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 "";
}

@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");
}
}
}

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：

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");
}
}
}
}

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 "";
}

注解修饰的类

@CustomizeComponent
public class ScanClass1 {
public void print() {
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 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());
}
}

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

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);
}
}

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();
}
}

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。