Spring boot 自定义注解中的方面

Question

I am creating a custom annotation NullCheck for method parameter to check value is null or not hello(@NullCheck String text) , but I am not able to invoke Aspect around the Annotation.

Main Class

package com.example.demo;
import org.springframework.boot.SpringApplication;
    import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
    import org.springframework.boot.autoconfigure.SpringBootApplication;
    import org.springframework.context.annotation.EnableAspectJAutoProxy;
    @SpringBootApplication
    @EnableAutoConfiguration
    @EnableAspectJAutoProxy
    public class DemoApplication {
        public static void main(String[] args) {
            SpringApplication.run(DemoApplication.class, args);
        }
    }

Controller class, just invoking an aspect for POC, not returning anything

package com.example.demo;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController; 
@RestController
@RequestMapping("/")
class HelloController {
    @GetMapping
    public void hello() {
        hello("hi");
    }
    private void hello(@NullCheck String text) {
        System.out.println(text);
    }
}

Annotation

package com.example.demo;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
@Documented
@Retention(RUNTIME)
@Target(ElementType.PARAMETER)
public @interface NullCheck { }

Aspect

package com.example.demo;
import org.aspectj.lang.JoinPoint;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
import org.springframework.stereotype.Component;
@Aspect
@Component
public class NullCheckAspect {

this is working

@Before("@annotation(org.springframework.web.bind.annotation.GetMapping)")

but this is not

@Before("@annotation(com.example.demo.NullCheck)")
    public void beforeAdvice(JoinPoint joinPoint) {
        System.out.println("Before method:" + joinPoint.getSignature());
    }
}

build.gradle

buildscript {
    ext {
        springBootVersion = '2.1.2.RELEASE'
    }
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
    }
}
apply plugin: 'java'
apply plugin: 'idea'
apply plugin: 'eclipse'
apply plugin: 'org.springframework.boot'
apply plugin: 'io.spring.dependency-management'
group = 'com.example'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '1.8'
idea {
    module {
        // if you hate browsing Javadoc
        downloadJavadoc = true
        // and love reading sources :)
        downloadSources = true
    }
}
bootJar {
    launchScript()
}
repositories {
    mavenCentral()
    jcenter()
}
bootJar {
    launchScript()
}
dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-actuator'
    implementation 'org.springframework.boot:spring-boot-starter-aop'
    implementation 'org.springframework.boot:spring-boot-starter-web'
    runtimeOnly 'org.springframework.boot:spring-boot-devtools'
}

what am I missing?

Answer 1

As per my understanding and after doing some search on google, you can get method parameter and its value with particular annotation with following code:

package com.example.demo;

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.reflect.MethodSignature;
import org.springframework.stereotype.Component;

@Component
@Aspect
public class NullCheckAspect {

    @Around("execution(* com.example.demo.HelloController.nullChecker(String))")
    public Object around(ProceedingJoinPoint pJoinPoint) throws Throwable {

        Object[] args = pJoinPoint.getArgs();

        Method method = MethodSignature.class.cast(pJoinPoint.getSignature()).getMethod();

        Annotation[][] parametersAnnotations = method.getParameterAnnotations();

        Map<String, Object> annotatedParameters = new HashMap<>();

        int i = 0;
        for(Annotation[] parameters : parametersAnnotations) {
                Object arg = args[i];
                String name = method.getParameters()[i++].getDeclaringExecutable().getName();
                for(Annotation parameter: parameters) {
                    if ((parameter instanceof NullCheck)) {
                        System.out.println("Found the null checker annotation with name: " + name);
                        System.out.println("Found the null checker annotation with arg:  " + arg);
                        annotatedParameters.put(name, arg);
                }
                }
        }
        System.out.println(annotatedParameters);

        return pJoinPoint.proceed(args);
    }
}

And with the interface annotation:

package com.example.demo;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import org.springframework.stereotype.Component;

@Component
@Retention(RUNTIME)
@Target(ElementType.PARAMETER)
public @interface NullCheck {
}

More details about my code you can check it at my github repository, I have created spring boot demo version and pushed it for you at https://github.com/krishnaiitd/learningJava/blob/master/springBoot/gs-spring-boot/src/main/java/com/example/demo/HelloController.java

This also include other type of aspects like tracking the time of a particular methods.

Hope this will help you to get the basic understanding of @Aspect in Spring boot.

Answer 2

I don't know why it is as it is but I had the same issue. This is can be solved easily by using Pointcuts, for example:

@Before("nulllCheckAnnotation()")
    public void beforeAdvice(JoinPoint joinPoint) {
        System.out.println("Before method:" + joinPoint.getSignature());
    }
}

@Pointcut("@annotation(com.example.demo.NullCheck)")
private void nulllCheckAnnotation() { }

Read more about pointcuts here if you interested: https://www.baeldung.com/spring-aop-pointcut-tutorial