为什么 AspectJ @Around 建议执行两次？

我有以下 AspectJ 示例，它是作为“hello world”风格的概念证明而完成的。中的建议代码StyleAspect似乎执行了两次，即使实际代码在SomeClass仅执行一次（根据需要）。

这是代码：

首先，一个名为 WithStyle 的注释：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface WithStyle {  
}

然后，使用 @WithStyle 注释拦截任何代码的方面

@Aspect
public class StyleAspect {

    @Around("@annotation(WithStyle)")
    public Object doItWithStyle(ProceedingJoinPoint pjp) throws Throwable {
        System.out.println("Doing it in style...");
        Object result = pjp.proceed();
        System.out.println("Done");
        
        return result;
    }
}

最后是一些带注释的代码

public class SomeClass {
    
    @WithStyle
    public void doIt() {
        System.out.println("I'm doing it....");
    }
}

当我运行它时，我得到以下输出：

--- exec-maven-plugin:1.2.1:exec (default-cli) @ AspectJTest ---
Doing it in style...
Doing it in style...
I'm doing it....
Done
Done

因此，虽然代码本身只执行一次，但方面中的代码似乎执行了两次。

这是调用代码：

public class Main {
    
    public static void main(String[] args) {
        SomeClass someClass = new SomeClass();
        someClass.doIt();
    }
}

为了完整起见，我将 pom 包含在 AspectJ 插件配置中

<?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>ie.philb</groupId>
    <artifactId>AspectJTest</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
        <project.build.java.target>1.8</project.build.java.target>
    </properties>
    
    <dependencies>
        <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjrt</artifactId>
            <version>1.9.6</version>
        </dependency>
        <dependency>
            <groupId>org.aspectj</groupId>
            <artifactId>aspectjweaver</artifactId>
            <version>1.9.6</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>aspectj-maven-plugin</artifactId>
                <version>1.11</version>
                <configuration>
                    <complianceLevel>1.8</complianceLevel>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
                <executions>
                    <execution>
                        <goals>
                            <goal>compile</goal>       <!-- use this goal to weave all your main classes -->
                            <goal>test-compile</goal>  <!-- use this goal to weave all your test classes -->
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
      
</project>

Your around() advice正在拦截call and execution注释方法的连接点@WithStyle (i.e., doIt()）。如果您添加一个System.out.println(pjp);对你而言：

@Aspect
public class StyleAspect {

    @Around("@annotation(WithStyle) ") 
    public Object doItWithStyle(ProceedingJoinPoint pjp) throws Throwable {
        System.out.println(pjp);
        System.out.println("Doing it in style...");
        Object result;
        try{
            result = pjp.proceed();
        }
        finally{
            System.out.println("Done");
        }
        return result;
    }
}

你会得到以下输出：

call(public void SomeClass.doIt()) <----
Doing it in style...
execution(public void SomeClass.doIt()) <----
Doing it in style...
I'm doing it....
Done
Done

您可以清楚地看到连接点call and execution方法的SomeClass.doIt()正在被拦截around advice doItWithStyle.

从拦截来看call, the around advice is weaving代码如下：

// around advice code  before the pjp.proceed();
someClass.doIt(); // during the pjp.proceed();
// around advice code  after the pjp.proceed();

最后：

 System.out.println("Doing it in style...");.
 someClass.doIt();
 System.out.println("Done");

从执行来看：

@WithStyle
public void doIt() {
    // around advice code  before the pjp.proceed();
    System.out.println("I'm doing it....");
  // around advice code  after the pjp.proceed();
}

最后：

@WithStyle
public void doIt() {
    System.out.println("Doing it in style...");
    System.out.println("I'm doing it....");
    System.out.println("Done");
}

产生输出：

Doing it in style... 
Doing it in style...
I'm doing it....
Done
Done

现在，如果您想避免around advice拦截两者call和execution该方法的doIt()。您需要进一步限制您的拦截的连接点around advice。只是拦截该方法call， 你可以做：

 @Around("@annotation(WithStyle) && call(* *(..))") 

对于方法execution:

@Around("@annotation(WithStyle) && execution(* *(..))") 

您可以通过调整方法的签名，根据方法的参数数量、返回类型、名称等进一步限制被拦截的连接点call or execution切入点。