Scanning Java annotations at runtime

java

reflection

spring

classgraph

byAnton Shumikhin·Aug 14, 2024

Harness Java Reflection to inspect annotations. Use a class's metadata with getDeclaredMethods(). Invoke isAnnotationPresent() to verify a specific annotation, and use getAnnotation() to retrieve it. Witness the magic:

public void scanAnnotations(Class<?> clazz) {
    for (Method m : clazz.getDeclaredMethods()) {
        // Engage your annotation detective mode
        if (m.isAnnotationPresent(YourAnnotation.class)) {
            // Congrats Sherlock, you found your clue!
            // Use m.getAnnotation(YourAnnotation.class) for insights
        }
    }
}

Replace YourAnnotation with your target annotation class, and feed your class to scanAnnotations.

The sword and the shield: Reflection and Libraries

Reflection is a powerful tool but can be cumbersome when scanning multiple classes or an entire project. Here, libraries and frameworks come to your rescue. Let's examine some of them.

All hail Spring: Scanning with ClassPathScanningCandidateComponentProvider

The Spring Framework provides the ClassPathScanningCandidateComponentProvider for automated scanning. It's fully configurable with include and exclude filters based on annotation types.

// Call upon the mighty ClassPathScanningCandidateComponentProvider
ClassPathScanningCandidateComponentProvider scanner =
    new ClassPathScanningCandidateComponentProvider(false);
scanner.addIncludeFilter(new AnnotationTypeFilter(YourAnnotation.class));

for (BeanDefinition bd : scanner.findCandidateComponents("your.base.package")) {
    // Victory is ours! We found a class  
    System.out.println(bd.getBeanClassName());
}

This snippet finds all classes with YourAnnotation within the provided package.

Expanding the arsenal: ClassGraph and others

ClassGraph is a robust tool for scanning annotations in complex environments. It supports classloaders and the JPMS module system.

// Summon ClassGraph and empower it with infinite knowledge
try (ScanResult scanResult =
  new ClassGraph().enableAllInfo().whitelistPackages("your.base.package").scan()) {
    // The universe unfolds its secrets
    for (ClassInfo classInfo : scanResult.getClassesWithAnnotation(YourAnnotation.class.getName())) {
        System.out.println(classInfo.getName());
    }
}

For lightweight solutions, consider Annovention. For enterprise applications, Java EE 5 auto-detects annotated classes.

Practical scenarios: Applying the tools

Maximizing Spring's potential

Spring's annotation scanning reduces boilerplate code, improving maintainability. See how it compares to raw reflection:

Method	Ease of Use
Java Reflection	Manual
Spring Framework	Automated

Leveraging ClassGraph for advanced cases

ClassGraph provides more control in complex scenarios. It shines when scanning across multiple frameworks, class loaders, and metadata within the JPMS module system.