【SpringBoot 拓展系列】一步步实现一个面向接口的网络访问实例(应用篇)

【WEB 系列】一步步实现一个面向接口的网络访问实例(应用篇)

一个自定义实现的面向接口的网络访问实例，主要使用以下知识点:

• ClassPathBeanDefinitionScanner 实现自定义 bean 扫描

• ImportBeanDefinitionRegistrar 来实现 bean 注册

• 代理：基于 Cglib 生成接口的代理类，实现接口增强

• RestTemplate: 实现网络访问

1. 背景

有使用过dubbo-rpc框架的小伙伴应该有这样的感受

• 服务提供方，写 API 接口，和具体的服务类，

• 消费者通过引入 API 包，再 Spring 的生态下，直接@Autowired注入接口来实现远程服务访问

如客户端定义一个 api 如下

@Api
public?interface?UserApi?{
????String?getName(int?id);

????String?updateName(String?user,?int?age);
}

对应消费者的使用姿势而言，直接注入即可（这里不展开 dubbo 的具体使用细节，主要是理解下这个用法）

@Service
public?class?ConsumerService?{
????@Autowired
????private?UserApi?indexApi;

那么我有一个大胆的想法，如果我的项目中有 http 请求，我是否可以直接定义一个对应的接口，然后实现一个类似上面的使用方式呢？

• 比如大名鼎鼎的 retrofit , openfeign

如果我们希望自己来实现这样的功能，应该怎么操作呢？

接下来我们以一个最简单、基础的实例，来演示下这个实现思路

部分代码参考自 https://github.com/LianjiaTech/retrofit-spring-boot-starter

2. 目标

首先明确以下我们希望实现的效果，我们假定，所有的请求都是 POST 表单，请求路径由类名 + 方法名来确定，如

@Api
public?interface?UserApi?{
????//?对应的url：?/UserApi/getName
????//?访问姿势形如?：curl -X POST '/UserApi/getName'?-d 'id=xxx'
????String?getName(int?id);

????//?对应的url:?/UserApi/updateName
????//?访问姿势形如:??curl?-X?POST?'/UserApi/updateName'?-d?'user=xxx&age=xxx'
????String?updateName(String?user,?int?age);
}

使用姿势直接像本地方法调用一样

@RestController
public?class?DemoRest?{
????@Autowired
????private?UserApi?indexApi;

????@GetMapping
????public?String?call(String?name,?Integer?age)?{
????????String?ans?=?indexApi.updateName(name,?age);
????????String?a2?=?indexApi.getName(1);
????????return?ans?+?"?|?"?+?a2;
????}
}

3. 实现方式

明确上面的目标之后，接下来的实现，第一步相对清晰，哪些接口是需要生成代理对象的呢？

3.1 @Api

定义一个Api注解，用来修饰需要接口类，表示这些接口需要生成代理类，通过 Http 的方法访问

@Inherited
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public?@interface?Api?{
}

3.2 接口扫描

下一步就是要扫描项目把接口上有@Api注解的都捞出来，需要创建代理类并注册到 Spring 容器

我们这里借助ClassPathBeanDefinitionScanner来实现扫描

@Slf4j
public?class?ApiScanner?extends?ClassPathBeanDefinitionScanner?{
????private?final?ClassLoader?classLoader;

????public?ApiScanner(BeanDefinitionRegistry?registry,?ClassLoader?classLoader)?{
????????super(registry,?false);
????????this.classLoader?=?classLoader;
????????registerFilter();
????}

????public?void?registerFilter()?{
????????//?表示要过滤出带有?Api?注解的类
????????addIncludeFilter(new?AnnotationTypeFilter(Api.class));
????}

????//?扫描包下待有?`@Api`?注解的接口，调用?processBeanDefinitions()?实现接口代理类生成注册
????@Override
????protected?Set<BeanDefinitionHolder>?doScan(String...?basePackages)?{
????????Set<BeanDefinitionHolder>?beanDefinitions?=?super.doScan(basePackages);
????????if?(beanDefinitions.isEmpty())?{
????????????logger.warn("No?@Api?interface?was?found?in?'"?+?Arrays.toString(basePackages)?+?"'?package.?Please?check?your?configuration.");
????????}?else?{
????????????processBeanDefinitions(beanDefinitions);
????????}
????????return?beanDefinitions;
????}

????/**
?????*?重写候选判断逻辑，捞出带有注解的接口
?????*
?????*?@param?beanDefinition
?????*?@return
?????*/
????@Override
????protected?boolean?isCandidateComponent(AnnotatedBeanDefinition?beanDefinition)?{
????????if?(beanDefinition.getMetadata().isInterface())?{
????????????try?{
????????????????Class<?>?target?=?ClassUtils.forName(beanDefinition.getMetadata().getClassName(),?classLoader);
????????????????return?!target.isAnnotation();
????????????}?catch?(Exception?ex)?{
????????????????logger.error("load?class?exception:",?ex);
????????????}
????????}
????????return?false;
????}

????private?void?processBeanDefinitions(Set<BeanDefinitionHolder>?beanDefinitions)?{
????????GenericBeanDefinition?definition;
????????for?(BeanDefinitionHolder?holder?:?beanDefinitions)?{
????????????definition?=?(GenericBeanDefinition)?holder.getBeanDefinition();
????????????if?(logger.isDebugEnabled())?{
????????????????logger.debug("Creating?ApiClient?with?name?'"?+?holder.getBeanName()
????????????????????????+?"'?and?'"?+?definition.getBeanClassName()?+?"'?Interface");
????????????}
????????????definition.getConstructorArgumentValues().addGenericArgumentValue(Objects.requireNonNull(definition.getBeanClassName()));
????????????//?beanClass全部设置为ApiFactoryBean
????????????definition.setBeanClass(ApiFactoryBean.class);
????????}
????}
}

上面的实现中，有几个细节需要注意

• addIncludeFilter(new AnnotationTypeFilter(Api.class));?只注册了一个根据@Api注解进行过滤的 Filter

• doScan: 扫描包，捞出满足条件的类

  • isCandidateComponent: 覆盖了父类的判断，用于过滤出我们需要的目标接口，没有它的话会发现捞出来的是一个空集合

  • processBeanDefinitions: 针对捞出来的目标，指定 FactoryBean(由它来创建 bean 对象)，构造方法的参数为 BeanClass

3.3 代理工厂类

上面再注册 bean 的时候，主要是借助 FactoryBean 来实现的，我们这里实现一个ApiFactoryBean，来负责为接口生成代理的访问类，再内部使用 RestTemplate 来执行 POST 请求

public?class?ApiFactoryBean<T>?implements?FactoryBean<T>?{
????private?Class<T>?targetClass;

????public?ApiFactoryBean(Class<T>?targetClass)?{
????????this.targetClass?=?targetClass;
????}

????@Override
????@SuppressWarnings("unchecked")
????public?T?getObject()?throws?Exception?{
????????return?ProxyUtil.newProxyInstance(targetClass,?new?InvocationHandler()?{
????????????@Override
????????????public?Object?invoke(Object?proxy,?Method?method,?Object[]?args)?throws?Throwable?{
????????????????if?(method.getName().equalsIgnoreCase("toString"))?{
????????????????????return?method.invoke(proxy,?args);
????????????????}

????????????????//?每次访问都创建了要给RestTemplate，可以考虑直接使用容器的bean对象,?方便与ribbon集成，实现负载均衡
????????????????RestTemplate?restTemplate?=?new?RestTemplate();
????????????????MultiValueMap<String,?Object>?params?=?new?LinkedMultiValueMap<>();

????????????????for?(int?index?=?0;?index?<?args.length;?index++)?{
????????????????????Parameter?p?=?method.getParameters()[index];
????????????????????params.add(p.getName(),?args[index]);
????????????????}

????????????????//?这里用于演示host是写死的，可以考虑根据配置来加载（比如?@Api?中指定host，或者?配置参数方式）
????????????????String?url?=?"http://127.0.0.1:8080/"?+?targetClass.getSimpleName()?+?"/"?+?method.getName();
????????????????String?response?=?restTemplate.postForObject(url,?params,?String.class);
????????????????if?(method.getReturnType()?==?String.class)?{
????????????????????return?response;
????????????????}

????????????????return?JSONObject.parseObject(response,?method.getReturnType());
????????????}
????????});
????}

????@Override
????public?Class<?>?getObjectType()?{
????????return?targetClass;
????}
}

代理类的实现中，有几个可以优化的地方

• restTemplate: 可以结合 ribbon 使用，从而实现更友好的负载策略

• host: 上面是直接写死的，推荐采用配置化策略来替代（最简单的就是在 application.yml 文件中加一个 api.host 的参数，从它来获取，项目源码中给出了实例）

代理生成工具类

public?class?ProxyUtil?{
????public?static?<T>?T?newProxyInstance(Class<?>?targetClass,?InvocationHandler?invocationHandler)?{
????????if?(targetClass?==?null)?{
????????????return?null;
????????}?else?{
????????????Enhancer?enhancer?=?new?Enhancer();
????????????enhancer.setSuperclass(targetClass);
????????????enhancer.setUseCache(true);
????????????enhancer.setCallback(new?ProxyUtil.SimpleMethodInterceptor(invocationHandler));
????????????return?(T)?enhancer.create();
????????}
????}

????private?static?class?SimpleMethodInterceptor?implements?MethodInterceptor,?Serializable?{
????????private?transient?InvocationHandler?invocationHandler;

????????public?SimpleMethodInterceptor(InvocationHandler?invocationHandler)?{
????????????this.invocationHandler?=?invocationHandler;
????????}

????????@Override
????????public?Object?intercept(Object?o,?Method?method,?Object[]?objects,?MethodProxy?methodProxy)?throws?Throwable?{
????????????return?this.invocationHandler.invoke(o,?method,?objects);
????????}
????}
}

3.4 bean 注册

Scanner 通常配合 Register 使用，实现 bean 的注册

@Slf4j
public?class?ApiRegister?implements?ImportBeanDefinitionRegistrar,?ResourceLoaderAware,?BeanFactoryAware,?BeanClassLoaderAware?{
????private?ResourceLoader?resourceLoader;
????private?BeanFactory?beanFactory;
????private?ClassLoader?classLoader;

????@Override
????public?void?setResourceLoader(ResourceLoader?resourceLoader)?{
????????this.resourceLoader?=?resourceLoader;
????}

????@Override
????public?void?setBeanFactory(BeanFactory?beanFactory)?throws?BeansException?{
????????this.beanFactory?=?beanFactory;
????}

????@Override
????public?void?setBeanClassLoader(ClassLoader?classLoader)?{
????????this.classLoader?=?classLoader;
????}

????@Override
????public?void?registerBeanDefinitions(AnnotationMetadata?importingClassMetadata,?BeanDefinitionRegistry?registry)?{
????????Set<String>?packages?=?getPackagesToScan(importingClassMetadata);
????????if?(log.isDebugEnabled())?{
????????????packages.forEach(pkg?->?log.debug("Using?auto-configuration?base?package?'{}'",?pkg));
????????}

????????ApiScanner?apiScanner?=?new?ApiScanner(registry,?classLoader);
????????if?(resourceLoader?!=?null)?{
????????????apiScanner.setResourceLoader(resourceLoader);
????????}

????????apiScanner.scan(packages.toArray(new?String[0]));
????}

????private?Set<String>?getPackagesToScan(AnnotationMetadata?metadata)?{
????????AnnotationAttributes?attributes?=
????????????????AnnotationAttributes.fromMap(metadata.getAnnotationAttributes(com.git.hui.boot.web.anno.ApiScanner.class.getName()));
????????String[]?basePackages?=?attributes.getStringArray("basePackages");
????????Class<?>[]?basePackageClasses?=?attributes.getClassArray("basePackageClasses");

????????Set<String>?packagesToScan?=?new?LinkedHashSet<>(Arrays.asList(basePackages));
????????for?(Class?clz?:?basePackageClasses)?{
????????????packagesToScan.add(ClassUtils.getPackageName(clz));
????????}

????????if?(packagesToScan.isEmpty())?{
????????????packagesToScan.add(ClassUtils.getPackageName(metadata.getClassName()));
????????}

????????return?packagesToScan;
????}
}

最后自定义一个扫描注解，让上面的 Register 生效

@Inherited
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Import(ApiRegister.class)
public?@interface?ApiScanner?{
????@AliasFor("basePackages")?String[]?value()?default?{};

????@AliasFor("value")?String[]?basePackages()?default?{};

????Class<?>[]?basePackageClasses()?default?{};
}

3.5 测试

上面就完成了我们的预期目标，接下来写一个 demo 测试一下

定义一个 api，以及提供 rest 的 Controller

项目 1

启用端口号 8080

@Api
public?interface?UserApi?{
????String?getName(int?id);

????String?updateName(String?user,?int?age);
}


@RestController
@RequestMapping(path?=?"UserApi")
public?class?UserRest?implements?UserApi?{
????@Override
????@RequestMapping(path?=?"getName")
????public?String?getName(int?id)?{
????????return?"一张三:?"?+?id;
????}

????@Override
????@PostMapping(path?=?"updateName")
????public?String?updateName(String?user,?int?age)?{
????????return?"update?"?+?user?+?"?age?to:?"?+?age;
????}
}

?

项目 2

UserApi 接口使用姿势，启用端口号 8081

@RestController
public?class?DemoRest?{
????@Autowired
????private?UserApi?indexApi;

????@GetMapping
????public?String?call(String?name,?Integer?age)?{
????????String?ans?=?indexApi.updateName(name,?age);
????????String?a2?=?indexApi.getName(1);
????????return?ans?+?"?|?"?+?a2;
????}
}

测试访问:

curl?'http://127.0.0.1:8081?name=yihui&age=18'

##?输出日志如下
update?yihui?age?to:?18?|?一张三:?1