一、代码 

package com.itheima.aop;

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Aspect // ⬅️注意此切面并未被 Spring 管理
public class MyAspect {

    private static final Logger log = LoggerFactory.getLogger(MyAspect.class);

    @Before("execution(* com.itheima.service.MyService.*())")
    public void before() {
        log.debug("before()");
    }
}

package com.itheima.service;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Service;

@Service
public class MyService {

    private static final Logger log = LoggerFactory.getLogger(MyService.class);

    final public void foo() {
        log.debug("foo()");
        this.bar();
    }

    public void bar() {
        log.debug("bar()");
    }
}

<?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>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.5.5</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.itheima</groupId>
    <artifactId>aspectj_02</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>aspectj_02</name>
    <description>aspectj_02</description>
    <properties>
        <java.version>8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>

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

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

        <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjweaver</artifactId>
        </dependency>

        <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjrt</artifactId>
        </dependency>


    </dependencies>

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

</project>

package com.itheima;

import com.itheima.service.MyService;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

/*
    注意几点
    1. 版本选择了 java 8, 因为目前的 aspectj-maven-plugin 1.14.0 最高只支持到 java 16
    2. 运行时需要在 VM options 里加入 -javaagent:C:/Users/manyh/.m2/repository/org/aspectj/aspectjweaver/1.9.7/aspectjweaver-1.9.7.jar
        把其中 C:/Users/manyh/.m2/repository 改为你自己 maven 仓库起始地址
 */
@SpringBootApplication
public class A10 {

    private static final Logger log = LoggerFactory.getLogger(A10.class);

    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(A10.class, args);
        MyService service = context.getBean(MyService.class);

        // ⬇️MyService 并非代理, 但 foo 方法也被增强了, 做增强的 java agent, 在加载类时, 修改了 class 字节码
        log.debug("service class: {}", service.getClass());
        service.foo();

//        context.close();

        /*
            学到了什么
            1. aop 的原理并非代理一种, agent 也能, 只要字节码变了, 行为就变了
         */
    }
}

二、测试

发现确实执行了切面类里面的方法，对目标方法增强了

查看编译后的Class文件，发现Class文件并没有被改动

用Arthas反编译，查看发现 在java类加载阶段改动了目标方法的字节码，从而实现对目标方法的增强