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 "";
}@Configuration
public classComponentAnnotationTest{
publicstaticvoidmain(String[] args){
AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext();
annotationConfigApplicationContext.register(ComponentAnnotationTest.class);
annotationConfigApplicationContext.refresh();
InjectClass injectClass = annotationConfigApplicationContext.getBean(InjectClass.class);
injectClass.print();
}
@MyComponent
public static classInjectClass{
publicvoidprint(){
System.out.println("hello world");
}
}
}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.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.
}
}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：
-
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"); } } } } -
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
-
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 ""; } -
注解修饰的类
@CustomizeComponent public class ScanClass1 { publicvoidprint() { System.out.println("scanClass1"); } } -
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 classBeanScannerConfigurerimplementsBeanFactoryPostProcessor, ApplicationContextAware{ private ApplicationContext applicationContext; publicvoidsetApplicationContext(ApplicationContext applicationContext)throws BeansException { this.applicationContext = applicationContext; } publicvoidpostProcessBeanFactory(ConfigurableListableBeanFactory beanFactory)throws BeansException { Scanner scanner = new Scanner((BeanDefinitionRegistry) beanFactory); scanner.setResourceLoader(this.applicationContext); scanner.scan("org.wcong.test.spring.scan"); } } - 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 classScannerextendsClassPathBeanDefinitionScanner{ 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()); } } - 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 itselfpublic static classFactoryBeanTest<T> implementsInitializingBean, FactoryBean<T> { private String innerClassName; publicvoidsetInnerClassName(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; } publicbooleanisSingleton(){ return true; } publicvoidafterPropertiesSet()throws Exception { } } public static classInterfaceProxyimplementsInvocationHandler{ 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 classMethodInterceptorImplimplementsMethodInterceptor{ public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy)throws Throwable { System.out.println("MethodInterceptorImpl:" + method.getName()); return methodProxy.invokeSuper(o, objects); } } - main函数
@Configuration public class CustomizeScanTest { publicstaticvoidmain(String[] args){ AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext(); annotationConfigApplicationContext.register(CustomizeScanTest.class); annotationConfigApplicationContext.refresh(); ScanClass1 injectClass = annotationConfigApplicationContext.getBean(ScanClass1.class); injectClass.print(); } }
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。
文／wcong（简书作者）
原文链接：http://www.jianshu.com/p/7c2948f64b1c
著作权归作者所有，转载请联系作者获得授权，并标注“简书作者”。
原文链接：http://www.jianshu.com/p/7c2948f64b1c
著作权归作者所有，转载请联系作者获得授权，并标注“简书作者”。
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