How to write an annotation/aspect to not enter a method but return null if a given condition is false?

Question

I currently have a requirement where I need to return null from 100s of methods if a given condition is false. I was thinking of using Java Annotations or Spring Aspects for thi

Answer 1

Without spring you could use pure AspectJ:

Demo.java : example of the method we want to modify

public class Demo {
    public String boom(String base) {
        return base;
    }
}

DemoAspect.aj : configuration file. Getting started. In AspectJ, pointcuts pick out certain join points in the program flow. To actually implement crosscutting behavior, we use advice. Advice brings together a pointcut (to pick out join points) and a body of code (to run at each of those join points): before, around, after...

public aspect DemoAspect {
    pointcut callBoom(String base, Demo demo) : 
            call(String Demo.boom(String)) && args(base) && target(demo);

    String around(String base, Demo demo) : callBoom(base, demo){
        if ("detonate".equals(base)) {
            return "boom!";
        } else {
            return proceed(base, demo);
        }
    }
}

DemoTest.java

public class DemoTest {
    @Test
    void name() {
        Demo demo = new Demo();
        assertEquals("boom!", demo.boom("detonate"));
        assertEquals("fake", demo.boom("fake"));
    }
}

Add dependencies into pom.xml

    
        org.aspectj
        aspectjrt
        1.8.13
    
    
        org.aspectj
        aspectjweaver
        1.8.13
    

And plugin, pay attention to the versions. For example 1.11 plugin expects 1.8.13 libraries.

    org.codehaus.mojo
    aspectj-maven-plugin
    1.11
    
        1.8
        1.8
        1.8
        true
        true
        ignore
        UTF-8 
    
    
        
            
                
                compile
                
                test-compile
            
        
    

Good example with more details, Baeldung. Official documentation.