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目录
1.扩展点使用实例
2.主要技术点
2.1 注解加持
2.2 注解解析
2.3 扩展点路由
在实际项目中，我们经常使用策略模式、或者状态模式来隔离同一接口下不同的实现逻辑，进而消除代码中ifelse硬编码分支，使代码结构更清晰，也大大提升了代码可读性；同时也满足了“开闭原则”，具备更高的可扩展性；
在cola架构中，给出了一种“扩展点”的思路，本质还是策略模式的实现方式，通过“扩展点注解”的组装方式将策略模式实现类注册到容器中，供后续场景逻辑决策使用；
1.扩展点使用实例
首先通过实例了解下，cola 扩展点的使用方式：
1.首先定义一个SomeExtPt接口，并实现ExtensionPointI接口
public interface SomeExtPt extends ExtensionPointI {public void doSomeThing();
}2.具体实现SomeExtPt接口，这里给出了2个实现类，如下：
@Extension(bizId = "A")
@Component
public class SomeExtensionA implements SomeExtPt {@Overridepublic void doSomeThing() {System.out.println("SomeExtensionA::doSomething");}}@Extension(bizId = "B")
@Component
public class SomeExtensionB implements SomeExtPt {@Overridepublic void doSomeThing() {System.out.println("SomeExtensionB::doSomething");}}3.测试方法：
@RunWith(SpringJUnit4ClassRunner.class)
@SpringBootTest(classes = Application.class)
public class ExtensionRegisterTest {@Resourceprivate ExtensionRegister register;@Resourceprivate ExtensionExecutor executor;@Testpublic void test() {SomeExtPt extA = new SomeExtensionA();register.doRegistration(extA);SomeExtPt extB = CglibProxyFactory.createProxy(new SomeExtensionB());register.doRegistration(extB);executor.executeVoid(SomeExtPt.class, BizScenario.valueOf("A"), SomeExtPt::doSomeThing);executor.executeVoid(SomeExtPt.class, BizScenario.valueOf("B"), SomeExtPt::doSomeThing);}}public class CglibProxyFactory {public static <T> T createProxy(T object) {Enhancer enhancer = new Enhancer();enhancer.setSuperclass(object.getClass());enhancer.setCallback(new ProxyCallback(object));return (T) enhancer.create();}public static class ProxyCallback implements MethodInterceptor {private Object target;public ProxyCallback(Object target) {this.target = target;}@Overridepublic Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {System.out.println("ProxyObject::before");Object object = proxy.invoke(target, args);System.out.println("ProxyObject::after");return object;}}
}2.主要技术点
2.1 注解加持
上述具体策略接口实现方法标注了扩展点注解：@Extension
@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Repeatable(Extensions.class)
@Component
public @interface Extension {String bizId() default BizScenario.DEFAULT_BIZ_ID;String useCase() default BizScenario.DEFAULT_USE_CASE;String scenario() default BizScenario.DEFAULT_SCENARIO;
}所有的具体实现方法都需要标注该注解，表明该类属于一个扩展点；同时，由于标注了@Component注解，表明每一个扩展点也是一个bean实例；
2.2 注解解析
扩展点注解的解析工作主要借助类ExtensionBootstrap和ExtensionRegister完成：
@Component
public class ExtensionBootstrap implements ApplicationContextAware {@Resourceprivate ExtensionRegister extensionRegister;private ApplicationContext applicationContext;@PostConstructpublic void init(){Map<String, Object> extensionBeans = applicationContext.getBeansWithAnnotation(Extension.class);extensionBeans.values().forEach(extension -> extensionRegister.doRegistration((ExtensionPointI) extension));// handle @Extensions annotationMap<String, Object> extensionsBeans = applicationContext.getBeansWithAnnotation(Extensions.class);extensionsBeans.values().forEach( extension -> extensionRegister.doRegistrationExtensions((ExtensionPointI) extension));}@Overridepublic void setApplicationContext(ApplicationContext applicationContext) throws BeansException {this.applicationContext = applicationContext;}
}@Component
public class ExtensionRegister {/*** 扩展点接口名称不合法*/private static final String EXTENSION_INTERFACE_NAME_ILLEGAL = "extension_interface_name_illegal";/*** 扩展点不合法*/private static final String EXTENSION_ILLEGAL = "extension_illegal";/*** 扩展点定义重复*/private static final String EXTENSION_DEFINE_DUPLICATE = "extension_define_duplicate";@Resourceprivate ExtensionRepository extensionRepository;public final static String EXTENSION_EXTPT_NAMING = "ExtPt";public void doRegistration(ExtensionPointI extensionObject) {Class<?> extensionClz = extensionObject.getClass();if (AopUtils.isAopProxy(extensionObject)) {extensionClz = ClassUtils.getUserClass(extensionObject);}Extension extensionAnn = AnnotationUtils.findAnnotation(extensionClz, Extension.class);BizScenario bizScenario = BizScenario.valueOf(extensionAnn.bizId(), extensionAnn.useCase(), extensionAnn.scenario());ExtensionCoordinate extensionCoordinate = new ExtensionCoordinate(calculateExtensionPoint(extensionClz), bizScenario.getUniqueIdentity());ExtensionPointI preVal = extensionRepository.getExtensionRepo().put(extensionCoordinate, extensionObject);if (preVal != null) {String errMessage = "Duplicate registration is not allowed for :" + extensionCoordinate;throw new ExtensionException(EXTENSION_DEFINE_DUPLICATE, errMessage);}}public void doRegistrationExtensions(ExtensionPointI extensionObject){Class<?> extensionClz = extensionObject.getClass();if (AopUtils.isAopProxy(extensionObject)) {extensionClz = ClassUtils.getUserClass(extensionObject);}Extensions extensionsAnnotation = AnnotationUtils.findAnnotation(extensionClz, Extensions.class);Extension[] extensions = extensionsAnnotation.value();if (!ObjectUtils.isEmpty(extensions)){for (Extension extensionAnn : extensions) {BizScenario bizScenario = BizScenario.valueOf(extensionAnn.bizId(), extensionAnn.useCase(), extensionAnn.scenario());ExtensionCoordinate extensionCoordinate = new ExtensionCoordinate(calculateExtensionPoint(extensionClz), bizScenario.getUniqueIdentity());ExtensionPointI preVal = extensionRepository.getExtensionRepo().put(extensionCoordinate, extensionObject);if (preVal != null) {String errMessage = "Duplicate registration is not allowed for :" + extensionCoordinate;throw new ExtensionException(EXTENSION_DEFINE_DUPLICATE, errMessage);}}}//String[] bizIds = extensionsAnnotation.bizId();String[] useCases = extensionsAnnotation.useCase();String[] scenarios = extensionsAnnotation.scenario();for (String bizId : bizIds) {for (String useCase : useCases) {for (String scenario : scenarios) {BizScenario bizScenario = BizScenario.valueOf(bizId, useCase, scenario);ExtensionCoordinate extensionCoordinate = new ExtensionCoordinate(calculateExtensionPoint(extensionClz), bizScenario.getUniqueIdentity());ExtensionPointI preVal = extensionRepository.getExtensionRepo().put(extensionCoordinate, extensionObject);if (preVal != null) {String errMessage = "Duplicate registration is not allowed for :" + extensionCoordinate;throw new ExtensionException(EXTENSION_DEFINE_DUPLICATE, errMessage);}}}}}/*** @param targetClz* @return*/private String calculateExtensionPoint(Class<?> targetClz) {Class<?>[] interfaces = ClassUtils.getAllInterfacesForClass(targetClz);if (interfaces == null || interfaces.length == 0) {throw new ExtensionException(EXTENSION_ILLEGAL, "Please assign a extension point interface for " + targetClz);}for (Class intf : interfaces) {String extensionPoint = intf.getSimpleName();if (extensionPoint.contains(EXTENSION_EXTPT_NAMING)) {return intf.getName();}}String errMessage = "Your name of ExtensionPoint for " + targetClz +" is not valid, must be end of " + EXTENSION_EXTPT_NAMING;throw new ExtensionException(EXTENSION_INTERFACE_NAME_ILLEGAL, errMessage);}}最终将扩展点和决策条件的映射关系存储到ExtensionRepository中：
@Component
public class ExtensionRepository {public Map<ExtensionCoordinate, ExtensionPointI> getExtensionRepo() {return extensionRepo;}private Map<ExtensionCoordinate, ExtensionPointI> extensionRepo = new HashMap<>();}2.3 扩展点路由
在实际业务场景调度过程中，会调用ExtensionExecutor的方法locateExtension完成扩展点的查找，最终执行扩展点逻辑；
@Component
public class ExtensionExecutor extends AbstractComponentExecutor {private static final String EXTENSION_NOT_FOUND = "extension_not_found";private Logger logger = LoggerFactory.getLogger(ExtensionExecutor.class);@Resourceprivate ExtensionRepository extensionRepository;@Overrideprotected <C> C locateComponent(Class<C> targetClz, BizScenario bizScenario) {C extension = locateExtension(targetClz, bizScenario);logger.debug("[Located Extension]: " + extension.getClass().getSimpleName());return extension;}/*** if the bizScenarioUniqueIdentity is "ali.tmall.supermarket"* <p>* the search path is as below:* 1、first try to get extension by "ali.tmall.supermarket", if get, return it.* 2、loop try to get extension by "ali.tmall", if get, return it.* 3、loop try to get extension by "ali", if get, return it.* 4、if not found, try the default extension** @param targetClz*/protected <Ext> Ext locateExtension(Class<Ext> targetClz, BizScenario bizScenario) {checkNull(bizScenario);Ext extension;logger.debug("BizScenario in locateExtension is : " + bizScenario.getUniqueIdentity());// first try with full namespaceextension = firstTry(targetClz, bizScenario);if (extension != null) {return extension;}// second try with default scenarioextension = secondTry(targetClz, bizScenario);if (extension != null) {return extension;}// third try with default use case + default scenarioextension = defaultUseCaseTry(targetClz, bizScenario);if (extension != null) {return extension;}String errMessage = "Can not find extension with ExtensionPoint: " +targetClz + " BizScenario:" + bizScenario.getUniqueIdentity();throw new ExtensionException(EXTENSION_NOT_FOUND, errMessage);}/*** first try with full namespace* <p>* example: biz1.useCase1.scenario1*/private <Ext> Ext firstTry(Class<Ext> targetClz, BizScenario bizScenario) {logger.debug("First trying with " + bizScenario.getUniqueIdentity());return locate(targetClz.getName(), bizScenario.getUniqueIdentity());}/*** second try with default scenario* <p>* example: biz1.useCase1.#defaultScenario#*/private <Ext> Ext secondTry(Class<Ext> targetClz, BizScenario bizScenario) {logger.debug("Second trying with " + bizScenario.getIdentityWithDefaultScenario());return locate(targetClz.getName(), bizScenario.getIdentityWithDefaultScenario());}/*** third try with default use case + default scenario* <p>* example: biz1.#defaultUseCase#.#defaultScenario#*/private <Ext> Ext defaultUseCaseTry(Class<Ext> targetClz, BizScenario bizScenario) {logger.debug("Third trying with " + bizScenario.getIdentityWithDefaultUseCase());return locate(targetClz.getName(), bizScenario.getIdentityWithDefaultUseCase());}private <Ext> Ext locate(String name, String uniqueIdentity) {final Ext ext = (Ext) extensionRepository.getExtensionRepo().get(new ExtensionCoordinate(name, uniqueIdentity));return ext;}private void checkNull(BizScenario bizScenario) {if (bizScenario == null) {throw new IllegalArgumentException("BizScenario can not be null for extension");}}}