Recently, when I was revising resumes and doing mock interviews for many people, some friends gave me feedback on Spring AOP interview questions, and I will ask them today.

The most powerful thing about Spring at the beginning is the two core functions of IOC/AOP. Today we will learn about the common annotations and execution sequence of Spring AOP.

Spring interview core points:

IOC, AOP, Bean injection, Bean life cycle, Bean circular dependency

First of all we Let’s review some commonly used annotations in Spring Aop:

• @Before Pre-notification: Execute before the target method
• @After Post notification: executed after the target method (always executed)
• @AfterReturning Post notification: execution method ends Execute before (not executed if exception occurs)
• @AfterThrowing Exception notification: Execute after exception
• @Around Around notification: Around target method execution

##FAQ

Sample code

Let’s quickly build a demo program of spring aop to discuss spring together Some details in aop.

Configuration file

For the convenience, I directly use spring-boot for quick projects To build, you can use the spring-boot project quick creation function of idea, or go to start.spring.io to quickly create a spring-boot application.

Because I often manually post some dependencies on the Internet, there are some problems such as dependency conflicts and service startup failure.

plugins {
    id 'org.springframework.boot' version '2.6.3'
    id 'io.spring.dependency-management' version '1.0.11.RELEASE'
    id 'java'
}

group 'io.zhengsh'
version '1.0-SNAPSHOT'

repositories {
    mavenCentral()
    maven { url 'https://repo.spring.io/milestone' }
    maven { url 'https://repo.spring.io/snapshot' }
}

dependencies {
    # 其實(shí)這里也可以不增加 web 配置，為了試驗(yàn)簡單，大家請忽略 
    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'org.springframework.boot:spring-boot-starter-actuator'
    implementation 'org.springframework.boot:spring-boot-starter-aop'
    
    testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

tasks.named('test') {
    useJUnitPlatform()
}

Interface class

First we need to define an interface. Here we can review the choice of JDK's default proxy implementation:

• If the target object implements the interface, the JDK dynamic proxy is used by default
• If the target object does not implement the interface, use dynamic proxy
• If the target object implements the interface and Cglib is forced, use cglib proxy

The logic of this piece is in DefaultAopProxyFactory If you are interested, you can take a look.

public interface CalcService {

    public int div(int x, int y);
}

Implementation class

Here we will simply do a division operation, which can simulate normal or easy errors.

@Service
public class CalcServiceImpl implements CalcService {

    @Override
    public int div(int x, int y) {
        int result = x / y;
        System.out.println("====> CalcServiceImpl 被調(diào)用了，我們的計(jì)算結(jié)果是：" + result);
        return result;
    }
}

aop interceptor

#To declare an interceptor, we need to add @Aspect and @Component to the current object. The author has only stepped on it before. Only one such pit has been added.

其實(shí)這塊我剛開始也不是很理解，但是我看了 Aspect 注解的定義我就清楚了

這里面根本就沒有 Bean 的定義。所以我們還是乖乖的加上兩個(gè)注解。

還有就是如果當(dāng)測試的時(shí)候需要開啟Aop 的支持為配置類上增加@EnableAspectJAutoProxy 注解。

其實(shí) Aop 使用就三個(gè)步驟：

• 定義 Aspect 定義切面
• 定義 Pointcut 就是定義我們切入點(diǎn)
• 定義具體的通知，比如: @After, @Before 等。

@Aspect
@Component
public class MyAspect {

    @Pointcut("execution(* io.zhengsh.spring.service.impl..*.*(..))")
    public void divPointCut() {

    }

    @Before("divPointCut()")
    public void beforeNotify() {
        System.out.println("----===>> @Before 我是前置通知");
    }

    @After("divPointCut")
    public void afterNotify() {
        System.out.println("----===>> @After  我是后置通知");
    }

    @AfterReturning("divPointCut")
    public void afterReturningNotify() {
        System.out.println("----===>> @AfterReturning 我是前置通知");
    }

    @AfterThrowing("divPointCut")
    public void afterThrowingNotify() {
        System.out.println("----===>> @AfterThrowing 我是異常通知");
    }

    @Around("divPointCut")
    public Object around(ProceedingJoinPoint proceedingJoinPoint) throws Throwable {
        Object retVal;
        System.out.println("----===>> @Around 環(huán)繞通知之前 AAA");
        retVal = proceedingJoinPoint.proceed();
        System.out.println("----===>> @Around 環(huán)繞通知之后 BBB");
        return retVal;
    }
}

測試類

其實(shí)我這個(gè)測試類，雖然用了 @Test 注解，但是我這個(gè)類更加像一個(gè) main 方法把：如下所示：

執(zhí)行結(jié)論

結(jié)果記錄：spring 4.x, spring-boot 1.5.9

無法現(xiàn)在依賴，所以無法試驗(yàn)

我直接說一下結(jié)論：Spring 4 中環(huán)繞通知是在最里面執(zhí)行的

結(jié)果記錄：spring 版本5.3.15 springboot 版本2.6.3

多切面的情況

多個(gè)切面的情況下，可以通過@Order指定先后順序，數(shù)字越小，優(yōu)先級越高。如下圖所示：

代理失效場景

下面一種場景會導(dǎo)致 aop 代理失效，因?yàn)槲覀冊趫?zhí)行 a 方法的時(shí)候其實(shí)本質(zhì)是執(zhí)行 AServer#a 的方法攔截器（MethodInterceptor）鏈, 當(dāng)我們在 a 方法內(nèi)直接執(zhí)行b(), 其實(shí)本質(zhì)就相當(dāng)于 this.b() , 這個(gè)時(shí)候由執(zhí)行 a方法是調(diào)用到 a 的原始對象相當(dāng)于是 this 調(diào)用，那么會導(dǎo)致 b() 方法的代理失效。這個(gè)問題也是我們開發(fā)者在開發(fā)過程中最常遇到的一個(gè)問題。

@Service
public class AService {
    
    public void a() {
        System.out.println("...... a");
        b();
    }
    
    public void b() {
        System.out.println("...... b");
    }

}