How does ComponentScan work?

Question

@ComponentScan will give you a list of all the classes with the @Component annotation in a package (or @Service/@Repository). To do this I imagine they use reflection to enumerate all the classes in a package and find the ones with that annotation.

However according to other StackOverflow answers you can not reliably enumerate all the classes in a package due to how the ClassLoaderworks. So how does @ComponentScan seemingly manage to accomplish this?

Michael

With the help of ClassPathScanningCandidateComponentProvider – Uniformed

The above class has a method findCandidateComponents accepting basePackage as an input argument and then ResourcePatternResolver instance reads all the classpath files with .class suffix. Hope this helps! – Uniformed

@harshavmb, if you turn your comment into an answer, I'll upvote it. – Boswell

@NoelYap, so nice of you! I feel it's too late to put it as an answer that I almost forgot Java & Spring. Sorry to disappoint! – Uniformed

Answer 1

@ComponentScan works differently. Workflow is put shortly this:

• Find all .class files in same folder and all subfolders
• Read .class file and wrap it into Resource object
• Check if class has annotations that will make it good candidate
• If class is good candidate, create bean from it.

Classes from Spring source code to look:

• ComponentScanAnnotationParser
• AnnotationConfigUtils
• ClassPathBeanDefinitionScanner
• BeanDefinitionReaderUtils
• DefaultListableBeanFactory

Acapulco