Interview Questions

Java Keywords
Key Concepts
Key Concepts #2
OOPS in java
Java Collections#1
Java Collections #2
Exceptions #1
Exceptions #2
Threads#1
Threads#2
InnerClass
Serialization
Immutable Class
Cloning in Java
Garbage Collection
JSP #1
JSP #2
Programming Q#1
Programming Q#2
Programming Q#3


Spring AOP Tutorial


Aspect-Oriented Programming (AOP) complements Object-Oriented Programming (OOP) by providing another way of thinking about program structure. The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect. Aspects enable the modularization of concerns such as transaction management that cut across multiple types and objects.

AOP concepts:

* Aspect: a modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in J2EE applications. In my words: a trigger which can affect the multiple classes a one point….

* Join point: a point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution. In my words: a locus of points where execution will happen.

* Advice: action taken by an aspect at a particular join point. Different types of advice include "around," "before" and "after" advice. (Advice types are discussed below.) In my words : the action to be taken at the point.

* Pointcut: a predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name). The concept of join points as matched by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut expression language by default. In my words a criteria used to locate point.

* Introduction: declaring additional methods or fields on behalf of a type. Spring AOP allows you to introduce new interfaces (and a corresponding implementation) to any advised object. For example, you could use an introduction to make a bean implement an IsModified interface, to simplify caching. (An introduction is known as an inter-type declaration in the AspectJ community.)

* Target object: object being advised by one or more aspects. Also referred to as the advised object. Since Spring AOP is implemented using runtime proxies, this object will always be a proxied object.

* AOP proxy: an object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). In the Spring Framework, an AOP proxy will be a JDK dynamic proxy or a CGLIB proxy.


Consider the example:

Lets declare an interface:

public interface Foo {

Foo getFoo(String fooName,int age);

void getAfter();

void getBefore(String myName);

}

A class implementing the interface:

public class DefaultFooService implements FooService {

public Foo getFoo(String name, int age) {

return new Foo(name, age);

}

public void getAfter() {}

public void getBefore(String myName) {}

}


Till here we have simple java implementation.Now let see come AOP concepts in picture.

Before - Now I want that before the getBefore() method is called I want to log message saying what is the parameter passed.
After - Also I want that once any method in the interface is called a message should be logged after it.
I have a class which will be called to satisfy the above criteria.

public class SimpleProfiler {

public void afterMethod() throws Throwable {

System.out.println("After the method call");

}

public void beforeMethod(String myName){

System.out.println("My name is "+myName);

}

}

The afterMethod() will log message after any method is called and beforeMethod() will log message before getBefore() is called.
The AOP is configured in xm'l'spring.xml'.This is how I can configure my spring.xml.

<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">

<!-- this is the object that will be proxied by Spring's AOP infrastructure -->

1 <bean id="fooService" class="DefaultFooService"/>

2

3 <!-- this is the actual advice itself -->

4 <bean id="profiler" class="SimpleProfiler"/>

5

6 <aop:config>

7 <aop:aspect ref="profiler">

14 <aop:pointcut id="aopafterMethod"

expression="execution(* FooService.*(..))"/>

15 <aop:after pointcut-ref="aopafterMethod"

method="afterMethod"/>

16 <aop:pointcut id="aopBefore"

expression="execution(* FooService.getBefore(String)) and args(myName)"/>

17 <aop:before pointcut-ref="aopBefore"

method="beforeMethod"/>

</aop:aspect>

</aop:config>

</beans>


Let see how we have configure the AOP .
  • Line 1 is used to create a proxy AOP object..

  • Line 7 we define the aspect “SimpleProfiler? class which will come into picture at different point-cuts.

  • Line 6 is used to configure the AOP.

  • Line 14 defines a pointcut in which an expression needs to mention. In this case the expressions say that “call afterMethod of SimpleProfiler class for any method declared inside the FooService interface.

  • Note Line 14 doesn’t define when to call afterMethod().This is done in line 15

  • Line 15 states that call afterMethod() for id aopAfterMethod

Similarly for beforeMethod we define in Line 16,17.
In above example we have

Aspect – SimpleProfiler.class
Point-cut – aopafterMethod,aopBefore
Advice <aop:before> <aop:after>

Now I am ready to run my main class and class methods of FooService.

 public class Boo {
   public static void main(final String[] args) throws Exception {
      BeanFactory ctx = new ClassPathXmlApplicationContext("spring.xml");
      FooService foo = (FooService) ctx.getBean("fooService");
      foo.getFoo("Pengo", 12);
      foo.getAfter();
      foo.getBefore("Harshit");
    }
 }


OutPut is
After the method call // ( log messagefor getAfter method )
My name is Harshit //(log message for getBefore)
After the method call //(log message for getAfter method)


Learn More
How to build various pointcut expression.