Learn how to find custom annotations from spring source code

Strategy 2020-11-06 21:09:19
learn custom annotations spring source


Look at a few basic notes

@AliasFor

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@Documented
public @interface AliasFor {
@AliasFor("attribute")
String value() default "";
@AliasFor("value")
String attribute() default "";
Class<? extends Annotation> annotation() default Annotation.class;
}

AliasFor This annotation is very strange ,value Its alias is attribute,attribute Its alias is value

So where is its behavior defined ? stay AnnotationTypeMapping We can find the answer

// It's used here AnnotationsScanner Of getDeclaredAnnotation Method to get all the AliasFor Method of annotation 
// AnnotationsScanner yes spring The non-public abstract class in , It can't be used directly in our code 
// Spring There is no subclass provided in 
private Map<Method, List<Method>> resolveAliasedForTargets() {
Map<Method, List<Method>> aliasedBy = new HashMap<>();
for (int i = 0; i < this.attributes.size(); i++) {
Method attribute = this.attributes.get(i);
AliasFor aliasFor = AnnotationsScanner.getDeclaredAnnotation(attribute, AliasFor.class);
if (aliasFor != null) {
Method target = resolveAliasTarget(attribute, aliasFor);
aliasedBy.computeIfAbsent(target, key -> new ArrayList<>()).add(attribute);
}
}
return Collections.unmodifiableMap(aliasedBy);
}
// For the sake of simplicity , I left out the rest of the source code , You can see , Here we use reflection Method Of getAnnotation Method to get an example 
private Method resolveAliasTarget(Method attribute, AliasFor aliasFor, boolean checkAliasPair) {
// ... ...
Method target = AttributeMethods.forAnnotationType(targetAnnotation).get(targetAttributeName);
// ... ...
if (isAliasPair(target) && checkAliasPair) {
AliasFor targetAliasFor = target.getAnnotation(AliasFor.class);
if (targetAliasFor != null) {
Method mirror = resolveAliasTarget(target, targetAliasFor, false);
if (!mirror.equals(attribute)) {
throw new AnnotationConfigurationException(String.format(
"%s must be declared as an @AliasFor %s, not %s.",
StringUtils.capitalize(AttributeMethods.describe(target)),
AttributeMethods.describe(attribute), AttributeMethods.describe(mirror)));
}
}
}
return target;
}

Through the study @AliasFor, We've learned that we can do it through the first activity Method, Or get the modified annotation .

According to this method , We can use the following code , Find the class DockingHandlers All are annotated in @DockIngMessage The method of decoration

// DockIngMessage It's a custom annotation 
Method[] methods = DockingHandlers.class.getMethods();
for (Method method : methods) {
DockIngMessage dockIngMessage = method.getAnnotation(DockIngMessage.class);
if (dockIngMessage != null) {
System.out.println(dockIngMessage.name());
}
}

@Bean

@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Bean {
@AliasFor("name")
String[] value() default {};
@AliasFor("value")
String[] name() default {};
@Deprecated
Autowire autowire() default Autowire.NO;
boolean autowireCandidate() default true;
String initMethod() default "";
String destroyMethod() default AbstractBeanDefinition.INFER_METHOD;
}

@Bean The note is Spring One of the more widely used annotations in , Let's see Spring How to find the source code @Bean Annotated

public static boolean isBeanAnnotated(Method method) {
return AnnotatedElementUtils.hasAnnotation(method, Bean.class);
}

Used AnnotatedElementUtils Tool class , Then we can modify the code above

Method[] methods = DockingHandlers.class.getMethods();
for (Method method : methods) {
if (AnnotatedElementUtils.hasAnnotation(method, DockIngMessage.class)) {
DockIngMessage dockIngMessage = AnnotatedElementUtils.getMergedAnnotation(method,DockIngMessage.class);
System.out.println(dockIngMessage.name());
}
}
// Compared with judgment != null , This kind of writing is relatively elegant 

as for Bean How does it work , We need to save it for later research Spring We'll talk about the container

@Controller

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Controller {
@AliasFor(annotation = Component.class)
String value() default "";
}

stay Controller Of test There's a piece of code in it

@Test
public void testWithComponentAnnotationOnly() {
ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false);
provider.addIncludeFilter(new AnnotationTypeFilter(Component.class));
provider.addExcludeFilter(new AnnotationTypeFilter(Repository.class));
provider.addExcludeFilter(new AnnotationTypeFilter(Service.class));
provider.addExcludeFilter(new AnnotationTypeFilter(Controller.class));
Set<BeanDefinition> candidates = provider.findCandidateComponents(TEST_BASE_PACKAGE);
assertThat(candidates.size()).isEqualTo(3);
assertThat(containsBeanClass(candidates, NamedComponent.class)).isTrue();
assertThat(containsBeanClass(candidates, ServiceInvocationCounter.class)).isTrue();
assertThat(containsBeanClass(candidates, BarComponent.class)).isTrue();
assertThat(containsBeanClass(candidates, FooServiceImpl.class)).isFalse();
assertThat(containsBeanClass(candidates, StubFooDao.class)).isFalse();
assertThat(containsBeanClass(candidates, NamedStubDao.class)).isFalse();
}

in other words , You can use the method of scanning the package to get the class modified by some annotation under a package .

summary

Find all classes decorated by an annotation and use ClassPathScanningCandidateComponentProvider scan .

Find a way to modify an annotation , Find that class first , And then you get all the methods , Use AnnotatedElementUtils.hasAnnotation It can be used to determine whether the method is modified by some annotation

Here is a simple example

ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false);
provider.addIncludeFilter(new AnnotationTypeFilter(DockingAnnotation.class));
Set<BeanDefinition> candidates = provider.findCandidateComponents("package_name");
for (BeanDefinition definition : candidates){
try {
Class clz = Class.forName(definition.getBeanClassName());
Method[] methods = clz.getMethods();
for (Method method : methods){
if (AnnotatedElementUtils.hasAnnotation(method,DockIngMessage.class)){
DockIngMessage dockIngMessage = AnnotatedElementUtils.getMergedAnnotation(method,DockIngMessage.class);
System.out.println(dockIngMessage.name());
}
}
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}

Fried chicken spicy chicken original article , Please quote source for reprint

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