Java, custom annotation, AspectJ, Maven: minimalist example

08 Apr 2017

Goal: some code must be executed before/after a method annotated with custom annotation. We need a custom annotation first:

package yourpackage;
 
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
 
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MyCustomAnnotation {
    public boolean isRun() default true;
}

An aspect:

package yourpackage;
 
import org.aspectj.lang.JoinPoint;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
 
@Aspect
public class MyCustomAspect {
    @Before("execution(* *.*(..)) && @annotation(MyCustomAnnotation)")
    public void advice(JoinPoint joinPoint) {
        System.out.printf("BINGO! advice() called before '%s'%n", joinPoint);
    }
}

pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
 
    <groupId>Group</groupId>
    <artifactId>Main</artifactId>
    <version>1.0-SNAPSHOT</version>
 
    <dependencies>
        <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjweaver</artifactId>
            <version>1.8.10</version>
        </dependency>
    </dependencies>
 
    <build>
        <plugins>
 
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>aspectj-maven-plugin</artifactId>
                <version>1.7</version>
                <configuration>
                    <complianceLevel>1.8</complianceLevel>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
                <executions>
                    <execution>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
 
            <plugin>
                <artifactId>maven-assembly-plugin</artifactId>
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <mainClass>yourpackage.AspectJRawTest</mainClass>
                        </manifest>
                    </archive>
                    <descriptorRefs>
                        <descriptorRef>jar-with-dependencies</descriptorRef>
                    </descriptorRefs>
                </configuration>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>single</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
 
        </plugins>
 
    </build>
</project>

Main class:

package yourpackage;
 
import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
 
public class AspectJRawTest {
 
    public static void main(String[] args) {
        System.out.println("custom annotation playground");
 
        ISomething something = new SomethingImpl();
 
        something.annotatedMethod();
        something.notAnnotatedMethod();
    }
 
}
 
interface ISomething {
    void annotatedMethod();
 
    void notAnnotatedMethod();
}
 
class SomethingImpl implements ISomething {
    @MyCustomAnnotation
    public void annotatedMethod() {
        System.out.println("I am annotated and something must be printed by an advice above.");
        CalledFromAnnotatedMethod ca = new CalledFromAnnotatedMethod();
    }
 
    public void notAnnotatedMethod() {
        System.out.println("I am not annotated and I will not get any special treatment.");
        CalledFromAnnotatedMethod ca = new CalledFromAnnotatedMethod();
    }
}
 
/**
 * Imagine this is your bean which needs to know if any annotations affected its construction
 */
class CalledFromAnnotatedMethod {
    CalledFromAnnotatedMethod() {
        List<Annotation> ants = new ArrayList<Annotation>();
        for (StackTraceElement elt : Thread.currentThread().getStackTrace()) {
            try {
                Method m = Class.forName(elt.getClassName()).getMethod(elt.getMethodName());
                ants.addAll(Arrays.asList(m.getAnnotations()));
            } catch (ClassNotFoundException ignored) {
            } catch (NoSuchMethodException ignored) {
            }
        }
        System.out.println(ants);
    }
}

Assemble using maven:
mvn clean compile assembly:single

An executable jar with dependencies included will be created.

Output: java -jar target/Main-1.0-SNAPSHOT-jar-with-dependencies.jar

custom annotation playground
BINGO! advice() called before 'execution(void yourpackage.SomethingImpl.annotatedMethod())'
I am annotated and something must be printed by an advice above.
I am not annotated and I will not get any special treatment.