深入Spring:自定义注解加载和使用

前言

在工作中经常使用Spring的相关框架，免不了去看一下Spring的实现方法，了解一下Spring内部的处理逻辑。特别是开发Web应用时，我们会频繁的定义@Controller，@Service等JavaBean组件，通过注解，Spring自动扫描加载了这些组件，并提供相关的服务。
Spring是如何读取注解信息，并注入到bean容器中的，本文就是通过嵌入Spring的Bean加载，来描述Spring的实现方法。完整的例子都在Github上了。

自定义注解

先看一个最简单的例子，在使用SpringWeb应用中的过程中，大家免不了会使用@Controller，@Service，@Repository等注解来定义JavaBean。那么怎么自己定义一个注解，Spring可以自动加载呢。所以就有了第一个例子。

@Target({ ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface MyComponent {
    String value() default "";
}
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@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");
    }
  }
}
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运行这个例子，就会发现，@MyComponent 注解的类，也被Spring加载进来了，而且可以当成普通的JavaBean正常的使用。查看Spring的源码会发现，Spring是使用ClassPathScanningCandidateComponentProvider扫描package,这个类有这样的注释

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.
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这个类的 registerDefaultFilters 方法有这样几行代码

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.  
   }
}
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这里就会发现Spring在扫描类信息的使用只会判断被@Component注解的类，所以任何自定义的注解只要带上@Component（当然还要有String value() default “”;的方法，因为Spring的Bean都是有beanName唯一标示的），都可以被Spring扫描到，并注入容器内。

定制功能

但上面的方法太局限了，没办法定制，而且也没有实际的意义。如何用特殊的注解来实现定制的功能呢，一般有两种方式：
1. 还是用上面的方法，在注入Spring的容器后，再取出来做自己定制的功能，Spring-MVC就是使用这样的方法。AbstractDetectingUrlHandlerMapping 中的 detectHandlers方法,这个方法取出了所有的bean，然后循环查找带有Controller的bean，并提取其中的RequestMapping信息

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");
                }
            }
        }
    }
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1. 不依赖@Component，自定义扫描。所以就有了第二个例子。

自定义扫描

结构比较复杂，可以参考完整的例子，这里是关键的几个类
1. 还是定义一个注解，只不过不再需要@Component了

@Target({ ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface CustomizeComponent {
     String value() default "";
}
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1. 注解修饰的类

@CustomizeComponent
public class ScanClass1 {
    public void print() {
        System.out.println("scanClass1");
    }
}
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1. BeanScannerConfigurer用于嵌入到Spring的加载过程的中，这里用到了BeanFactoryPostProcessor 和 ApplicationContextAware。
   Spring提供了一些的接口使程序可以嵌入Spring的加载过程。这个类中的继承ApplicationContextAware接口，Spring会读取ApplicationContextAware类型的的JavaBean，并调用setApplicationContext(ApplicationContext applicationContext)传入Spring的applicationContext。
   同样继承BeanFactoryPostProcessor接口，Spring会在BeanFactory的相关处理完成后调用postProcessBeanFactory方法，进行定制的功能。

@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");
    }
  }
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1. Scanner继承的ClassPathBeanDefinitionScanner是Spring内置的Bean定义的扫描器。
   includeFilter里定义了类的过滤器，newAnnotationTypeFilter(CustomizeComponent.class)表示只取被CustomizeComponent修饰的类。
   doScan里扫面了包底下的读取道德BeanDefinitionHolder，自定义GenericBeanDefinition相关功能。

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());
      }
}
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1. FactoryBean是Spring中比较重要的一个类。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
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普通的JavaBean是直接使用类的实例，但是如果一个Bean继承了这个借口，就可以通过getObject()方法来自定义实例的内容，在FactoryBeanTest的getObject()就通过代理了原始类的方法，自定义类的方法。

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);
      }
}
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1. main函数

@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();
    }
 }
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至此一个完整的例子就完成了，这里主要用到了BeanFactoryPostProcessor，ApplicationContextAware，FactoryBean等Spring内置的接口，来嵌入Spring的加载和使用过程，这样就实现了自定义注解，和自定义代理了。