How to copy Spring's component-scan - spring

Question

I want to search for some annotations in a Spring based web application, like #Entity. Therefore I need the same functionality like Spring involves when the server starts up and it looks for all classes that are annotated with #Component. In my case I don't create singleton's, it's just important for me to collect all those classes annotated with #Entity.
Is there any possibility to use existing Spring tools for that? I want to search exactly in the same namespace as Spring does for the #Component annotations.

Answer 1

Sure, look at parse() method in org.springframework.context.annotation.ComponentScanBeanDefinitionParser. This method is called when Spring encounters <context:component-scan/> in the XML configuration. Probably You can strip it a bit to better suit your needs, but it should serve as a comprehensive example.
The class You should be particularly interested in is org.springframework.context.annotation.ClassPathBeanDefinitionScanner. From JavaDoc:
Candidate classes are detected through configurable type filters. The default filters include classes that are annotated with Spring's #Component, #Repository, #Service, or #Controller stereotype.
BTW if you need less general solution, maybe your persistence provider has some API to fetch all entity classes?

Answer 2

Spring's built-in classpath scanning infrastructure (ClassPathBeanDefinitionScanner/ ComponentScanBeanDefinitionParser) is geared up for registering classes as BeanDefinitions within an Spring appcontext.
If you're just looking to obtain a list of classes annotated with a given annotation (rather than actually register them in Spring as bean definitions) take a look at the Google Reflections library.
Reflections allows you to scan your classpath using various filters, including an annotation filter.
Reflections reflections = new Reflections("my.project.prefix");
Set<Class<? extends SomeClassOrInterface>> subTypes = reflections.getSubTypesOf(SomeClassOrInterface.class);
Set<Class<?>> annotated = reflections.getTypesAnnotatedWith(SomeAnnotation.class);

Answer 3

Spring based solution
Use spring AnnotationTypeFilter and pass Entity.class as annotationType
using ResourcePatternResolver load all resouces(.class) under given pacakage
Use SimpleMetadataReaderFactory to get MetadataReader
for each resource you can call match on AnnotationTypeFilter using MetadataReader
metadataReader.getAnnotationMetadata().getClassName() will provide FQN of class
usage
AnnotatedClassFinder entityScanner = new AnnotatedClassFinder(Entity.class);
entityScanner.setPackages(Arrays.asList("org.myapp.domain"));
Collection<Class<?>> entities = entityScanner.findMarkedClassOfType();
public class AnnotatedClassFinder {
private static final String CLASS_RESOURCE_PATTERN = "**/*.class";
private List<String> packages;
private final ResourceLoader resourceLoader = new DefaultResourceLoader();
private final ResourcePatternResolver resourcePatternResolver = ResourcePatternUtils
.getResourcePatternResolver(resourceLoader);
private final MetadataReaderFactory metadataReaderFactory = new SimpleMetadataReaderFactory();
private final TypeFilter annotationFilter;
public AnnotatedClassFinder(final Class<? extends Annotation> annotationToScanFor) {
annotationFilter = new AnnotationTypeFilter(annotationToScanFor);
}
public Set<Class<?>> findMarkedClassOfType() {
if (packages == null) {
return new HashSet<Class<?>>();
}
final Set<Class<?>> annotatedClasses = new HashSet<Class<?>>();
try {
for (final String p : packages) {
final String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX
+ ClassUtils.convertClassNameToResourcePath(SystemPropertyUtils.resolvePlaceholders(p)) + "/"
+ CLASS_RESOURCE_PATTERN;
final Resource[] resources = resourcePatternResolver.getResources(packageSearchPath);
for (final Resource resource : resources) {
if (resource.isReadable()) {
final MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource);
if (annotationFilter.match(metadataReader, metadataReaderFactory)) {
annotatedClasses.add(Class.forName(metadataReader.getAnnotationMetadata().getClassName()));
}
}
}
}
return annotatedClasses;
} catch (final IOException ex) {
throw new RuntimeException("I/O failure during classpath scanning", ex);
} catch (final ClassNotFoundException ex) {
throw new RuntimeException("Class loading failure during classpath scanning", ex);
}
}
public void setPackages(final List<String> packages) {
this.packages = packages;
}
}