
In my last article about **Aspect Oriented Programming** (AOP), I explained [how to use Spring AOP for profiling method execution time][1]. When the article was [posted][2] in DZone, it got a comment from [an user][3] saying that

[1]: https://veerasundar.com/blog/spring-aop-example-profiling-method-execution-time-tutorial "Spring AOP Example: Profiling method execution time tutorial"
[2]: http://www.dzone.com/links/spring_aop_example_profiling_method_execution_tim.html
[3]: http://www.dzone.com/links/users/profile/388907.html

> Profiling and logging seem to be to only imaginable AOP use cases.

But, I think **it is not**. Because AOP is an extensive area where a lot of examples can be shown as use cases of AOP. Here, 'Use cases' are nothing but the **cross cutting concerns** in your application. That is, the functionalities that are used by many part of your application. Logging and Profiling are the best examples and can be easily explained for the cross cutting concerns. That is why in most of the AOP tutorials you can see a logging example or profiling.

Anyway, I tried to do some search on the various AOP cross-cutting concerns (i.e. the _use cases_) that can be found in an J2EE applications. Here is the list of them. Feel free to add if I'm missing/wrongly added something.

## List of AOP Use Cases or Cross cutting concerns:

- **Transaction Management** - in an enterprise application, you can manage the transactional requests using AOP. For example, using the *Around *advice, you can wrap the request processing method and if the method fails for some reason, in your Advice, you can roll back the particular transaction.
- **Access control or Security **- in case you want to restrict the access to your method to a set of users, you can add these checks to the *Before *advice and verify the access credentials of the users before letting them to access your methods.
- **Managing Quotas for your API **- if your API is being used by many third party applications with predefined quotas, you can limit the access based the usage in your AOP advice. If the usage is below the allowed limit, the Advice can let the user to call the API or blocks it otherwise. This is something similar to the Google App Engine / Amazon services quota usages. But I'm not sure how Google/Amazon has implemented the quota limiting.
- **Exception Wrapping** - if you want to wrap a exception, thrown from your business methods, into a Common exception object and throw it to the top level layers, you can use *After throwing *advice to do the task. For example, if you want to wrap all the SQLExceptions into DAOLayerException object and throw it back to Service layer, instead of doing the exception wrapping in every single DAO method, you can implement the exception wrapping in an _After throwing advice_ and it's done.
- **Logging and Profiling - **off course, how can I leave these two? The evergreen examples(!) for the AOP.

I understand that this list is not an extensive one. So, if you have any more use cases to add this list, please leave a comment.


Use cases of Aspect Oriented Programming

I have already written about the _Spring Aspect Oriented Programming (AOP)_ with the use of simple logging example. But, somehow I felt that the example code was little confusing for the newcomers. So, I decided to write a new Spring AOP example code that explains how to use Spring AOP for profiling method execution time.

## Using Spring AOP to profile method execution time:

The example I chose is very simple: our application has a business class and a business method. We are going to profile how much time does it take to execute the business method. We will use Spring AOP for profiling the execution time. Since, profiling is a best example for _cross cutting concern_, it's a good decision to use Spring AOP for implementing it. I'll explain these in step by step. Meanwhile, you can [download the source code][1](excluding dependent JAR file, for which you can see the download links below).

[1]: http://www.box.net/shared/9nygu7t80t "Download source code for Spring AOP example"

And, here's the library versions I'm using: [Spring 2.5.4][2], JDK 1.6, [Commons-logging-1.0.4][3], [aspectjrt-1.2][4], [aspectjweaver-1.5.3][5].

[2]: http://s3.amazonaws.com/dist.springframework.org/release/SPR/spring-framework-2.5.4.zip "Download Spring 2.5.4 JAR"
[3]: http://mirrors.ibiblio.org/pub/mirrors/maven/commons-logging/jars/commons-logging-1.0.4.jar "Download commons logging"
[4]: http://mirrors.ibiblio.org/pub/mirrors/maven/aspectj/jars/aspectjrt-1.2.jar
[5]: http://mirrors.ibiblio.org/pub/mirrors/maven/aspectj/jars/aspectjweaver-1.5.3.jar

## 1. Setting up the project structure:

Launch your preferred IDE (I use Eclipse) and _Create New Java Project_. And then create the file structure as shown below (off course, no need to follow the package naming convention.

![Spring AOP - Project Structure][7]

[7]: https://veerasundar.com/img/2010/01/spring-aop-project-structure.png "spring-aop-project-structure"

You need to download all the JARs that are in _Referenced Libraries_ and add them to the project's class path. You can find these JARs here: [Spring][8], [Commons-logging][9], [aspectjrt][10], [aspectjweaver][11]. Also, please note that the file **Business.java** is an Interface. Other than this, all other java files are plain Classes.

[8]: http://s3.amazonaws.com/dist.springframework.org/release/SPR/spring-framework-2.5.4.zip "Download Spring"
[9]: http://mirrors.ibiblio.org/pub/mirrors/maven/commons-logging/jars/commons-logging-1.0.4.jar "Download Commons-logging"
[10]: http://mirrors.ibiblio.org/pub/mirrors/maven/aspectj/jars/aspectjrt-1.2.jar "Download AspectJrt"
[11]: http://mirrors.ibiblio.org/pub/mirrors/maven/aspectj/jars/aspectjweaver-1.5.3.jar "download Aspectjweaver"

## 2. Writing our Business Logic:

We will first write our business logic and then we will add Spring AOP to profile our business methods. Open _Business.java_ interface and copy the below code into it.

```java

    package com.veerasundar.spring.aop;

    public interface Business {
            void doSomeOperation();
    }

```

Now, open the *BusinessImpl.java* and copy the below code into it.

```java

    package com.veerasundar.spring.aop;

    public class BusinessImpl implements Business {

            public void doSomeOperation() {
                    System.out.println("I do what I do best, i.e sleep.");
                    try {
                            Thread.sleep(2000);
                    } catch (InterruptedException e) {
                            System.out.println("How dare you to wake me up?");
                    }
                    System.out.println("Done with sleeping.");
            }

    }

```

I guess this code is self explanatory. Our business method just sleeps for 2 seconds (good business, isn't it!?) and then write some text on the console.

## 3. Writing a Spring Aspect to profile business method:

Lets write a Aspect which will profile our business method. I'm gonna use **@Around** advice (Lost in the jargons? [Spring AOP basics][12]). Open the *BusinessProfiler.java* file and copy the below code into it.

 [12]: http://static.springsource.org/spring/docs/2.5.4/reference/aop.html#aop-introduction-defn "Spring AOP basics"

```java

    package com.veerasundar.spring.aop;

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

    @Aspect
    public class BusinessProfiler {

            @Pointcut("execution(* com.veerasundar.spring.aop.*.*(..))")
            public void businessMethods() { }

            @Around("businessMethods()")
            public Object profile(ProceedingJoinPoint pjp) throws Throwable {
                    long start = System.currentTimeMillis();
                    System.out.println("Going to call the method.");
                    Object output = pjp.proceed();
                    System.out.println("Method execution completed.");
                    long elapsedTime = System.currentTimeMillis() - start;
                    System.out.println("Method execution time: " + elapsedTime + " milliseconds.");
                    return output;
            }

    }

```

Here's what this code does:

1.  Using *@AspectJ* annotation, we have declared that this class is an Aspect.
2.  Using *@Pointcut* annotation, we have defined a pointcut that will match the execution of all public method inside com.veerasundar.spring.aop package**.** [[read more about pointcut][13]]
3.  Using *@Around* annotation, we have defined a Around advice which will be invoked *before* and *after* our business method. i.e. @Around advice will wrap our business method. As you can see in the above code, *pjp.proceed()***.** This is when our business method gets called from @Around advice.

 [13]: http://static.springsource.org/spring/docs/2.5.4/reference/aop.html#aop-pointcuts "Spring Aspect Oriented Programming - Pointcut examples"

## 4. Configuring Spring AOP and @AspectJ support:

Open the *applicationContext.xml* file and copy the below code into it.

```xml

    <?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="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
    http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd">

        
        <aop:aspectj-autoproxy />

        <bean id="businessProfiler" class="com.veerasundar.spring.aop.BusinessProfiler" />
        <bean id="myBusinessClass" class="com.veerasundar.spring.aop.BusinessImpl" />
    </beans>

```

Here's what we are doing in the above XML file:

1.  We have added the [required AOP schemas][14] on the top of the XML file.
2.  Using , we've enabled the @AspectJ support to our application.
3.  And then we defined two normal Spring beans - one for our Business class and the other for Business Profiler (i.e. our aspect).

 [14]: http://static.springsource.org/spring/docs/2.5.4/reference/xsd-config.html#xsd-config-body-schemas-aop "XML Schemas required for Spring AOP support"

That's it. With all the above code is done, we have successfully added Spring AOP support to our project and configured it. In the next step, lets test our code.

## 5. Testing the Spring AOP profiler:

Open the file *SpringAOPDemo.java* and copy the below lines into it.

```java

    package com.veerasundar.spring.aop;

    import org.springframework.context.ApplicationContext;
    import org.springframework.context.support.ClassPathXmlApplicationContext;

    public class SpringAOPDemo {

            /**
             * @param args
             */
            public static void main(String[] args) {
                    ApplicationContext context = new ClassPathXmlApplicationContext(
                                    "applicationContext.xml");
                    Business bc = (Business) context.getBean("myBusinessClass");
                    bc.doSomeOperation();
            }

    }

```

In the above code we are loading our Business bean from Spring Context and then calling our business method. If you run this class, it will produce the following output on the console.

```java

    Going to call the method.
    I do what I do best, i.e sleep.
    Done with sleeping.
    Method execution completed.
    Method execution time: 2000 milliseconds.

```
I hope this tutorial helps you to understand the basics of Spring Aspect Oriented programming. If you have any questions regarding the above example, feel free to leave a comment**.**

Spring AOP Example: Profiling method execution time tutorial


If you are into Java enterprise application development, then get yourself ready to hear the terms '*Spring*', '*Dependency Injection*' or '*IoC*' a lot. Because, without these terms (and technologies) enterprise development wouldn't be this much simpler. Spring, the framework which became very popular because of it's capability to inject the dependencies to our Java objects in a very efficient manner. Spring's lightweight dependency injection container manages all the defined beans and their dependencies.

## So, what is this 'dependency injection' thing ?

**Dependency Injection** (or DI, in short) is nothing but the dependencies of your Java objects are managed by a **container** so that your object can concentrate on it's functionality rather than searching for it's dependencies. Here, it's the container's (for example Spring's container) responsibility to inject all the dependent objects that your class need. You just need to tell the container what you want and the container takes care of the rest.

Confusing, huh !?

## A Ground-breaking(!) HelloWorld2 App:

Let's take a simple example - a Web2.0 hello world - 'HelloWorld2'². You are writing a 'HelloWorld2'² class which has the capability to tweet the message you are giving. From this, it is clear that your 'HelloWorld2'² will need a TwitterService object to complete its task. So, usually you will declare a TwitterService variable in your 'HelloWorld2'² class, and you will instantiate a new 'TwitterService' object. The 'HelloWorld2'² may look like this:

```java
    class HelloWorld2
    {
    	TwitterService ts = new TwitterService();

    	public void sayHello(String name)
    	{
    		ts.tweet("Hello "   name);
    	}
    }
```

In the above, you are declaring and instantiating the TwitterService object in your HelloWorld2 code itself. In other words, _your code_ is responsible for getting it's dependent objects, in this case the TwitterService object.

Let's see a different version of HelloWorld2.

## The 'Dependency Injected' version of 'HelloWorld2'²:

Now, have a look at this code.

```java
    class HelloWorld2
    {
    	TwitterService ts = null;

    	public void setTs(TwitterService ts){
    		this.ts = ts;
    	}

    	public void sayHello(String name)
    	{
    		ts.tweet("Hello "   name);
    	}
    }
```

What's new in this version?! We've removed the TwitterService object creation, instead created a new setter method setTs() (I know, it's a bad name!). But, where is our mission-critical TwitterService object is getting created?

That's where the dependency Injection comes into play. The creation of objects will be _outsourced_ to the container. Let's outsource this task to the Spring container using a Spring configuration file.

In the above configuration file, we are defining the TwitterService as a _Spring managed_ bean and then, in the 'helloWorld' bean definition, we are telling the Spring container to _inject_ 'twitterService' bean for the property 'ts'. When the Spring container reads this XML file, it will create the 'helloWorld' bean and using the setter method for the variable 'ts', it will inject the 'twitterService' into 'helloWorld' . This technique is called as **_setter method injection_**(duh!).

Apart from this XML based configuration, Spring supports annotations based DI configuration also. I'll be explaining that in my next article.


#spring (3)