Spring 实现策略模式--自定义注解方式解耦if...else

策略模式定义定义一簇算法类，将每个算法分别封装起来，让他们可以互相替换，策略模式可以使算法的变化独立于使用它们的客户端
场景使用策略模式，可以避免冗长的if-else 或 switch分支判断
实现

策略的定义
策略的定义需要定义一个策略接口和一组实现这个接口的策略类，因为所有的策略类都实现相同的接口

public interface Strategy{	void algorithm();}public class ConcreteStrategyA implements Strategy { @Override public void algorithm() { //具体的算法... }}public class ConcreteStrategyB implements Strategy { @Override public void algorithm() { //具体的算法... }}

策略的创建
在使用的时候，一般会通过类型来判断创建哪个策略来使用，在策略上下文中，可以使用map维护好策略类
策略的使用
策略模式包含一组可选策略，在使用策略时，一般如何确定使用哪个策略呢？最常见的是运行时动态确定使用哪种策略。程序在运行期间，根据配置、计算结果、网络等这些不确定因素，动态决定使用哪种策略

public class StrategyContext{	private static final Map<String, Strategy> strategies = new HashMap<>();static {strategies.put("A", new ConcreteStrategyA());strategies.put("B", new ConcreteStrategyB());}private static Strategy getStrategy(String type) {if (type == null || type.isEmpty()) {throw new IllegalArgumentException("type should not be empty.");}return strategies.get(type);	}public void algorithm(String type){Strategy strategy = this.getStrategy(type);strategy.algorithm();}}

UML

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策略模式的创建和使用--Spring和自定义注解在介绍策略模式时，在上下文中使用了map存储好的策略实例，在根据type获取具体的策略，调用策略算法。
当需要添加一种策略时，需要修改context代码，这违反了开闭原则：对修改关闭，对扩展开放。
要实现对扩展开放，就要对type和具体的策略实现类在代码中进行关联，可以使用自定义注解的方式，在注解中指定策略的type 。
策略上下文实现类实现 BeanPostProcessor 接口，在该接口中编写策略类型与bean的关系并维护到策略上下文中。

package com.masterlink.strategy;import lombok.extern.slf4j.Slf4j;import org.springframework.aop.support.AopUtils;import org.springframework.beans.BeansException;import org.springframework.beans.factory.config.BeanPostProcessor;import org.springframework.core.Ordered;import org.springframework.core.annotation.AnnotatedElementUtils;import org.springframework.stereotype.Component;import java.util.Collections;import java.util.Set;import java.util.concurrent.ConcurrentHashMap;@Slf4j@Componentpublic class StrategyDemoBeanPostProcessor implements BeanPostProcessor, Ordered {private final Set<Class<?>> nonAnnotatedClasses = Collections.newSetFromMap(new ConcurrentHashMap<>(64));private final StrategyContext strategyContext;private StrategyDemoBeanPostProcessor(StrategyContext context) {this.strategyContext = context;}@Overridepublic int getOrder() {return LOWEST_PRECEDENCE;}@Overridepublic Object postProcessAfterInitialization(final Object bean, final String beanName) throws BeansException {if (!this.nonAnnotatedClasses.contains(bean.getClass())) {// 获取使用 @StrategyDemo 注解的Class信息Class<?> targetClass = AopUtils.getTargetClass(bean);Class<Strategy> orderStrategyClass = (Class<Strategy>) targetClass;StrategyDemo ann = findAnnotation(targetClass);if (ann != null) {processListener(ann, orderStrategyClass);}}return bean;}@Overridepublic Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {return bean;}protected void processListener(StrategyDemo annotation,Class<Strategy> classes) {// 注册策略this.strategyContext.registerStrategy(annotation.type(), classes);}private StrategyDemo findAnnotation(Class<?> clazz) {StrategyDemo ann = AnnotatedElementUtils.findMergedAnnotation(clazz, StrategyDemo.class);return ann;}}

@Componentpublic class StrategyContext implements ApplicationContextAware {private final Map<String, Class<Strategy>> strategyClassMap = new ConcurrentHashMap<>(64);private final Map<String, Strategy> beanMap = new ConcurrentHashMap<>(64);private ApplicationContext applicationContext;/*** 注册策略* @param type* @param strategyClass*/public void registerStrategy(String type, Class<Strategy> strategyClass){if (strategyClassMap.containsKey(type)){throw new RuntimeException("strategy type:"+type+" exist");}strategyClassMap.put(type, strategyClass);}/*** 执行策略* @param type*/public void algorithm(String type){Strategy strategy = this.getStrategy(type);strategy.algorithm();}private Strategy getStrategy(String type) {if (type == null || type.isEmpty()) {throw new IllegalArgumentException("type should not be empty.");}Class<Strategy> strategyClass = strategyClassMap.get(type);return createOrGetStrategy(type, strategyClass);}private Strategy createOrGetStrategy(String type,Class<Strategy> strategyClass ){if (beanMap.containsKey(type)){return beanMap.get(type);}Strategy strategy = this.applicationContext.getBean(strategyClass);beanMap.put(type, strategy);return strategy;}@Overridepublic void setApplicationContext(ApplicationContext applicationContext) throws BeansException {this.applicationContext = applicationContext;}}

实用案例在我们的平台中，有一部分是使用的netty框架编写的tcp服务，在服务端，需要将二进制转换为对象，在协议设计阶段，定义第一个字节表示对象类型，比如int,String等，第二三个字节，表示数据长度，后面的字节位传输内容。
比如，
0x01, 0x00, 0x04,0x00, 0x00, 0x00, 0x09 ，解析出来的内容是int类型数字9 。
0x02, 0x00, 0x03,0x31, 0x32, 0x33,解析出的内容是String类型，内容是 123 。
在不使用策略模式的时候，需要将第一个字节解析出来，然会使用if--else判断类型，对后继的字节进行解析。
在实际的实现过程中，是使用了策略模式，并且使用注解的方式表示数据类型，实现过程如下。
定义策略接口和注解定义 CodecStrategyType 注解和编码解码器的策略接口 CodecStrategy

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface CodecStrategyType {/*** 编码解码类型* @return*/byte type();}public interface CodecStrategy<T> {T decoding(byte[] buffer);}/** 通用解码接口 */public interface Codec {Object decoding(byte[] bytes);}

策略实现实现两种类型的解码器： Integer 和 String

/** * integer解码 */@CodecStrategyType(type = (byte)0x01)@Servicepublic class IntgerCodecStrategy implements CodecStrategy<Integer> {@Overridepublic Integer decoding(byte[] buffer) {int value;value = https://tazarkount.com/read/(int) ((buffer[3] & 0xFF)| ((buffer[2] & 0xFF)<<8)| ((buffer[1] & 0xFF)<<16)| ((buffer[0] & 0xFF)<<24));return value;}}@CodecStrategyType(type = (byte)0x02)@Servicepublic class StringCodecStrategy implements CodecStrategy {@Overridepublic String decoding(byte[] bufferr) {return new String(bufferr);}}

策略上下文和策略注册策略上下文类 CodecStrategyContext 提供了统一解码入口，将 byte[] 转换为 Object 类型，同时提供策略的注解接口 void registerStrategy(Byte type, Class<CodecStrategy<?>> strategyClass) ，注册解码类型对应的策略实现类。
策略上下文类同时还提供了策略Bean的创建，根据类型从Spring 的 ApplicationContext 获取策略bean ，并缓存到map 。
策略Bean处理类 CodecStrategyTypeBeanPostProcessor 中解析 CodecStrategyType 注解中指定的类型。

@Componentpublic class CodecStrategyContext implements ApplicationContextAware, Codec {private final Map<Byte, Class<CodecStrategy<?>>> strategyClassMap = new ConcurrentHashMap<>(64);private final Map<Byte, CodecStrategy<?>> beanMap = new ConcurrentHashMap<>(64);private ApplicationContext applicationContext;/*** 注册策略* @param type* @param strategyClass*/public void registerStrategy(Byte type, Class<CodecStrategy<?>> strategyClass){if (strategyClassMap.containsKey(type)){throw new RuntimeException("strategy type:"+type+" exist");}strategyClassMap.put(type, strategyClass);}/*** 执行策略*/@Overridepublic Object decoding(byte[] bytes){Byte type = bytes[0];CodecStrategy<?> strategy =this.getStrategy(type);byte l1 = bytes[1];byte l2= bytes[2];short length =(short) ((l2 & 0xFF)| ((l1 & 0xFF)<<8));byte[] contentBytes = new byte[length];arraycopy(bytes,3,contentBytes,0, length);return strategy.decoding(contentBytes);}private CodecStrategy<?> getStrategy(Byte type) {Class<CodecStrategy<?>> strategyClass = strategyClassMap.get(type);return createOrGetStrategy(type, strategyClass);}private CodecStrategy<?> createOrGetStrategy(Byte type, Class<CodecStrategy<?>> strategyClass ){if (beanMap.containsKey(type)){return beanMap.get(type);}CodecStrategy<?> strategy = this.applicationContext.getBean(strategyClass);beanMap.put(type, strategy);return strategy;}@Overridepublic void setApplicationContext(ApplicationContext applicationContext) throws BeansException {this.applicationContext = applicationContext;}}

@Componentpublic class CodecStrategyTypeBeanPostProcessor implements BeanPostProcessor, Ordered {private final Set<Class<?>> nonAnnotatedClasses = Collections.newSetFromMap(new ConcurrentHashMap<>(64));private final CodecStrategyContext strategyContext;private CodecStrategyTypeBeanPostProcessor(CodecStrategyContext context) {this.strategyContext = context;}@Overridepublic int getOrder() {return LOWEST_PRECEDENCE;}@Overridepublic Object postProcessAfterInitialization(final Object bean, final String beanName) throws BeansException {if (!this.nonAnnotatedClasses.contains(bean.getClass())) {// 获取使用 @StrategyDemo 注解的Class信息Class<?> targetClass = AopUtils.getTargetClass(bean);Class<CodecStrategy<?>> orderStrategyClass = (Class<CodecStrategy<?>>) targetClass;CodecStrategyType ann = findAnnotation(targetClass);if (ann != null) {processListener(ann, orderStrategyClass);}}return bean;}@Overridepublic Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {return bean;}protected void processListener(CodecStrategyType annotation,Class<CodecStrategy<?>> classes) {// 注册策略this.strategyContext.registerStrategy(annotation.type(), classes);}private CodecStrategyType findAnnotation(Class<?> clazz) {CodecStrategyType ann = AnnotatedElementUtils.findMergedAnnotation(clazz, CodecStrategyType.class);return ann;}}

使用和测试测试Integer和String类型的策略：

0x01, 0x00, 0x04,0x00, 0x00, 0x00, 0x09 ，解析出来的内容是int类型数字9 。
0x02, 0x00, 0x03,0x31, 0x32, 0x33,解析出的内容是String类型，内容是 123 。

@ExtendWith(SpringExtension.class)@ContextConfiguration(classes = {CodecStrategyTest.CodecStrategyTestConfig.class})public class CodecStrategyTest {@ResourceCodec codec;@Testpublic void testInterDecoding(){byte[] buffer = new byte[]{0x01,0x00,0x04, 0x00, 0x00,0x00, 0x09};Integer decoding = (Integer)codec.decoding(buffer);assertThat(decoding).isNotNull().isEqualTo(9);}@Testpublic void testStringDecoding(){byte[] buffer = new byte[]{0x02, 0x00, 0x03, 0x31, 0x32,0x33};String decoding = (String)codec.decoding(buffer);assertThat(decoding).isNotNull().isEqualTo("123");}@ComponentScan({"com.masterlink.strategy"})@Configurationpublic static class CodecStrategyTestConfig {}}

扩展复杂类型【Spring 实现策略模式--自定义注解方式解耦if...else】自定义复杂类型User类，对应协议类型为 0xA0 ，第2 、3 字节表示整个对象的字段长度，紧接着是 Integer 类型的age 和 String 类型的name,
比如0xA0, 0x00 0x100x00, 0x04, 0x00, 0x00, 0x00, 0x17,0x00, 0x08,0x5A,0x68,0x61,0x6E,0x67,0x53, 0x61,0x6E ，对应的user对象是
{"age": 23,"name": "ZhangSan"}@Datapublic class User {private Integer age;private String name;}实现解码策略类已知 User 中的基础类型依赖了 Integer 和 String ，所以在User的解码策略类中，依赖了 IntgerCodecStrategy 和 StringCodecStrategy

@CodecStrategyType(type = (byte) (0xA0))@Servicepublic class UserCodeStrategy implements CodecStrategy<User> {private final StringCodecStrategy stringCodecStrategy;private final IntgerCodecStrategy intgerCodecStrategy;public UserCodeStrategy(StringCodecStrategy stringCodecStrategy, IntgerCodecStrategy intgerCodecStrategy) {this.stringCodecStrategy = stringCodecStrategy;this.intgerCodecStrategy = intgerCodecStrategy;}@Overridepublic User decoding(byte[] buffer) {byte ageL1 = buffer[0];byte ageL2 = buffer[1];short ageLength =(short) ((ageL2 & 0xFF)| ((ageL1 & 0xFF)<<8));byte[] ageBytes = new byte[ageLength];System.arraycopy(buffer,2, ageBytes,0,ageLength);byte nameL1 = buffer[0+ageLength];byte nameL2 = buffer[1+ageLength];short nameLength =(short) ((nameL2 & 0xFF)| ((nameL1 & 0xFF)<<8));byte[] nameBytes = new byte[nameLength];System.arraycopy(buffer,2+ageLength+2, nameBytes,0,nameLength);User user = new User();user.setAge(intgerCodecStrategy.decoding(ageBytes));user.setName(stringCodecStrategy.decoding(nameBytes));return user;}}

测试通过测试可以发现很轻松的就扩展了一个复杂类型的解码算法，这样随着协议的增加，可以做到对修改代码关闭，对扩展代码开放，符合开闭原则。

@Testpublic void testUserDecoding(){byte[] buffer = new byte[]{(byte)0xA0, (byte)0x00 ,(byte)0x10 ,(byte)0x00, (byte)0x04,(byte)0x00, (byte)0x00, (byte)0x00, (byte)0x17, (byte)0x00,(byte)0x08, (byte)0x5A, (byte)0x68, (byte)0x61, (byte)0x6E,(byte)0x67, (byte)0x53, (byte)0x61, (byte)0x6E};User user = (User)codec.decoding(buffer);assertThat(user).isNotNull();assertThat(user.getAge()).isEqualTo(23);assertThat(user.getName()).isEqualTo("ZhangSan");}

总结

使用策略模式，可以避免冗长的if-else 或 switch分支判断
掌握自定义注解的是使用方式
与使用 @Service("name") 注解相比，自定义注解方式支撑和扩展的类型或更灵活

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