前言

在工作中經常使用Spring的相關框架，免不了去看一下Spring的實現方法，瞭解一下Spring內部的處理邏輯。特別是開發Web應用時，我們會頻繁的定義@Controller，@Service等JavaBean元件，通過註解，Spring自動掃描載入了這些元件，並提供相關的服務。
Spring是如何讀取註解資訊，並注入到bean容器中的，本文就是通過嵌入Spring的Bean載入，來描述Spring的實現方法。完整的例子都在Github上了。

自定義註解

先看一個最簡單的例子，在使用SpringWeb應用中的過程中，大家免不了會使用@Controller，@Service，@Repository

@Target({ ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface MyComponent {
    String value() default "";
}

@Configuration
public class ComponentAnnotationTest {
  public static void main(String[] args) {
    AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext();
annotationConfigApplicationContext.register(ComponentAnnotationTest.class);
    annotationConfigApplicationContext.refresh();
    InjectClass injectClass = annotationConfigApplicationContext.getBean(InjectClass.class);
        injectClass.print();
  }
  @MyComponent
  public static class InjectClass {
    public void print() {
        System.out.println("hello world");
    }
  }
}
 

執行這個例子，就會發現，@MyComponent 註解的類，也被Spring載入進來了，而且可以當成普通的JavaBean正常的使用。檢視Spring的原始碼會發現，Spring是使用ClassPathScanningCandidateComponentProvider掃描package,這個類有這樣的註釋

A component provider that scans the classpath from a base package. 
It then applies exclude and include filters to the resulting classes to find candidates.
 

這個類的 registerDefaultFilters 方法有這樣幾行程式碼

protected void registerDefaultFilters() {   
   this.includeFilters.add(new AnnotationTypeFilter(Component.class));
   ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader();
   try {    
      this.includeFilters.add(new AnnotationTypeFilter(((Class<? extends Annotation>) ClassUtils.forName("javax.annotation.ManagedBean", cl)), false)); 
      logger.debug("JSR-250 'javax.annotation.ManagedBean' found and supported for component scanning"); 
   }   catch (ClassNotFoundException ex) {     
     // JSR-250 1.1 API (as included in Java EE 6) not available - simply skip.   
   }   
   try {      
      this.includeFilters.add(new AnnotationTypeFilter(((Class<? extends Annotation>) ClassUtils.forName("javax.inject.Named", cl)), false));      
      logger.debug("JSR-330 'javax.inject.Named' annotation found and supported for component scanning");   
   }  
   catch (ClassNotFoundException ex) {     
   // JSR-330 API not available - simply skip.  
   }
}

這裡就會發現Spring在掃描類資訊的使用只會判斷被@Component註解的類，所以任何自定義的註解只要帶上@Component（當然還要有String value() default "";的方法，因為Spring的Bean都是有beanName唯一標示的），都可以被Spring掃描到，並注入容器內。

定製功能

但上面的方法太侷限了，沒辦法定製，而且也沒有實際的意義。如何用特殊的註解來實現定製的功能呢，一般有兩種方式：

1. 還是用上面的方法，在注入Spring的容器後，再取出來做自己定製的功能，Spring-MVC就是使用這樣的方法。AbstractDetectingUrlHandlerMapping 中的 detectHandlers方法,這個方法取出了所有的bean，然後迴圈查詢帶有Controller的bean，並提取其中的RequestMapping資訊

   protected void detectHandlers() throws BeansException {
        if (logger.isDebugEnabled()) {
            logger.debug("Looking for URL mappings in application context: " + getApplicationContext());
        }
        String[] beanNames = (this.detectHandlersInAncestorContexts ?
                BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
                getApplicationContext().getBeanNamesForType(Object.class));
   
        // Take any bean name that we can determine URLs for.
        for (String beanName : beanNames) {
            String[] urls = determineUrlsForHandler(beanName);
            if (!ObjectUtils.isEmpty(urls)) {
                // URL paths found: Let's consider it a handler.
                registerHandler(urls, beanName);
            }
            else {
                if (logger.isDebugEnabled()) {
                    logger.debug("Rejected bean name '" + beanName + "': no URL paths identified");
                }
            }
        }
    }

2. 不依賴@Component，自定義掃描。所以就有了第二個例子。

自定義掃描

結構比較複雜，可以參考完整的例子，這裡是關鍵的幾個類

1. 還是定義一個註解，只不過不再需要@Component了

   @Target({ ElementType.TYPE })
   @Retention(RetentionPolicy.RUNTIME)
   @Documented
   public @interface CustomizeComponent {
     String value() default "";
   }

2. 註解修飾的類

   @CustomizeComponent
   public class ScanClass1 {
    public void print() {
        System.out.println("scanClass1");
    }
   }

3. BeanScannerConfigurer用於嵌入到Spring的載入過程的中，這裡用到了BeanFactoryPostProcessor 和 ApplicationContextAware。
   Spring提供了一些的介面使程式可以嵌入Spring的載入過程。這個類中的繼承ApplicationContextAware介面，Spring會讀取ApplicationContextAware型別的的JavaBean，並呼叫setApplicationContext(ApplicationContext applicationContext)傳入Spring的applicationContext。
   同樣繼承BeanFactoryPostProcessor介面，Spring會在BeanFactory的相關處理完成後呼叫postProcessBeanFactory方法，進行定製的功能。

   @Component
   public static class BeanScannerConfigurer implements  BeanFactoryPostProcessor, ApplicationContextAware {
    private ApplicationContext applicationContext;
   
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
      this.applicationContext = applicationContext;
    }
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
      Scanner scanner = new Scanner((BeanDefinitionRegistry) beanFactory);
      scanner.setResourceLoader(this.applicationContext);
      scanner.scan("org.wcong.test.spring.scan");
    }
   }

4. Scanner繼承的ClassPathBeanDefinitionScanner是Spring內建的Bean定義的掃描器。
   includeFilter裡定義了類的過濾器，newAnnotationTypeFilter(CustomizeComponent.class)表示只取被CustomizeComponent修飾的類。
   doScan裡掃面了包底下的讀取道德BeanDefinitionHolder，自定義GenericBeanDefinition相關功能。

   public final static class Scanner extends ClassPathBeanDefinitionScanner {
      public Scanner(BeanDefinitionRegistry registry) {
          super(registry);
      }
      public void registerDefaultFilters() {
          this.addIncludeFilter(new AnnotationTypeFilter(CustomizeComponent.class));
      }
      public Set<BeanDefinitionHolder> doScan(String... basePackages) {
          Set<BeanDefinitionHolder> beanDefinitions =   super.doScan(basePackages);
          for (BeanDefinitionHolder holder : beanDefinitions) {
              GenericBeanDefinition definition = (GenericBeanDefinition) holder.getBeanDefinition();
              definition.getPropertyValues().add("innerClassName", definition.getBeanClassName());
              definition.setBeanClass(FactoryBeanTest.class);
          }
          return beanDefinitions;
      }
      public boolean isCandidateComponent(AnnotatedBeanDefinition beanDefinition) {
         return super.isCandidateComponent(beanDefinition) && beanDefinition.getMetadata()
   .hasAnnotation(CustomizeComponent.class.getName());
      }
   }

5. FactoryBean是Spring中比較重要的一個類。它的描述如下

   Interface to be implemented by objects used within a BeanFactory which are themselves factories. 
   If a bean implements this interface, it is used as a factory for an object to expose, not directly as a bean* instance that will be exposed itself

   普通的JavaBean是直接使用類的例項，但是如果一個Bean繼承了這個藉口，就可以通過getObject()方法來自定義例項的內容，在FactoryBeanTest的getObject()就通過代理了原始類的方法，自定義類的方法。

   public static class FactoryBeanTest<T> implements InitializingBean, FactoryBean<T> {
      private String innerClassName;
      public void setInnerClassName(String innerClassName) {
          this.innerClassName = innerClassName;
      }
      public T getObject() throws Exception {
          Class innerClass = Class.forName(innerClassName);
          if (innerClass.isInterface()) {
              return (T) InterfaceProxy.newInstance(innerClass);
          } else {
              Enhancer enhancer = new Enhancer();
              enhancer.setSuperclass(innerClass);
              enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
              enhancer.setCallback(new MethodInterceptorImpl());
              return (T) enhancer.create();
          }
      }
      public Class<?> getObjectType() {
          try {
                return Class.forName(innerClassName);
          } catch (ClassNotFoundException e) {
                e.printStackTrace();
          }
          return null;
      }
      public boolean isSingleton() {
          return true;
      }
      public void afterPropertiesSet() throws Exception {
      }
   }
   public static class InterfaceProxy implements InvocationHandler {
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
          System.out.println("ObjectProxy execute:" + method.getName());
          return method.invoke(proxy, args);
      }
      public static <T> T newInstance(Class<T> innerInterface) {
          ClassLoader classLoader = innerInterface.getClassLoader();
          Class[] interfaces = new Class[] { innerInterface };
          InterfaceProxy proxy = new InterfaceProxy();
          return (T) Proxy.newProxyInstance(classLoader, interfaces, proxy);
      }
     }
     public static class MethodInterceptorImpl implements MethodInterceptor {
          public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
          System.out.println("MethodInterceptorImpl:" + method.getName());
          return methodProxy.invokeSuper(o, objects);
      }
   }

6. main函式

   @Configuration
   public class CustomizeScanTest {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext();                
        annotationConfigApplicationContext.register(CustomizeScanTest.class);
        annotationConfigApplicationContext.refresh();
        ScanClass1 injectClass = annotationConfigApplicationContext.getBean(ScanClass1.class);
        injectClass.print();
    }
   }

至此一個完整的例子就完成了，這裡主要用到了BeanFactoryPostProcessor，ApplicationContextAware，FactoryBean等Spring內建的介面，來嵌入Spring的載入和使用過程，這樣就實現了自定義註解，和自定義代理了。