AspectJ: Applying Advice to Methods Based on an Annotation

I am new to AspectJ (not new to AOP though) and have been recently doing some R&D on using AspectJ for intercepting methods annotated with an annotation.

What I Tried …
Use AspectJ to intercept all the public methods (belonging to classes of specified packages) based on an annotation applied.

What I Needed …
Aspect J Development Tool plugin for eclipse. [equipped my eclipse 3.7 with AJDT plugin]
Searched for AspectJ getting started tutorials available on the internet. [Visit this link  for a good AspectJ getting started tutorial]
Searched how to write a pointcut for intercepting a method based on annotation.

This is how I did it …

Created a simple annotation named MyAnnotation.

MyAnnotation.java

package com.technicalmusings.examples.aspectj.annotation;

public @interface MyAnnotation {}


Created three classes Hello1, Hello2 and Hello3 having public and private methods annotated with MyAnnotation. The intent was to apply a before advice to all the public methods (annotated with MyAnnotation) present in the package com.technicalmusings.examples and its sub-packages. Therefore these three classes have been kept in three different packages. Please refer to the class organization in the screenshot below:

AspectJProjectOrganization

Hello1 class has a method without annotation as well to ensure that the pointcuts are doing exactly what they are supposed to.

Hello1.java

package com.technicalmusings.examples.hello1;
import com.technicalmusings.examples.aspectj.annotation.MyAnnotation;

public class Hello1 {

    public static void main(String[] args) {
        methodOfHello1WithoutAnnotation();
        greetPubliclyFromHello1();
    }

    public static void methodOfHello1WithoutAnnotation() {
        System.out.println("Greetings from a method without annotation from Hello1 class!");
    }

    @MyAnnotation
    public static void greetPubliclyFromHello1() {
        System.out.println("Greetings from public method of Hello1 class!");
    }
}


Hello2.java

package com.technicalmusings.examples.aspectj.hello2;
import com.technicalmusings.examples.aspectj.annotation.MyAnnotation;

public class Hello2 {

    public static void main(String[] args) {
        greetPubliclyFromHello2();
        greetPrivatelyFromHello2();
    }

    @MyAnnotation
    public static void greetPubliclyFromHello2() {
        System.out.println("Greetings from public method of Hello2 class!");
    }

    @MyAnnotation
    private static void greetPrivatelyFromHello2() {
        System.out.println("Greetings from private method of Hello2 class!");
    }
}

Hello3.java

package com.technicalmusings.partofexamples;
import com.technicalmusings.examples.aspectj.annotation.MyAnnotation;

public class Hello3 {

    public static void main(String[] args) {
        greetPrivatelyFromHello3();
        greetPubliclyFromHello3();
    }

    @MyAnnotation
    public static void greetPubliclyFromHello3() {
        System.out.println("Greetings from public method of Hello3 class!");
    }

    @MyAnnotation
    private static void greetPrivatelyFromHello3() {
        System.out.println("Greetings from private method of Hello3 class!");
    }
}

Now the important part, the AspectJ stuff…

Created an aspect, HelloAspect, with one after advice and one before advice (just to differentiate between the two scenario)
a) The after advice is applied to all the methods annotated with MyAnnotation (irrespective of package)
b) The before advice is applied to all the mehods anotated with MyAnnotation belonging to classes of specified package or subpackages

HelloAspect.aj

package com.technicalmusings.examples;

public aspect HelloAspect {

    // Pointcut for all public methods
    pointcut publicPointcut(): execution(public * *(..));

    // Pointcut for all the methods with the specified annotation
    pointcut annotationPointcutAll()
        : execution(@com.technicalmusings.examples.aspectj.annotation.MyAnnotation * *(..));

    // Pointcut for all the methods with the specified annotation within specified package
    pointcut annotationPointcutPackage()
        : execution( @com.technicalmusings.examples.aspectj.annotation.MyAnnotation
         * com.technicalmusings.examples..*(..));

    after() : annotationPointcutAll() && publicPointcut() {
        System.out.println("annotationPointcutAll: Intercepted the after call of : " 
        + thisJoinPoint.getSignature());
    }

    before() : annotationPointcutPackage() && publicPointcut() {
        System.out.println("annotationPointcutPackage: Intercepted the before call of : "
        + thisJoinPoint.getSignature());
    }
}

By executing the classes Hello1, Hello2 and Hello3 individually, we can see how the advices are being applied. The after advice which does not have the package filter should include the method of Hello3 class whereas the other should not.

Below snapshot from ‘Cross References’ view of eclipse shows how the HelloAspect is advising the methods.

HelloAdvice

Enjoy Learning,
Kamlesh

String Utility Classes in Java

Manipulating strings is a part and parcel of a programmer’s day to day life. There are many string related operations which we use quite often. Some of the string operations are provided by Java (classes like String, StringBuilder etc.) and some we devise ourselves. For example, as a good practice the first thing we do while working with string is to check it for null and emptiness as shown below:

if (myString != null || !"".equals(myString)) {
// work with myString
}

For the above and some other frequently used cases of string manipulation we can use the string utility classes already provided by the third party libraries. The string utility class available in apache commons project, org.apache.commons.lang3.StringUtils, is one of the widely used string utility classes. This class has many utility methods which take care of commonly used string operations and most of these are null safe.

For example we can use the isBlank, isNotBlank methods from the StringUtils class to validate the string for empty and null as shown below:

if(StringUtils.isNotBlank(myString)) {
//work with myString
}

The java documentation of the StringUtils.isBlank method says:

public static boolean isBlank(CharSequence cs)
Checks if a CharSequence is whitespace, empty ("") or null.

StringUtils.isBlank(null) = true
StringUtils.isBlank("") = true
StringUtils.isBlank(" ") = true
StringUtils.isBlank("bob") = false
StringUtils.isBlank(" bob ") = false

Based on requirements we can also consider using isEmpty and isNotEmpty.

You can check the API documentation of org.apache.commons.lang3.StringUtils and have a look on other utility methods it provides.

If you are already using Spring framework in your application, you can consider using the StringUtils class available in Spring Framework for the utility methods it provides. This class has been mainly used inside Spring Framework but as it is available, we can use this. Some of the methods I liked from this class are:

// Convert an array or a collection to a comma delimited string (say, for creating a csv file) and vice versa
public static java.lang.String arrayToCommaDelimitedString(java.lang.Object[] arr)
public static java.lang.String[] commaDelimitedListToStringArray(java.lang.String str)

public static java.lang.String collectionToCommaDelimitedString(java.util.Collection<?> coll)
public static java.util.Set<java.lang.String> commaDelimitedListToSet(java.lang.String str)

//Trim an array of strings
public static java.lang.String[] trimArrayElements(java.lang.String[] array)

For more information on Spring Framework’s StringUtil class, check it’s API documentation.

Enjoy Learning,
Kamlesh