Adding micrometer dependency causes weird Spring proxy issue

Question

I have a simple Spring Boot application with private @Scheduled method:

@SpringBootApplication
@EnableScheduling
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @Scheduled(fixedRate = 1000)
    private void scheduledTask() {
        System.out.println("Scheduled task");
    }
}

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>com.example</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>demo</name>
    <description>Demo project for Spring Boot</description>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.8.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <!--<dependency>
            <groupId>io.micrometer</groupId>
            <artifactId>micrometer-spring-legacy</artifactId>
            <version>1.1.1</version>
        </dependency>
        <dependency>
            <groupId>io.micrometer</groupId>
            <artifactId>micrometer-registry-prometheus</artifactId>
            <version>1.1.1</version>
        </dependency>-->

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>

        <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjrt</artifactId>
            <version>1.8.11</version>
        </dependency>
        <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjweaver</artifactId>
            <version>1.8.11</version>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>


</project>

When micrometer dependencies and commented out everything works as expected, but when I uncomment them, following exception occurs:

Caused by: java.lang.IllegalStateException: Need to invoke method 'scheduledTask' found on proxy for target class 'DemoApplication' but cannot be delegated to target bean. Switch its visibility to package or protected.
    at org.springframework.aop.support.AopUtils.selectInvocableMethod(AopUtils.java:133) ~[spring-aop-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.scheduling.annotation.ScheduledAnnotationBeanPostProcessor.processScheduled(ScheduledAnnotationBeanPostProcessor.java:343) ~[spring-context-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.scheduling.annotation.ScheduledAnnotationBeanPostProcessor.postProcessAfterInitialization(ScheduledAnnotationBeanPostProcessor.java:326) ~[spring-context-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsAfterInitialization(AbstractAutowireCapableBeanFactory.java:423) ~[spring-beans-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1633) ~[spring-beans-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:555) ~[spring-beans-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    ... 15 common frames omitted

Does anyone have an idea what's going on underneath?

Edit: I think there is some kind of conflict between mictometer and aspectJ dependencies, the issue occurs only if both are present on classpath.

Answer 1

Spring advice is usually implemented when possible with JDK proxies, which require an interface to implement dynamically (which does the interesting logic and then delegates to your business code). In a case like this, you have an actual class, so the best Spring can do is to subclass it.

However, there are two conflicting requirements here: Spring wants to apply advice that captures metrics for your code, but since the method is private, it isn't accessible from a subclass. (I'm actually moderately surprised it's detecting and calling your scheduled task with a private method.)

Changing your method to protected allows Spring to do this at runtime (not actual Java code, but the equivalent generated bytecode):

class DemoApplicationWithAdvice extends DemoApplication {
    @Override
    protected void scheduledTask() {
        // record start time
        super.scheduledTask();
        // write metric with execution time
    }
}