DZone Introduction to AOP article

I have written an introductory article to JBoss AOP on DZone. It can be found here.

TDC 2008

The Developers Conference or TDC is a Brazilian conference about Java and Agile Development Methods that will take place in Sao Paulo. The event lasts two days and will start tomorrow.
If you are going to attend to this conference, it would be great to chat about the JBoss technologies. I will be there on Saturday, and will do a mini-presentation about JBoss AS 5 at the Red Hat stand (the time of the presentation is to be defined and will be informed at the RH stand).
See you there!

Dynamic AOP Tutorial - Part 2

In this second part of the dynamic AOP tutorial, we will show how you can perform dynamic AOP through our API. This is currently the only available way of using dynamic AOP in standalone applications.

The JBoss AOP API


Aspects, Interceptors and Advices

All aspects, advices, and interceptors are represented by factories. Notice that, internally, JBoss AOP does not differentiate advices from interceptors. They are both represented as interceptor instances, which results in uniform, transparent handling of interceptors and advices. This way, an interceptor chain is used for intercepting a joinpoint every time it is executed, and may contain even typed advices.
For that, JBoss AOP provides a factory capable of transforming advices into interceptors, the AdviceFactory.
Take a look at the main classes used to represent those elements, all located in the org.jboss.aop.advice package:

• AspectFactory: responsible for creating an instance of an aspect. While you can provide your own implementation of this interface, the most commonly used implementation is the GenericAspectFactory, that creates aspect instances given the class of the aspect to be created.
• AspectDefinition: this is how aspect factories are managed internally. Whenever you declare an aspect, JBoss AOP creates an AspectDefinition. This definition contains the aspect factory, the aspect scope, and the aspect name.
• InterceptorFactory: responsible for creating instances of interceptors. To represent a simple interceptor, the GenericInterceptorFactory is used; the AdviceFactory represents advices.

The next example illustrates how you use those classes to represent an advice:
// aspect class
public class MyAspect
{
  public void helloWorld()
  {
    System.out.println(“Hello world!”);
  }

  public Object helloWorld2(Invocation invocation) throws Throwable
  {
    System.out.println(“Hello world!”);
    return invocation.invokeNext();
  }
}
-----
  public static void main(String [] args)
  {
    AspectFactory aspectFactory = new GenericAspectFactory(MyAspect.class, null);
    AspectDefinition aspectDefinition = new AspectDefinition(“myAspect”, Scope.PER_INSTANCE, aspectFactory);
    AdviceFactory adviceFactory = new AdviceFactory(aspectDefinition, “helloWorld”, AdviceType.BEFORE);
    ...
  }

In the first part of the example, we declare the MyAspect class, with two advices: helloWorld and helloWorld2. In the second part, we represent the aspect MyAspect and the advice helloWorld using the classes we have just learned. First, an aspect factory is created. The constructor of GenericAspectFactory must receive the class of the aspect, and an xml element. This second parameter allows to pass extra configuration to the aspect class. Since we are not loading anything from an xml file, the second parameter must be null.
Next, we create an aspect definition with the factory, defining also the name of our aspect (could be any unique name you wanted), and the scope of the aspect. Finally, we create an advice factory. This factory's constructor receives the aspect definition, the name of the advice, and the type of the advice. A second constructor is also provided. If you are declaring around advices, you can omit the advice type, as follows:
// declare the helloWorld2 advice as being of the default advice type, which is around
new AdviceFactory(aspectDefinition, “helloWorld2”);
Naturally, creating those elements is not enough, you must deploy them through our API. We will see how to do this in the next topics.

Binding Aspects to Pointcuts

Advices and interceptors are useless unless they are bound to a pointcut expression, indicating when they should be called. This takes us to the most important class of the dynamic AOP API, the org.jboss.aop.advice.AdviceBinding. This class contains a pointcut expression and a series of interceptor factories. Optionally, it can also contain a cflow expression.
It provides one constructor for dynamic AOP operations:
AdviceBinding(String pointcutExpression, String cflow)

Designed specially for dynamic AOP operations, this constructor creates a binding, defining the pointcut expression and the cflow expression you intend to use. If there is no cflow expression, the second parameter should be null. While there is a second constructor provided by AdviceBinding. It is for internal use only, so always use the one above.

The following methods add advices and interceptors to a binding:

• addInterceptor(Class clazz)
  Adds an interceptor to the binding. Always use this method when you are binding interceptors.
• addInterceptorFactory(InterceptorFactory interceptorFactory)
  Adds an interceptor factory to the binding. You must use it to add advices, in the form of AdviceFactory instances (as we have seen above, this class is an implementation of InterceptorFactory, and creates interceptor adapters to call your advice).
  

AspectManager

Most of the functionalities provided by JBoss AOP have an common entry point: the org.jboss.aop.AspectManager class. This class is our facade and provides all sort of AOP functionalities. Among them, you can find the dynamic AOP methods:

• addAspectDefinition(AspectDefinition definition): deploys an aspect
• addAdviceBinding(AdviceBinding binding): deploys a binding
• removeAdviceBinding(String bindingName): removes a binding
• removeAdviceBindings(Collection bindings): removes a collection of bindings.
  

Advisor and InstanceAdvisor

Every weaved class implements the org.jboss.aop.Advised interface. Through this interface, you can access the class Advisor, a sort of AspectManager with scope limited to that single class, and the InstanceAdvisor, with an even more specific scope, a single instance.
Take a look at the methods you can call on the Advised interface:

• Advisor _getAdvisor()
• InstanceAdvisor _getInstanceAdvisor()
  

The InstanceAdvisor class provides dynamic AOP operations that affect only the instance it advises. Its main methods are:

• insertInterceptor(Interceptor)
  Inserts an interceptor in the beginning of all interceptor chains associated with the advised instance.
• appendInterceptor(Interceptor)
Adds the interceptor to the end of the instance's chains
• removeInterceptor(String)
Removes an interceptor that has been inserted or appended. The parameter specifies the name of the interceptor to be removed.
  

Putting it all together

Take a look at the following example:


AdviceBinding binding = new AdviceBinding(“execution(public !static void Pojo->*(int, long))”, null);
binding.addInterceptor(MyInterceptor.class);
AspectManager.getInstance().addAdviceBinding(binding);

This example adds a binding dynamically. It is the API equivalent of the xml binding below:

<aop>
  <binding pointcut="execution(public !static void Pojo->*(int, long))">
    <interceptor class="MyInterceptor.class/"/>
  </binding>
</aop>
Now, an example with advices:

// declares the aspect class
AspectFactory aspectFactory = new GenericAspectFactory(MyAspect.class, null);
AspectDefinition aspectDefinition = new AspectDefinition(“myAspect”, Scope.PER_INSTANCE, aspectFactory);
AspectManager manager = AspectManager.instance();
// deploys the aspect
manager.addAspectDefinition(aspectDefinition);
// creates the advice factory
AdviceFactory adviceFactory = new AdviceFactory(aspectDefinition, “helloWorld”, AdviceType.BEFORE);
// creates the binding
AdviceBinding binding = new AdviceBinding(“execution(public !static void Pojo->*(int, long))”, null);
binding.addInterceptorFactory(adviceFactory);
// adds the binding
manager.addAdviceBinding(binding);

The example above declares and deploys an aspect, then creates an advice factory to represent an advice of this aspect. This advice factory is added to a new advice binding, that is registered at AspectManager by a call to AspectManager.addAdviceBinding method. It is the API counterpart of the xml below:
<aop>
  <aspect name="”myAspect”" class="”MyAspect”/">
  <binding pointcut="”execution(public !static void Pojo->*(int, long))”>
    <before aspect="”myAspect”" name="”helloWorld”/">
  </binding>
</aop>
To remove the advice binding, you can call the removeAdviceBinding method:
manager.removeAdviceBinding(binding.getName());

Notice that the AdviceBinding.addInterceptor and AdviceBinding.addInterceptorFactory methods can be called only before you deploy your binding. Doing so after that will cause inconsistencies.

Our last example adds an interceptor to a single instance of the class Pojo:
// create Pojo
Pojo pojo = new Pojo();
// retrieve its instance advisor
InstanceAdvisor instanceAdvisor = ((Advised) pojo)._getInstanceAdvisor();
// append an interceptor to the chains of this instance only
instanceAdvisor.append(new MyInterceptor());
// executing someMethod will trigger MyInterceptor
pojo.someMethod();

If you create another instance of Pojo, this instance will not be intercepted by MyInterceptor, as the interceptor has been added to the InstanceAdvisor of pojo:
Pojo otherPojo = new Pojo();
pojo.someMethod(); // will not be intercepted by MyInterceptor

Wrapping up

This second part of the tutorial showed the main classes involved in the dynamic AOP API. To add an aspect at runtime you have to

1. declare the aspect, by adding an AspectDefinition to AspectManager
2. create a binding, with the pointcut expression of your choice, and add interceptor factories to it
3. add the binding to AspectManager (it is at this point that the binding will be deployed to your system)
   

You have two options to add interceptors:

• create an AdviceBinding containing interceptors and add it to AspectManager as above
• use the InstanceAdvisor API
  

Next Steps

-> Take a look at our dynamic AOP example. It is available as part of our releases. Run the example and play with it:

• add advices to the advice binding, instead of interceptors;
• create an advice binding that contains interceptors and advices mixed.
  

-> Check the JBoss AOP Javadoc to learn more about all methods provided by the InstanceAdvisor class. Edit the dynamic AOP example, this time using the InstanceAdvisor methods you learned. Try removing an interceptor at runtime.
-> Get engaged! Help us design a state of the art dynamic AOP API, which will be part of a future release:
http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4166298#4166298

Dynamic AOP Tutorial - Part 1

The goal of this tutorial is to show how to use dynamic AOP with JBoss AOP. Notice that, by dynamic AOP, we mean the dynamic weaving of aspects, and not dynamic pointcut expressions (such as cflow), as other tools refer to them using the same term.

There are two ways of using dynamic AOP with JBoss AOP: hot deployment (when running your application in AS); or through our API.

In the next topics, we will show what are the prerequisites for dynamic AOP, and how to use hot deployment of aspects at the server. In the second part of our tutorial, we will see how to use our API to perform dynamic operations.

What is Dynamic AOP?

Dynamic AOP or dynamic weaving consists of adding/removing aspects to/from your system at runtime, without need for recompilation or rebooting.

In practice, this means that you can enable and disable aspects at runtime, including aspects previously unknown to your system. There are several scenarios where this functionality comes in handy. For example, to debug failures that show up only after your application has been running for a long time. In a situation like this, a lot of time could be saved as dynamic AOP will not require the system to be rebooted in order to attach an aspect to debug your application. Furthermore, dynamic AOP allows the runtime addition of extensions to a system that cannot be rebooted frequently. Another example of applicability is to fix an aspect that contains a bug, or that needs to have its functionality extended at runtime.

JBoss AOP supports dynamic AOP by rebuilding interceptor chains associated with a joinpoint when aspects are added and/or removed. If hot swap is enabled, JBoss AOP also instruments the code at runtime to insert/remove invocations to these chains whenever appropriate. Otherwise, the calls to chains are inserted during compilation or class loading (depending on whether you are using compile time or load time weaving).

Preparing your Application for Dynamic AOP

In order to perform dynamic AOP operations, JBoss AOP must know beforehand which joinpoints need to be instrumented, so that the invocations to interceptor chains are added to the code before it gets executed.
This restriction applies even with hot swap enabled. Due to security constraints of the JVM, the list of methods, constructors, and fields of a class cannot change at runtime. Nor can their signatures. For weaving, JBoss AOP inserts auxiliary fields and methods into your class bytecodes. Given the JVM constraints, this step must be performed before your code gets executed and, hence, the joinpoints to be instrumented must be known at the first weaving stage (compile time or loadtime weaving).

To indicate which joinpoints must be instrumented for dynamic AOP, you must use prepare expressions. A prepare expression can be declared either in an xml file or as an annotation. Look at the example below:

<aop>
  <prepare expr="all(Pojo)"/>
</aop>

This is the content of a jboss-aop.xml file. This file contains a single declaration of a prepare expression, but it could contain as many as you wish. Naturally, it could contain as well tags for non-dynamic AOP purposes, such as pointcuts, bindings, introductions, etc.

Prepare expressions can also be declared in the form of annotations:

@Aspect(scope = Scope.PER_VM)
public class MyAspect
{
  @Prepare ("all(Pojo)")
  public static Pointcut preparePOJO;

  @PointcutDef("execution(Pojo->new())")
  public static Pointcut aPointcut;
}

Notice above the similarity in the syntax of the declaration of a prepare expression, with the @Prepare annotation, and of a pointcut expression, with the @PointcutDef expression. Except for the annotation used, their syntax is the same.

The annotation @Prepare can also be used to annotate plain-old Java classes:

@Prepare
public class Pojo
{
  ...
}

In this case, it indicates that the method execution and field access joinpoints contained in Pojo class must be prepared for runtime weaving of aspects.

Joinpoints matched by plain pointcut expressions can also be the target of dynamic AOP operations. JBoss AOP treats joinpoints matched by pointcuts and joinpoints matched by prepare expression in a very similar way.

The main difference between the two types of expressions lies in their semantic meaning. When you run across a prepare expression, you immediately know that it is being declared for dynamic AOP purposes. On the other hand, if you see a pointcut expression you expect it to be used in one or more bindings. For readability, always prefer to use prepare expressions over plain pointcuts to indicate dynamic AOP usage.

In addition, JBoss AOP does not affect the control flow of joinpoints matched by prepare expressions when using hot swap. The control flow of those joinpoints will be affected only after a dynamic aspect addition is performed, which means you will have a better performance when prepare expressions are used appropriately.

Hot Deployment of Aspects

Hot deployment is the most straightforward way of using dynamic AOP with JBoss AOP, and is available for JBoss AS.

In JBoss AS, all deployment units, such as ears, wars, jars, can be hot deployed. JBoss AS will automatically load the applications added to the deploy folder, and unload them when their units are removed from the deploy folder.

The JBoss AOP deployment units have the .aop suffix. By deploying such an unit at runtime, the aspects contained in it will be automatically deployed to all prepared joinpoints of the previously deployed applications. If you are running the server with load time weaving, all the applications deployed after this moment will also be instrumented by JBoss AOP to insert the aspects contained in the deployment unit, according to the bindings and pointcuts it contains.
Similarly, when a .aop file is removed from the deploy dir, the bindings it contains will be automatically removed from the running applications.

As an alternative to using .aop units, you can use other deployment units and xml files to hot deploy aspects to the server. In this case, the aspect classes are bundled into a non-aop deployment unit (such as a .jar or an .ear) and deployed to the server. The binding declarations are deployed to the server separately in an xml file whose name must end with -aop.xml. The effect of this two-part deployment is the same as you get by deploying an .aop file, but with one advantage. This type of deployment allows you to redeploy only the *-aop.xml file, without causing the aspect classes to be reloaded. The simple removal of your *-aop.xml file will cause the bindings to be undeployed. And the addition of a new *-aop.xml file will cause the bindings declared in it to be deployed to the server.

Wrapping up

In a nutshell, we have seen that:

• only joinpoints matched by prepare and pointcut expressions during compile/load time weaving can be the target of dynamic operations.


• to use dynamic aop in the AS, you can deploy and undeploy .aop files.


• an alternative is to deploy xml files ending in -aop.xml, containing the declarations of bindings and pointcut expressions.

Next Steps

Take a look at the injboss example, available as part of the JBoss AOP documentation. It shows the different ways of packaging aspectized applications to be run in JBoss AS. Try running those examples and playing with dynamic AOP to see it in action! (For example, run $ ant deploy-basic-lt-war and try removing the jboss-aop.xml file from your server deploy dir in order to have the aspects undeployed)

In the next part of the tutorial, we will show how to use the JBoss AOP API to perform dynamic AOP operations. Stay tuned!

Typed Advices Tutorial - Part 3

We have been very busy with implementing the main issues left for 2.0.0.GA. But, finally, I have digged sometime to finish this tutorial. Here is the third and final part. Enjoy!

Typed Advices Tutorial - Part 3

So far I have talked about the general form of the JBoss AOP typed advices, and how to declare them in your jboss-aop.xml file. We also saw how to declare them through annotations. This is all you need to know to get started. This final part of the tutorial goes deeper into the options available for typed advices. We will show a complete list of annotated-parameters available, and talk about the possibility of overloading advices.

Annotated Parameters Revisited

In the previous part, we saw examples that used @Arg, @Target and @Args annotated-parameters. Below you will find the complete list of available parameter annotations:

• @Target: this annotation indicates that the advice parameter receives the target of the joinpoint.


• @Caller: available when intercepting call joinpoints, this annotation is used on the advice parameter that contains the caller.


• @Arg: contains one of the joinpoint arguments. It has an optional attribute, index, if you need to indicate the exact joinpoint argument you are referring to (this happens when the attributes have the same type and JBoss AOP cannot infer which argument your @Arg-annotated parameter refers to).


• @Args: this annotation is used on parameters of the type Object[], that contain the full list of the joinpoint arguments. Use this only when necessary, as there is an overhead incurred in creating this array. This annotation differs from @Arg when it comes to flexibility (your advice will be compatible with any joinpoint, regardless of the type and the number of the joinpoint arguments ) and when you need to edit the value of an argument.


• @Return: available only for after and finally advices. Parameters with this annotation will contain the value returned by the advice.


• @Thrown: this is available only for after-throwing and finally advices. It allows you to receive the exception thrown by the joinpoint.


• @JoinPoint: this annotation is used on reflective parameters, containing joinpoint beans, which allow access to any reflective information regarding the joinpoint. For example, if the joinpoint is a method execution, you can have access to the Method object that represents that object, and to the Class object that represents the class declaring that method. For around advices, the joinpoint beans are the Invocation beans. For the other type of advices, the joinpoint beans are the info beans. While the first ones allow extra features, like meta-data recording, they are also heavier when compared with the info beans.

Bellow you will find an example of a finally advice that uses @JoinPoint, @Thrown, @Return and @Caller:
<aop>
  <aspect class="MyAspect"/>
  <bind pointcut="call(public int POJO1->someMethod(..)) AND within(POJO2)">
    <finally aspect="MyAspect" name="myAdvice"/>
  </bind>
</aop>
----
public class MyAspect
{
  public myAdvice(@JoinPoint MethodCall callBean, @Caller POJO2 caller, @Return int returnedValue, @Thrown Throwable thrown)
  {
    System.out.println(“The execution of joinpoint:”);
    System.out.println(callBean.toString());
    System.out.println(“Executed by caller: “ + caller);
    if (thrown == null)
      System.out.println(“Returned “ + returnedValue);
    else
      System.out.println(“Threw an exception: + thrown);
  }
}


Overloaded Advices

JBoss AOP Advices can be overloaded. This is useful when you want to apply the same advice to several different scenarios. Take a look at the example below:
<aop>
  <aspect class="MyAspect"/>
  <bind pointcut="execution(* POJO->someMethod(..) OR execution(POJO->new(..))">
    <before aspect="MyAspect" name="loggerAdvice"/>
  </bind>
</aop>
----
public class MyAspect
{
  public void loggerAdvice(@JoinPoint JoinPointBean joinPointInfo)
  {
    if (joinPointInfo instanceof MethodExecution)
    {
      System.out.println(“Intercepting method execution:” + ((MethodExecution) joinPointInfo).getMethod().getDeclaringClass().getName() + “->” + ((MethodInfo) joinPointInfo).getMethod().getName());
    }
    else if (joinPointInfo instanceof ConstructorExecution)
    {
      System.out.println(“Intercepting constructor execution:” + ((ConstructorExecution)joinPointInfo).getConstructor().getDeclaringClass().getName() + “->new” );
    }
  }
}

This advice checks the type of joinPointBean every time it gets executed, so it can do a type casting and extract the information it needs. The cost of performing these operations every time the advice gets called can be avoided with the use of overloaded advices. Look at the overloaded implementation of loggerAdvice below:

public class MyAspect
{
  public void loggerAdvice(@JoinPoint MethodExecution methodExecution)
  {
    System.out.println(“Intercepting method execution:” + ((MethodExecution) joinPointInfo).getMethod().getDeclaringClass().getName() + “->” + ((MethodInfo) joinPointInfo).getMethod().getName());
  }

  public void loggerAdvice(@JoinPoint ConstructorExecution constructorExecution)
  {
    System.out.println(“Intercepting constructor execution:” + ((ConstructorExecution)joinPointInfo).getConstructor().getDeclaringClass().getName() + “->new” );
  }
}

The overloaded version of loggerAdvice allows a more efficient interception, because JBoss AOP will define when to call which advice method only once per joinpoint, avoiding the extra cost of checking the type of the joinpoint every time it executes. As a plus, you get a cleaner code.


Sintax and Semantics Rules

There are other rules when writing your typed advices. For example, your advice must match the joinpoints it will intercept. You cannot write an advice that receives a target whose type is Collection and declare it to be String. However, you are not supposed to worry about all the rules involved in writing advices. While you can read all the rules at our documentation, there are error messages for each rule you might break. This way, if you break a rule, JBoss AOP will point you what is wrong so you can easily fix it.

As an example, I am going to do a slight change to the myAdvice example given above:

<aop>
  <aspect class="MyAspect"/>
  <bind pointcut="call(public int POJO1->someMethod(..) AND within(POJO2)">
    <finally aspect="MyAspect" name="myAdvice"/>
  </bind>
</aop>
----
public class MyAspect
{
  public myAdvice(@JoinPoint JoinPointBean callBean, @Caller POJO1 caller, @Return int returnedValue, @Thrown Throwable thrown)
  {...}
}

An error has been inserted to the advice above: it receives a caller of type POJO1, while the pointcut expression clearly states that the type of the caller is POJO2. Unless POJO2 extends POJO1, we are going to get an error from JBoss AOP like the one below:

org.jboss.aop.advice.NoMatchingAdviceException: No matching finally advice called 'myAdvice' could be found in MyAspect for joinpoint Method called by Constructor[calling=public POJO2(boolean) throws java.lang.Exception,called=public int POJO1.someMethod(boolean) throws java.lang.Exception]
  On method 'public void MyAspect.myAdvice(org.jboss.aop.joinpoint.MethodCall,POJO1,int,java.lang.Throwable)'
   @Caller-annotated parameter is not assignable from expected type class POJO2


Next steps

This tutorial introduced you to the new typed advices of JBoss AOP. You can see concrete examples in the tutorial that comes bundled with the JBoss AOP distribution. A good way of starting to write typed advices is playing with those examples, changing the code and seeing what are the results.

Besides, you can also consult our Reference Guide, that will describe typed advices in detail, including a complete description of all the rules involving them (Chapter 4. Advices). There you will also find a table of the possible parameter annotations and which types of advices support them. At the end of Chapter 3 (Joinpoint and Pointcut Expressions), you can also find a complete table containing the two joinpoint bean types available: invocation and info beans.

If you have any questions that are not answered at our documentation, open a thread in our user forum and we will answer as fast as we can.

Typed Advices Tutorial - Part 2

Today we will see what are the signature rules a valid typed advice must follow.


Introduction to Annotated Parameters

Typed advices can receive as parameters several context values, besides reflective objects. In this introduction, we will talk only about parameters that receive the joinpoint target and the argument values. The other options will be seen in the next part of this tutorial.


Take a look at the following example:

<aop>
  <aspect class="MyAspect"/>
  <bind pointcut="public int POJO->someMethod(..)">
    <before aspect="MyAspect" name="myAdvice"/>
  </bind>
</aop>
----
public class MyAspect
{
  public myAdvice(@Target POJO pojo, @Arg int arg1, @Arg long arg2)
  {
    System.out.println("Hello world!");
  }
}

This example binds a before advice with the pointcut expression "public int POJO->someMethod(..)". The advice receives as parameters the target of POJO.someMethod execution, and two arguments which should be of type int and long. Now, take a look at the class POJO below:

public class POJO
{
  public void someMethod(int arg1, long arg2){}
  public void someMethod(int arg1, int arg2, long arg3) {}
  public void someMethod(String arg1, int arg2, long arg3) {}
  // public void someMethod(String arg1, int arg2) {}
}

This is what would happen to our advice when it intercepts each one of the methods above:

- someMethod(int arg1, long arg2)
JBoss AOP invokes the advice passing as parameters the POJO target, and the two arguments received by someMethod

- someMethod(int arg1, int arg2, long arg3)
The same as before, but the advice will receive the values of the first and third arguments, skipping int arg2.

- someMethod(String arg1, int arg2, long arg3)
Now the advice will receive the values of arg2 and arg3 as the @Arg-annotated parameters.

Would the last method of POJO, someMethod(String, int), be uncommented, JBoss AOP will throw an InvalidAdviceException telling you that it cannot find a way of applying your advice to a method that does not receive a long-typed argument.

Now, look at the second version of our advice below:

public class MyAspect
{
  public myAdvice(@Target POJO pojo, @Args Object[] args)
  {
     if(args.length > 0 && args[0] instanceof Integer)
     {
       // the intercepted method will receive a new int argument value
       args[0] = Integer.valueOf(args[0].intValue() - 1);
     }
  }
}

This advice receives the list of the intercepted method arguments regardless of the number and type of arguments declared by this method. This would allow us to uncomment POJO.someMethod(String, int) without getting an InvalidAdviceException. Besides, any changes performed to the values contained in the args array will be propagated to the joinpoint. This allows myAdvice to edit the value of a joinpoint argument, as shown above.

Flexibility and Parameter Annotations

So what you are probably asking yourself is why do you have to use the annotations @Target, @Arg and @Args to indicate the meaning of each of your advice's parameters.

The answer to this question is flexibility. Take a look at the two different advices below:

public void advice1(@Target Object target)
public void advice2(@Arg Object arg)

As you can see, you do not have to specify the exact type of the target of the joinpoint, as we did before. Instead of @Target POJO, our example could have used @Target Object as the advice parameter, which is useful when your advice is going to be applied to several different target types. This is what advice1 above does: it does not specify the type of the target. JBoss AOP will successfully apply this advice to any joinpoint, regardless of the type of the target.

The second advice, advice2, also does not specify the type of the argument it wants to receive. So it will receive as parameter the first non-primitive argument of the intercepted joinpoint.

Now, if we remove the parameter annotations from advice1 and advice2, we will not be able of differentiating between those advices, except for their names. Hence, JBoss AOP will not be able of knowing what value it should provide to the advice parameter. This gets more complex when you have more than one advice parameter, as the example below:

public void advice1(@Target Object target, @Arg Object arg)
public void advice2(@Arg Object arg, @Target Object target)

Both advices receive the joinpoint target and the first non-primitive joinpoint argument value. How could we differentiate between them and decide what value to pass to each of those advice parameters without the annotations?

This makes the parameter annotation usage essential to achieve total flexibility regarding what to receive as parameter value, and in which type and order. So, when writing your typed advices you must follow the rule below:

The parameters of a typed advice must be annotated.

Advice Return Type

All the typed advices we have seen so far had a void return type.

This is not mandatory. After and finally advices are allowed to have a non-void return type. Take a look at the following example:

<aop>
  <aspect class="MyAspect">
  <bind pointcut="">*(..)">
    <after aspect="MyAspect" name="myAdvice"/>
  </bind>
</aop>
----
public MyAspect
{
  public String myAdvice()
  {
     return "Hello world!";
  }
}

The advice above intercepts all methods of POJO that return a String and it overwrites the method return value, by returning the string "Hello World".

In practice, this is useful for aspects that perform extra actions on the joinpoint return type. For example, an aspect that implements a remote call layer would want to deserialize the return value of a remote call before providing the result to the caller.

Throwing Exceptions

Your advice is allowed to declare to throw any exceptions you like. JBoss AOP will wrap that exception inside a RuntimeException if this exception is not declared by the intercepted joinpoint. On the other hand, if the joinpoint declares to throw that exception type, JBoss AOP will just throw the exception as is. Either way, the basis application will get the exception as if the intercepted joinpoint had thrown it.

Part III Preview

In the next part, we will see the complete list of parameter annotations available. Besides, we will talk about overloaded advices. See you!

Typed Advices Tutorial - Part 1

As we get closer and closer to our 2.0.0.GA release, I found it would be interesting to post a tutorial on typed advices, one of the new features of the next GA release.

The tutorial, targeted to users already familiar with the basics of JBoss AOP, will be split into parts. The plan is to post a new part every Monday. Enjoy!

Typed Advices Tutorial - Part 1

In this part, we will see what are the new types of advices and how to declare a binding using typed advices.


5 Different Types

JBoss AOP now supports 5 types of advices:

• before: advices of this type are executed before the joinpoint.
  
• around: this is equivalent to the single advice type previously supported by JBoss AOP. Around advices work like the interceptors, wrapping the joinpoint execution. They are invoked by JBoss AOP as if they were the original joinpoint, and it is up to them to proceed execution to the joinpoint itself, through the method invokeNext() of the class Invocation.
  
• after: advices of the type after are executed after the joinpoint returns normally.
  
• throwing: advices of this type are invoked only after the joinpoint throws an exception. If the joinpoint returns normally, these advices will not be called.
  
• finally: these advices are invoked after the joinpoint execution, regardless of the way it returns.
  

The before, after, throwing and finally advices are more lightweight when compared with around advices. Hence, they must be chosen whenever possible over around advices.

Binding Typed Advices

In the previous versions of JBoss AOP, the XML tag advice was the only choice to declare an advice. This tag must be inside a bind tag, like the example below:

<bind pointcut="execution(* *->(..))">
   <advice aspect="Aspect" name="myAdvice"/>
</bind>

Notice that "Aspect" must have been declared before the bind tag, in an aspect tag.

To declare a typed advice, the XML would be similar to the above, except for the fact that the tag advice will be replaced by a tag with the name of the type of the advice you want to bind.

The example below binds five advices, one of each type. As you can see, the tags before, after, around, throwing, and finally have the same attributes as the tag advice.

<bind pointcut="execution(* *->(..))">
   <before aspect="Aspect" name="myBeforeAdvice"/>
   <around aspect="Aspect" name="myAdvice"/>
   <after aspect="Aspect" name="myAfterAdvice"/>
   <throwing aspect="Aspect" name="myThrowingAdvice"/>
   <finally aspect="Aspect" name="myFinallyAdvice"/>
</bind>

Notice that the tag advice is also supported in the new JBoss AOP version. As this tag does not indicate the type of the advice being declared, the default type, around, will be used. This keeps compatibility with previous versions, garanteeing that no AOP code will be broken when upgrading to the new release.

To allow declaration of typed advice bindings through annotations, a new attribute has been added to the annotation @Bind:

@Bind(pointcut="execution(* *->(..))", type=AdviceType.BEFORE)

The example above shows an advice annotation that binds the annotated advice (hidden in the example) to the same pointcut we saw in the previous examples. It also specifies the type of the advice as being before. Similarly, we can declare advices of all the other four types. The annotation parameter type is optional and its default value is AdviceType.AROUND.

Notice that the class that contains the advice must be annotated with @Aspect in order to be recognized as an aspect by JBoss AOP.

An Example of Typed Advice

The following Java method is a simple example of typed advice:

public void myAdvice()
{
   System.out.println("Hello world!");
}

The signature of typed advices is pretty flexible, allowing even void advices with no parameters at all. The only type of advice that cannot have the above signature is the throwing advice. As we will see, this advice has a mandatory parameter. This means that the method above could be a before, around, after, or finally advice.

In the next part of this tutorial we will see more about the signature rules, showing how an advice can receive pretty much any joinpoint information as a parameter.

See you next week!