SPI(Service Provider Interface)--通过接口获取服务

spi 现在已有实现

1. jdk 提供实现
2. dubbo里的spi实现

一、jdk实现

• 配置
  1. 定义接口
  2. 定义实现类
  3. 配置资源文件 classpath下创建（META-INF/services/接口全面：META-INF/services/spring.design.mode.test4.spi.DogService）

　

• 调用方法

        ServiceLoader<DogService> loaders = ServiceLoader.load(DogService.class);
        for (DogService d : loaders) {
            d.sleep();
        }

• 测试结果

黑色dog。。。汪汪叫，不睡觉...
白色dog。。。呼呼大睡觉...

• 代码下载

　　　　https://files.cnblogs.com/files/z-test/spi-jdk.rar

二、dubbo 里的spi实现。

• 用法介绍

         //得到一个自适应实现类，用@Adaptive 注解的类，没有就自动生产一个自适应类，可以根据调用方法的参数，动态获取处理类
         ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();

         //得到默认的实现类
         ExtensionLoader.getExtensionLoader(Protocol.class).getDefaultExtension();

         //通过名字得到实现类。得到实现类，先注入实现类，然后，使用它的包装类，进行包装。所以 返回的实例，也可能是包装类的实例
         ExtensionLoader.getExtensionLoader(Protocol.class).getExtension("name");

         //通过url里的 key对应的参数获取实现类。1.先加载默认的激活实现，2加载key对应的value里的值。
         ExtensionLoader.getExtensionLoader(Protocol.class).getActivateExtension(url, key);

创建对应名称的扩展类代码

    @SuppressWarnings("unchecked")
    private T createExtension(String name) {
        Class<?> clazz = getExtensionClasses().get(name);
        if (clazz == null) {
            throw findException(name);
        }
        try {
            T instance = (T) EXTENSION_INSTANCES.get(clazz);
            if (instance == null) {
                EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
                instance = (T) EXTENSION_INSTANCES.get(clazz);
            }
            injectExtension(instance);
            Set<Class<?>> wrapperClasses = cachedWrapperClasses;
            if (wrapperClasses != null && !wrapperClasses.isEmpty()) {
                for (Class<?> wrapperClass : wrapperClasses) {
                    instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
                }
            }
            return instance;
        } catch (Throwable t) {
            throw new IllegalStateException("Extension instance(name: " + name + ", class: " +
                    type + ")  could not be instantiated: " + t.getMessage(), t);
        }
    }

• 配置
  • 文件位置

//加载顺序 DUBBO_INTERNAL_DIRECTORY，DUBBO_DIRECTORY，SERVICES_DIRECTORY
    //即用户可以覆盖调源码里的实现。

    private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";

    private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";

    private static final String SERVICES_DIRECTORY = "META-INF/services/";

• • 配置文件内容如下 （文件名：接口全名；内容是key=value：自定义名称：接口实现类全名）

• 注解解释
  • @SPI("dubbo") 标记接口，提供一个默认的实现 一便于getDefaultExtension()得到默认实现类
  • @Adaptive
    • 　　1.出现在实现类上，.getAdaptiveExtension() 可以得到对应的实现类，
    • 　　2.出现在 接口里的对应的方法上  强制要求对应的方法有URL参数，或者参数里包含URL对象。（否则执行报错）
      • 实现类是com.alibaba.dubbo.rpc.Protocol  动态获取协议 根据url.getProtocol() == null ? "dubbo" : url.getProtocol() 获取协议名称，url参数protocol 为空时使用默认名称即@SPI("dubbo")中的名称

      •     public int getDefaultPort() {
                throw new UnsupportedOperationException(
                        "method public abstract int com.alibaba.dubbo.rpc.Protocol.getDefaultPort() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
            }
        
            public com.alibaba.dubbo.rpc.Exporter export(com.alibaba.dubbo.rpc.Invoker arg0)
                    throws com.alibaba.dubbo.rpc.RpcException {
                if (arg0 == null)
                    throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument == null");
                if (arg0.getUrl() == null)
                    throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument getUrl() == null");
                com.alibaba.dubbo.common.URL url = arg0.getUrl();
                String extName = (url.getProtocol() == null ? "dubbo" : url.getProtocol());
                if (extName == null)
                    throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url("
                            + url.toString() + ") use keys([protocol])");
                com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol) ExtensionLoader
                        .getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
                return extension.export(arg0);
            }

      • 接口对应方法中含有com.alibaba.dubbo.rpc.Invocation的根据 url.getMethodParameter(methodName, "cache", "lru");“cache“为参数名，lru 为@spi中指定的默认值

      •     public com.alibaba.dubbo.cache.Cache getCache(com.alibaba.dubbo.common.URL arg0,
                    com.alibaba.dubbo.rpc.Invocation arg1) {
                if (arg0 == null)
                    throw new IllegalArgumentException("url == null");
                com.alibaba.dubbo.common.URL url = arg0;
                if (arg1 == null)
                    throw new IllegalArgumentException("invocation == null");
                String methodName = arg1.getMethodName();
                String extName = url.getMethodParameter(methodName, "cache", "lru");
                if (extName == null)
                    throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.cache.CacheFactory) name from url("
                            + url.toString() + ") use keys([cache])");
                com.alibaba.dubbo.cache.CacheFactory extension = (com.alibaba.dubbo.cache.CacheFactory) ExtensionLoader
                        .getExtensionLoader(com.alibaba.dubbo.cache.CacheFactory.class).getExtension(extName);
                return extension.getCache(arg0, arg1);
            }

      • 其他的实现为url.getParameter("channel.handler", "all") 如果@Adaptive({Constants.DISPATCHER_KEY, "dispather", "channel.handler"})  url.getParameter("dispatcher",url.getParameter("dispather", url.getParameter("channel.handler", "all")));

      •     public com.alibaba.dubbo.remoting.ChannelHandler dispatch(com.alibaba.dubbo.remoting.ChannelHandler arg0,
                    com.alibaba.dubbo.common.URL arg1) {
                if (arg1 == null)
                    throw new IllegalArgumentException("url == null");
                com.alibaba.dubbo.common.URL url = arg1;
                String extName = url.getParameter("dispatcher",
                        url.getParameter("dispather", url.getParameter("channel.handler", "all")));
                if (extName == null)
                    throw new IllegalStateException(
                            "Fail to get extension(com.alibaba.dubbo.remoting.Dispatcher) name from url(" + url.toString()
                                    + ") use keys([dispatcher, dispather, channel.handler])");
                com.alibaba.dubbo.remoting.Dispatcher extension = (com.alibaba.dubbo.remoting.Dispatcher) ExtensionLoader
                        .getExtensionLoader(com.alibaba.dubbo.remoting.Dispatcher.class).getExtension(extName);
                return extension.dispatch(arg0, arg1);
            }

  • @Activate(group = {Constants.PROVIDER, Constants.CONSUMER})  激活的类，适用于getActivateExtension(url, key)  1.先查找适配的@activie对应的类，2，查找url里key的对应的value所对应的类。

• 动态注入属性（查找set方法，根据set方法参数类型和变量名查找可用对象注入到 spi生产的对象里面。可以注入spi里的对象和springcontext里的对象）

　　在 接口类对应的配置文件里，有两个实现类，spi 和spring

@Adaptive
public class AdaptiveExtensionFactory implements ExtensionFactory {

    private final List<ExtensionFactory> factories;

    public AdaptiveExtensionFactory() {
        ExtensionLoader<ExtensionFactory> loader = ExtensionLoader.getExtensionLoader(ExtensionFactory.class);
        List<ExtensionFactory> list = new ArrayList<ExtensionFactory>();
        for (String name : loader.getSupportedExtensions()) {
            list.add(loader.getExtension(name));
        }
        factories = Collections.unmodifiableList(list);
    }

    @Override
    public <T> T getExtension(Class<T> type, String name) {
        for (ExtensionFactory factory : factories) {
            T extension = factory.getExtension(type, name);
            if (extension != null) {
                return extension;
            }
        }
        return null;
    }

}

spring 获取需要注入的对象

public class SpringExtensionFactory implements ExtensionFactory {

    private static final Set<ApplicationContext> contexts = new ConcurrentHashSet<ApplicationContext>();

    public static void addApplicationContext(ApplicationContext context) {
        contexts.add(context);
    }

    public static void removeApplicationContext(ApplicationContext context) {
        contexts.remove(context);
    }

    @Override
    @SuppressWarnings("unchecked")
    public <T> T getExtension(Class<T> type, String name) {
        for (ApplicationContext context : contexts) {
            if (context.containsBean(name)) {
                Object bean = context.getBean(name);
                if (type.isInstance(bean)) {
                    return (T) bean;
                }
            }
        }
        return null;
    }

}

spi

public class SpiExtensionFactory implements ExtensionFactory {

    @Override
    public <T> T getExtension(Class<T> type, String name) {
        if (type.isInterface() && type.isAnnotationPresent(SPI.class)) {
            ExtensionLoader<T> loader = ExtensionLoader.getExtensionLoader(type);
            if (!loader.getSupportedExtensions().isEmpty()) {
                return loader.getAdaptiveExtension();
            }
        }
        return null;
    }

}

大家可以看看源码，根据用法介绍里的方法，跟进看看。