else { throw new SecurityException("Access Denied");}
}

What does AOP solve?

ee-cross-layer-aop

AOP concepts

Key words

join point - any identifiable places in the program
pointcut - defined collection of join points
advice - logic implementation executed before, after or instead of reaching pointcut
aspect - combination of advices and pointcuts; decides what should be executed in which place
introduction - adding to new fields, methods, interfaces implementations to the class
target object - object enriched with new logic
AOP proxy - proxy object which encapsulates target object and adds new logic by the given aspects
weaving - process of adding aspect to target object
- compile time
- load time
- runtime

OOP vs AOP

OOP (Object Orianted Programming) composes application into object relation
AOP (Aspect Oriented Programming) composes application into aspects
AOP completes OOP and allows
- static hierarchy object enrichment
- introduce new functionalities
- grouping similar concepts in decoupled classes
Aspect programming and reflection concept fulfill separation of concerns

aop vs oop

OOP	AOP
1. Class – code unit that encapsulates methods and attributes	1. Aspect – code unit that encapsulates pointcuts, advice, and attributes
2. Method signature – define the entry points for the execution of method bodies	2. Pointcut – define the set of entry points (triggers) in which advice is executed
3. Method bodies – implementation of the business logic concerns	3. Advice – implementation of cross cutting concern
4. Compiler – convert source code to object code	4. Weaver – construct code (source or object) with advice

Spring AOP

implemented in pure java
no need for a special compilation process
supports only method execution join points
only runtime weaving is available
AOP proxy
- JDK dynamic proxy
- CGLIB proxy
configuration
- @AspectJ annotation-style
- Spring XML configuration-style

Aspect

Aspect declare:

@Aspect
public class EmptyAspect {
}

XML Configuration:

<!--<context:annotation-config />-->
<aop:aspectj-autoproxy proxy-target-class="false | true"/>
<bean class="org.springframework.aop.aspectj.annotation.AnnotationAwareAspectJAutoProxyCreator"></bean>
<bean class="example.EmptyAspect"/>

proxy-target-class=”false” - used JDK Proxy
proxy-target-class=”true” - used CGLIB

Pointcut

Sample pointcut declaration and composition:

@Aspect
public class ItemStatusTracker {

 @Pointcut("execution(* approve(..))")
 public void ifApprove() {}

 @Pointcut("execution(* reject(..))")
 public void ifReject() {}
 
 @Pointcut("ifApprove() || ifReject()")
 public void ifStateChange() {}
}

Execution examples:

@Aspect
public class ItemStatusTracker {

    @Pointcut("execution(public * * (..))")
    public void anyPublicMethods() {}
    
    @Pointcut("execution(* get*(..))")
    public void anyMethodsThatStartsWithGet() {}
    
    @Pointcut("execution(* bank.BankService.*(..))")
    public void anyMethodDefinedByAppropriateInterface() {}
    
    @Pointcut("execution(* com.epam.pmc.service.*.*(..))")
    public void anyMethodDefinedByAppropriatePackage() {}

}

More examples you can find on: http://static.springsource.org/spring/docs/3.0.x/spring-frameworkreference/html/aop.html#aop-pointcuts-examples

Advice

associated with a pointcut expression
- a simple reference to a named pointcut
- a pointcut expression declared in place
runs
- before
- after returning
- after throwing
- after (finally)
- around

Examples:

@Aspect
public class BankAspect {
    
    @Pointcut("execution(public * * (..))")
    public void anyPublicMethod() {}
    
    @Before("anyPublicMethod()")
    public void logBefore(JoinPoint joinPoint) {
      //to do something
    }
    
    @AfterReturning(
      pointcut="execution(* get*(..))",
      returning="retVal")
    public void logAfter(JoinPoint joinPoint, Object retVal) {
    //to do something
    }
    
    @AfterThrowing(
      pointcut = "execution(* bank..*ServiceImpl.add*(..))",
      throwing = "exception")
    public void afterThrowing(Exception exception) {
      //to do something
    }
    
    @Around("@annotation(bank.Cached)")
    public Object aroundCache(ProceedingJoinPoint joinPoint){
      //to do something before
      Object retVal = joinPoint.proceed();
      //to do something after
}
}

XML Configuration

<?xml version="1.0" encoding="UTF-8"?>
<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="…">
    <aop:config>
    <aop:aspect id="bankAspectId" ref="bankAspect">
        <aop:pointcut id="anyPublicMethod"
            expression="execution(public * * (..))"/>
        <aop:before pointcut-ref="anyPublicMethod" method="logBefore"/>
    </aop:aspect>
    </aop:config>
    <bean id="bankAspect" class="bank.BankAspect"/>
</beans>

How does it work

Spring creates Proxy for each enriched object. Caller is working on Proxy so beware of using instanceof and == operators.
Creating proxy objects is transparent for beans and callers.
Proxy enrich target object with aspects and the delegates executions.

Proxy overview:

proxy

Proxy chaining:

proxy_chaining

AOP Proxy

Spring AOP supports two types of proxy that are being created at runtime:

JDK dynamic proxy - can only proxy by interface (so your target class needs to implement an interface, which will also be implemented by the proxy class).
CGLIB proxy - CGLIB (and javassist) can create a proxy by subclassing. In this scenario the proxy becomes a subclass of the target class. No need for interfaces.

So Java Dynamic proxies can proxy: public class Foo implements iFoo where CGLIB can proxy: public class Foo.

Reference: http://stackoverflow.com/questions/10664182/what-is-the-difference-between-jdk-dynamic-proxy-and-cglib

Spring AOP vs AspectJ

Spring AOP

no need for a special compilation process
support only method execution pointcuts
advise the execution of operations on Spring beans

AspectJ

need AspectJ compiler or setup LTW
support all pointcuts
advice all domain objects

Custom Annotation example

Configuration:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       xsi:schemaLocation="
           http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-3.2.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-3.1.xsd
           http://www.springframework.org/schema/aop 
       http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
       http://www.springframework.org/schema/mvc
       http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <!-- Scans for application @Components to deploy -->
    <context:annotation-config/>
    <context:component-scan base-package="io.github.kospiotr.*" />
    
    <!-- enable AOP -->
    <aop:aspectj-autoproxy/>

</beans>

Define Annotation:

@Retention(RetentionPolicy.RUNTIME)
public @interface LogDuration {
   String value();
}

Aspect definition:

@Aspect
public class AopExample{

    //for any method with @LogDuration, no matter what the return type, name, or arguments are, call this method 
    @Around("execution(@io.github.kospiotr.aop.LogDuration * *(..)) && @annotation(logDurationAnnotation)")
    public Object logDuration(ProceedingJoinPoint joinPoint, LogDuration logDurationAnnotation) throws Throwable {
       
        //capture the start time 
        long startTime = System.currentTimeMillis();
        
        //execute the method and get the result
        Object result = joinPoint.proceed();
        
        //capture the end time
        long endTime = System.currentTimeMillis();
        
        //calculate the duration and print results
        long duration = endTime - startTime;
        System.out.println(logDurationAnnotation.value()+": "+duration+"ms"); //you should use a logger  
        
        //return the result to the caller
        return result; 
    }

}

Use Annotation:

@Component
public class Example{

     @LogDuration("Hello World API") 
     public String getHelloWorld(){
          try {
            Thread.sleep(3000);
          } catch (InterruptedException e) {
            throw new RuntimeException("Sleep Interrupted", e);
          } 
          return "Hello World";
     } 

}

Output:

Hello World API: 3002ms