Enable Placeholder Expressions in Custom SpringBoot Annotations

Environment: SpringBoot 2.3.8.RELEASE with JDK 1.8.

This tutorial shows how to enable ${...} placeholder expressions in a custom annotation within a SpringBoot application.

Dependencies

<code>&lt;dependency&gt;
    &lt;groupId&gt;org.aspectj&lt;/groupId&gt;
    &lt;artifactId&gt;aspectjrt&lt;/artifactId&gt;
&lt;/dependency&gt;
&lt;dependency&gt;
    &lt;groupId&gt;org.aspectj&lt;/groupId&gt;
    &lt;artifactId&gt;aspectjweaver&lt;/artifactId&gt;
    &lt;scope&gt;runtime&lt;/scope&gt;
&lt;/dependency&gt;
</code>

Custom Annotation

<code>@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@Documented
public @interface Manufactur {
    String value() default ""; // manufacturer code
}
</code>

AOP Aspect

The aspect implements

EnvironmentAware

to obtain the

Environment

object, parses the annotation on the target method, resolves the placeholder, and stores the result in a

ThreadLocal

.

<code>@Component
@Aspect
public class ManufacturAspect implements EnvironmentAware {
    private static final Logger logger = LoggerFactory.getLogger(ManufacturAspect.class);
    private Environment environment;

    @Pointcut("@annotation(com.pack.annotation.Manufactur)")
    private void info() {}

    @Before("info()")
    public void execBefore(JoinPoint jp) {
        MethodSignature sign = (MethodSignature) jp.getSignature();
        Method method = sign.getMethod();
        Manufactur manu = method.getAnnotation(Manufactur.class);
        String value = manu.value();
        logger.info("获取到注解值：{}", value);
        BusinessService.code.set(this.environment.resolvePlaceholders(value));
    }

    @Override
    public void setEnvironment(Environment environment) {
        this.environment = environment;
    }
}
</code>

Service Usage

<code>@Service
public class BusinessService {
    public static ThreadLocal<String> code = new ThreadLocal<>();
    private static Logger logger = LoggerFactory.getLogger(BusinessService.class);

    @Manufactur("${manufactur.code}-#{1 + 3}")
    public String invoke(String id) {
        String sno = code.get();
        logger.info("自定义注解动态获取属性值：{}", sno);
        // todo
        return sno;
    }
}
</code>

Controller Test

<code>@RestController
@RequestMapping("/business")
public class BusinessController {
    @Resource
    private BusinessService bs;

    @GetMapping("/{id}")
    public Object home(@PathVariable String id) {
        return bs.invoke(id);
    }
}
</code>

The resolved value is stored in the

ThreadLocal

by the AOP aspect and can be accessed in the service method.

With these steps, the custom annotation now supports placeholder resolution.