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，了解一下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組件，通過注解，Spring自動掃描加載了這些組件，並提供相關的服務。
Spring是如何讀取注解信息，並注入到bean容器中的，本文就是通過嵌入Spring的Bean加載，來描述Spring的實現方法。完整的例子都在Github上了。
自定義注解
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等注解來定義JavaBean。那么怎么自己定義一個注解，Spring可以自動加載呢。所以就有了第一個例子。
@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");
}
}
}
運行這個例子，就會發現，@MyComponent 注解的類，也被Spring加載進來了，而且可以當成普通的JavaBean正常的使用。查看Spring的源碼會發現，Spring是使用ClassPathScanningCandidateComponentProvider掃描package,這個類有這樣的注釋
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 方法有這樣幾行代碼
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.
}
}
這里就會發現Spring在掃描類信息的使用只會判斷被@Component注解的類，所以任何自定義的注解只要帶上@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。
定制功能
但上面的方法太局限了，沒辦法定制，而且也沒有實際的意義。如何用特殊的注解來實現定制的功能呢，一般有兩種方式：
- 還是用上面的方法，在注入Spring的容器后，再取出來做自己定制的功能，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，然后循環查找帶有Controller的bean，並提取其中的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
結構比較復雜，可以參考完整的例子，這里是關鍵的幾個類
- 還是定義一個注解，只不過不再需要@Component了
@Target({ ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface CustomizeComponent {
String value() default "";
}
- 注解修飾的類
@CustomizeComponent
public class ScanClass1 {
public void print() {
System.out.println("scanClass1");
}
}
- BeanScannerConfigurer用於嵌入到Spring的加載過程的中，這里用到了BeanFactoryPostProcessor 和 ApplicationContextAware。
Spring提供了一些的接口使程序可以嵌入Spring的加載過程。這個類中的繼承ApplicationContextAware接口，Spring會讀取ApplicationContextAware類型的的JavaBean，並調用setApplicationContext(ApplicationContext applicationContext)傳入Spring的applicationContext。
同樣繼承BeanFactoryPostProcessor接口，Spring會在BeanFactory的相關處理完成后調用postProcessBeanFactory方法，進行定制的功能。
@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");
}
}
- Scanner繼承的ClassPathBeanDefinitionScanner是Spring內置的Bean定義的掃描器。
includeFilter里定義了類的過濾器，newAnnotationTypeFilter(CustomizeComponent.class)表示只取被CustomizeComponent修飾的類。
doScan里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());
}
}
- 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是直接使用類的實例，但是如果一個Bean繼承了這個借口，就可以通過getObject()方法來自定義實例的內容，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);
}
}
- main函數
@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，這里主要用到了BeanFactoryPostProcessor，ApplicationContextAware，FactoryBean等Spring內置的接口，來嵌入Spring的加載和使用過程，這樣就實現了自定義注解，和自定義代理了。