从Spring源码中学习如何查找自定义注解

chaojilaji 2020-11-06 21:09:19
java


看几个基础的注解

@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这个注解很奇怪,value的别名是attribute,attribute的别名是value

那么它的行为在哪里被定义的呢?在AnnotationTypeMapping中我们可以找到答案

// 这里使用了AnnotationsScanner的getDeclaredAnnotation方法来获取所有的AliasFor注解的方法
// AnnotationsScanner 是spring中的非公开抽象类,在我们的代码中不能直接进行使用
// Spring中没有提供子类
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);
}
// 为了简洁,我将源代码中其余部分省略掉了,可以看到,这里使用用反射得到的Method的getAnnotation方法得到实例
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;
}

通过学习@AliasFor,我们知道了可以通过先活动Method,再或得其修饰的注解的方法。

根据这样的方法,我们可以使用下面的代码,找到类DockingHandlers中所有被注解@DockIngMessage修饰的方法

// DockIngMessage 是自定义的注解
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注解是Spring中用得比较广泛的注解之一,来看看Spring源码中是怎么查找@Bean注解的

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

使用了AnnotatedElementUtils工具类,那么我们就可以把上面的代码改造一下

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());
}
}
// 相比于判断 != null , 这样的写法相对优雅了许多

至于Bean到底是怎么生效的,我们需要留到以后研究Spring容器的时候再讨论

@Controller

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

在Controller的test里面有这么一段代码

@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();
}

也就是说,可以利用扫包的方式来获取某个包下被某个注解修饰的类。

总结

查找某注解修饰的所有类就使用 ClassPathScanningCandidateComponentProvider 进行扫描。

查找某注解修饰的方法,就先找到那个类,然后得到所有的方法,使用AnnotatedElementUtils.hasAnnotation判断方法是否被某注解修饰即可

下面是一个简单的例子

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();
}
}

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https://my.oschina.net/u/3773302/blog/4702811

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