STAGE 33 OF 37

Stage 33: observe the self-invocation boundary

Learning path / Stage 33 of 37

Before you start: use JDK 21+ and Maven 3.9+. Extract this stage’s project and open a terminal in its pom.xml folder. Complete the setup below, including a database connection where required, then follow the commands in this lesson’s Run this example section. Console lessons finish after printing their result; web lessons keep running until Ctrl+C. Follow the lesson’s run section for profile and database settings.

Observe extra behaviour around an existing product service without mixing it into the business operation.

Download the complete Stage 33 project · Open the project guide

Before you start

Use JDK 21 or newer and Maven 3.9 or newer. Verify with java -version and mvn -version. Extract this project into its own folder; do not merge its files with another stage.

This lesson reuses the product catalog and Todo responsibilities from Stages 8–14. Only the feature described below is new. Read the ordered explanation before opening the complete files.

Caller → Spring proxy → Advice → ProductService → Original result

What we change and why

  1. Keep business logic in the service

    ProductService.count returns the product count. The aspect contains the additional logging concern; the service does not call the aspect itself.

  2. Enable proxy-based interception

    @EnableAspectJAutoProxy installs Spring’s annotation-driven AOP support. We request class-based proxies to keep the concrete service lookup simple. AspectJ annotations and pointcut expressions are used with Spring runtime proxies, not AspectJ bytecode weaving.

  3. Name the AOP pieces

    The service method execution is the join point. The pointcut selects executions. The advice is the extra method that runs. The aspect groups these rules. In Spring AOP, interception applies to method execution through eligible managed proxies, not arbitrary source-code lines.

  4. Select one annotated method

    Our @LogExecutionTime annotation is retained at runtime so the pointcut can inspect it. Only methods carrying that annotation are selected.

  5. Preserve the original operation

    Around advice calls proceed exactly once, returns its result and allows exceptions to propagate. finally records elapsed time even if the method fails. Timing output varies; this is an observation example rather than a performance benchmark.

  6. Keep the demonstration counter in context

    TimingAspect.invocations exists only to verify these sequential console calls. This mutable counter is not a production metric for concurrent requests.

  7. Compare external and internal calls

    The injected service reference is a proxy. service.count passes through it. countInternally enters the target and this.count calls the target directly, bypassing the proxy. The internal call still returns 2 but adds no timed invocation.

  8. Use a separate collaborator when interception is required

    Move the separately advised operation into another injected service rather than relying on this calls, private methods or self-injection tricks. The transaction lessons reuse this boundary. Caching and method security also have proxy-related boundaries; servlet security filters are a different mechanism.

Run this example

Open a terminal in the extracted folder containing pom.xml. Run one command at a time.

Terminal · Windows, macOS and Linux
mvn test
mvn spring-boot:run

This is a console lesson. It does not open a web port. Application.main starts a Spring context, calls the demonstration and closes the context. Read the terminal, not a browser. Spring Boot’s Maven plugin is used as a convenient launcher; these examples show Spring Framework directly.

Expected result

Expected output · ignore framework startup messages
Timed count: <varying number> ns
External result: 2
Internal result: 2
Timed invocations: 1

Object identifiers and elapsed times can vary. Compare the relationship between values rather than copying an identifier.

Understand each complete file

Create the following files at exactly these paths, or use the ZIP. Source files include their imports. The test checks the behaviour described in the lesson.

pom.xml
pom.xml · complete file
<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 https://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>4.1.1</version><relativePath/></parent>
  <groupId>com.edupoly</groupId><artifactId>spring-stage-33-proxy-boundary</artifactId><version>1.0.0</version>
  <properties><java.version>21</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-test</artifactId><scope>test</scope></dependency>
    <dependency><groupId>org.springframework</groupId><artifactId>spring-aop</artifactId></dependency>
    <dependency><groupId>org.aspectj</groupId><artifactId>aspectjweaver</artifactId></dependency>
  </dependencies>
  <build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin></plugins></build>
</project>
src/main/java/com/edupoly/lesson/AppConfig.java
src/main/java/com/edupoly/lesson/AppConfig.java · complete file
package com.edupoly.lesson;

import org.springframework.context.annotation.*;
@Configuration @ComponentScan("com.edupoly.lesson") @EnableAspectJAutoProxy(proxyTargetClass = true)
public class AppConfig { }
src/main/java/com/edupoly/lesson/LogExecutionTime.java
src/main/java/com/edupoly/lesson/LogExecutionTime.java · complete file
package com.edupoly.lesson;

import java.lang.annotation.*;
@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
public @interface LogExecutionTime { }
src/main/java/com/edupoly/lesson/TimingAspect.java
src/main/java/com/edupoly/lesson/TimingAspect.java · complete file
package com.edupoly.lesson;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.*;
import org.springframework.stereotype.Component;
@Aspect @Component public class TimingAspect {
    private int invocations;
    public int invocations() { return invocations; }
    @Around("@annotation(com.edupoly.lesson.LogExecutionTime)")
    public Object time(ProceedingJoinPoint point) throws Throwable {
        invocations++;
        long start = System.nanoTime();
        try { return point.proceed(); }
        finally { System.out.println("Timed " + point.getSignature().getName() + ": " + (System.nanoTime() - start) + " ns"); }
    }
}
src/main/java/com/edupoly/lesson/ProductService.java
src/main/java/com/edupoly/lesson/ProductService.java · complete file
package com.edupoly.lesson;

import org.springframework.stereotype.Service;
@Service public class ProductService {
    @LogExecutionTime public int count() { return 2; }
    public int countInternally() { return this.count(); }
}
src/main/java/com/edupoly/lesson/Lesson.java
src/main/java/com/edupoly/lesson/Lesson.java · complete file
package com.edupoly.lesson;

import org.springframework.stereotype.Component;
@Component public class Lesson {
    private final ProductService service; private final TimingAspect aspect;
    public Lesson(ProductService service, TimingAspect aspect) { this.service = service; this.aspect = aspect; }
    public void run() {
        System.out.println("External result: " + service.count());
        System.out.println("Internal result: " + service.countInternally());
        System.out.println("Timed invocations: " + aspect.invocations());
    }
}
src/main/java/com/edupoly/lesson/Application.java
src/main/java/com/edupoly/lesson/Application.java · complete file
package com.edupoly.lesson;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
public class Application {
    public static void main(String[] args) throws Exception {
        try (var context = new AnnotationConfigApplicationContext(AppConfig.class)) {
            context.getBean(Lesson.class).run();
        }
    }
}
src/test/java/com/edupoly/lesson/LessonTest.java
src/test/java/com/edupoly/lesson/LessonTest.java · complete file
package com.edupoly.lesson;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

class LessonTest {
    @Test void advicePreservesReturnValueAndUsesProxy() {
        try (var context = new AnnotationConfigApplicationContext(AppConfig.class)) {
            var service = context.getBean(ProductService.class);
            assertTrue(org.springframework.aop.support.AopUtils.isAopProxy(service));
            assertEquals(2, service.count());
            var aspect = context.getBean(TimingAspect.class);
            assertEquals(1, aspect.invocations());
            assertEquals(2, service.countInternally());
            assertEquals(1, aspect.invocations());
        }
    }
}

Try a failure, then correct it

Call countInternally and expect a second timing line: it does not appear. Move the count operation into a separately injected service if that call needs advice.

Words you can now explain

WordMeaning in this example
ProxyThe injected intermediary applying advice before reaching the target.
Self-invocationA this.method() call within the target bypasses its proxy.

Practice assignment

Add another service method. Predict whether the current pointcut matches it, then run the example to check. Keep the service return value unchanged.

Read the official documentation →

Concepts used in this stage

Use the right column for a quick reminder, or open a concept above for its full explanation.

Java 21 · Spring Boot 4.1.1 · Edupoly