Simple Introduction to AOP - Session 3

Another way of defining an Aspect is using @AspectJ annotaions - which is natively understood by Spring:

package org.bk.inventory.aspect;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.After;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
import org.aspectj.lang.annotation.Pointcut;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Aspect
public class AuditAspect {

    private static Logger logger = LoggerFactory.getLogger(AuditAspect.class);

    @Pointcut("execution(* org.bk.inventory.service.*.*(..))")
    public void serviceMethods(){
        //
    }

    
    @Before("serviceMethods()")
    public void beforeMethod() {
        logger.info("before method");
    }

    @Around("serviceMethods()")
    public Object aroundMethod(ProceedingJoinPoint joinpoint) {
        try {
            long start = System.nanoTime();
            Object result = joinpoint.proceed();
            long end = System.nanoTime();
            logger.info(String.format("%s took %d ns", joinpoint.getSignature(), (end - start)));
            return result;
        } catch (Throwable e) {
            throw new RuntimeException(e);
        }
    }
      
    @After("serviceMethods()")
    public void afterMethod() {
        logger.info("after method");
    }    
}

The @Aspect annotation on the class identifies it as an aspect definition. It starts by defining the pointcuts:

@Pointcut("execution(* org.bk.inventory.service.*.*(..))")
    public void serviceMethods(){}

The above basically identifies all the methods of all types in org.bk.inventory.service package, this pointcut is identified by the name of the method on which the annotation is placed - in this case "serviceMethods". Next, the advice is defined using the @Before(serviceMethods()), @After(serviceMethods()) and @Around(serviceMethods()) annotation and the specifics of what needs to happen is the body of the methods with those annotations. Spring AOP natively understands the @AspectJ annotations, if this Aspect is defined as a bean:

<bean id="auditAspect" class="org.bk.inventory.aspect.AuditAspect" />

Spring would create a dynamic proxy to apply the advice on all the target beans identified as part of the pointcut notation.

Links to all sessions on AOP:
AOP Session 1 - Decorator Pattern using Java Dynamic Proxies
AOP Session 2 - Using Spring AOP - xml based configuration
AOP Session 3 - Using Spring AOP - @AspectJ based configuration - with/without compile time weaving
AOP Session 4 - Native AspectJ with compile time weaving
AOP Session 5 - Comprehensive Example

Simple Introduction to AOP - Session 2

Here, I will show how the cross-cutting concern that was introduced in the previous session, can be implemented using Spring AOP - Spring offers multiple ways of implementing Aspects - XML configuration based, @AspectJ based. In this specific example, I will use XML configuration file based way of defining the aspect

Spring AOP works in the context of a Spring container, so the service implementation that was defined in the previous session needs to be a Spring bean, I am defining it using the @Service annotation:

@Service
public class DefaultInventoryService implements InventoryService{
...
}

Now, I want to record the time taken for each of the method calls of my DefaultInventoryService - I am first going to modularize this as an "advice":

package org.bk.inventory.aspect;

import org.aspectj.lang.ProceedingJoinPoint;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class AuditAdvice {

    private static Logger logger = LoggerFactory.getLogger(AuditAdvice.class);

    public void beforeMethod() {
        logger.info("before method");
    }

    public void afterMethod() {
        logger.info("after method");
    }

    public Object aroundMethod(ProceedingJoinPoint joinpoint) {
        try {
            long start = System.nanoTime();
            Object result = joinpoint.proceed();
            long end = System.nanoTime();
            logger.info(String.format("%s took %d ns", joinpoint.getSignature(), (end - start)));
            return result;
        } catch (Throwable e) {
            throw new RuntimeException(e);
        }
    }
        
}

This advice is expected to capture the time taken by the methods in DefaultInventoryService. So now to wire this advice to the DefaultInventoryService spring bean:

 <bean id="auditAspect" class="org.bk.inventory.aspect.AuditAdvice" />

 <aop:config>
  <aop:aspect ref="auditAspect">
   <aop:pointcut id="serviceMethods" expression="execution(* org.bk.inventory.service.*.*(..))" />

   <aop:before pointcut-ref="serviceMethods" method="beforeMethod" />  
   <aop:around pointcut-ref="serviceMethods" method="aroundMethod" />
   <aop:after-returning pointcut-ref="serviceMethods" method="afterMethod" /> 
  </aop:aspect>
 </aop:config>

This works by first defining the "pointcut" - the places(in this example, the service methods) to add the cross cutting concern(capturing the method execution time in this example) to. Here I have defined it using a pointcut expression -

execution(* org.bk.inventory.service.*.*(..))

, which is essentially selecting all methods of all the types in the org.bk.inventory.service package. Once the pointcut is defined, it defines what needs to be done around the pointcut(the advice), using the expression:

<aop:around pointcut-ref="serviceMethods" method="aroundMethod" />

This basically says, that around every method of any service type, execute the aroundMethod of AspectAdvice that was defined earlier. Now, if the service methods are executed, I would see the advice getting invoked during the method execution, the following is a sample output if DefaultInventoryService, createInventory method is called:

org.bk.inventory.service.InventoryService - Create Inventory called
org.bk.inventory.aspect.AuditAdvice - Inventory org.bk.inventory.service.InventoryService.create(Inventory) took 82492 ns

Spring's AOP implementation works by generating a dynamic proxy at runtime for all the target beans, based on the defined pointcut.
Links to all sessions on AOP:
AOP Session 1 - Decorator Pattern using Java Dynamic Proxies
AOP Session 2 - Using Spring AOP - xml based configuration
AOP Session 3 - Using Spring AOP - @AspectJ based configuration - with/without compile time weaving
AOP Session 4 - Native AspectJ with compile time weaving
AOP Session 5 - Comprehensive Example

A simple introduction to AOP - Session 1

Why use AOP, a simple way to answer this question is to show an implementation of a cross cutting concern without using AOP.

Consider a simple service and it's implementation:

public interface InventoryService {
    public Inventory create(Inventory inventory);
    public List list();
    public Inventory findByVin(String vin);
    public Inventory update(Inventory inventory);
    public boolean delete(Long id);
    public Inventory compositeUpdateService(String vin, String newMake);
}

and its default implementation:

public class DefaultInventoryService implements InventoryService{
    
    @Override
    public Inventory create(Inventory inventory) {
        logger.info("Create Inventory called");
        inventory.setId(1L);
        return inventory; 
    }

    @Override
    public List list() {
        return new ArrayList();
    }

    @Override
    public Inventory update(Inventory inventory) {
        return inventory;
    }

    @Override
    public boolean delete(Long id) {
        logger.info("Delete Inventory called");
        return true;
    }
....

This is just one service. Assume that there are many more services in this project.

So now, if there were a requirement to record the time taken by each of the service methods, the option without AOP would be something along the following lines. Create a decorator for the service:

public class InventoryServiceDecorator implements InventoryService{
    private static Logger logger = LoggerFactory.getLogger(InventoryServiceDecorator.class);
    private InventoryService decorated;

    @Override
    public Inventory create(Inventory inventory) {
        logger.info("before method: create");
        long start = System.nanoTime();
        Inventory inventoryCreated = decorated.create(inventory);
        long end = System.nanoTime();
        logger.info(String.format("%s took %d ns", "create", (end-start)) );
        return inventoryCreated;
    }

This decorator would essentially intercept the call on behalf of the decorated, record the time taken for the method call while delegating the call to the decorated object.

Imagine doing this for all the methods and all the services in the project. This is the scenario that AOP addresses, it provides a way for the cross cutting concerns(Recording time for the service method calls for eg.) to be modularized - to be packaged up separately without polluting the core of the classes.

To end the session, a different way to implement the decorator would be using the dynamic proxy feature of Java:

public class AuditProxy implements java.lang.reflect.InvocationHandler {
    
    private static Logger logger = LoggerFactory.getLogger(AuditProxy.class);
    private Object obj;

    public static Object newInstance(Object obj) {
        return java.lang.reflect.Proxy.newProxyInstance(obj.getClass().getClassLoader(), obj
                .getClass().getInterfaces(), new AuditProxy(obj));
    }

    private AuditProxy(Object obj) {
        this.obj = obj;
    }

    public Object invoke(Object proxy, Method m, Object[] args) throws Throwable {
        Object result;
        try {
            logger.info("before method " + m.getName());
            long start = System.nanoTime();
            result = m.invoke(obj, args);
            long end = System.nanoTime();
            logger.info(String.format("%s took %d ns", m.getName(), (end-start)) );
        } catch (InvocationTargetException e) {
            throw e.getTargetException();
        } catch (Exception e) {
            throw new RuntimeException("unexpected invocation exception: " + e.getMessage());
        } finally {
            logger.info("after method " + m.getName());
        }
        return result;
    }
}

So now, when creating an instance of InventoryService, I would create it through the AuditProxy dynamic proxy:

InventoryService inventoryService = (InventoryService)AuditProxy.newInstance(new DefaultInventoryService());

the overridden invoke method of java.lang.reflect.InvocationHandler would intercept all calls to the InventoryService created in this manner, where the cross cutting concern of auditing the method call time is recorded. This way the cross cutting concern is modularized to one place(AuditProxy), but still needs to be explicitly known by the clients of InventoryService, when instantiating InventoryService.

In the next few sessions, I will demonstrate how this can be more cleanly accomplished using Spring AOP, Spring AOP with @AspectJ, AspectJ, @AspectJ with compile time weaving and finish it off with a comprehensive example.

Links to all sessions on AOP:
AOP Session 1 - Decorator Pattern using Java Dynamic Proxies
AOP Session 2 - Using Spring AOP - xml based configuration
AOP Session 3 - Using Spring AOP - @AspectJ based configuration - with/without compile time weaving
AOP Session 4 - Native AspectJ with compile time weaving
AOP Session 5 - Comprehensive Example