Question

I want to have all my DAO methods to return an empty collection instead of null. How can I do this with AOP spring?

Answer 1

This should work:

@Aspect
@Service
public class DaoAspect {

    @Around("execution(java.util.List com.example.*Dao.get*())")
    public Object aroundGetDaoMethods(ProceedingJoinPoint joinPoint) throws Throwable {
        final Object retVal = joinPoint.proceed();
        return retVal != null ? retVal : Collections.emptyList();
    }

}

Adjust pointcut to match only methods you wish to intercept. Also you need to add AspectJ JARs:

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-aop</artifactId>
    <version>3.1.2.RELEASE</version>
</dependency>
<dependency>
    <groupId>org.aspectj</groupId>
    <artifactId>aspectjrt</artifactId>
    <version>1.6.6</version>
</dependency>
<dependency>
    <groupId>org.aspectj</groupId>
    <artifactId>aspectjweaver</artifactId>
    <version>1.6.6</version>
</dependency>

and enable CLASSPATH scanning:

<aop:aspectj-autoproxy/>