dubbo中是spi机制

spi（service provider interface），服务提供者接口，目的是定义一些接口规范，让一些厂商去实现接口，在程序运行时按照一定的规则选择使用哪个厂商提供的接口服务实现。最常见的例子就是连接数据库的驱动Driver接口，有mysql，oracle的驱动实现。
这篇文章讲的是spi机制在dubbo扩展服务中的使用

dubbo的扩展实现类放在图中的路径里面

其中文件名表示一个可扩展点的全类名或者wrapper的全类名，一个可扩展点就是我们说的服务接口，跟数据库驱动的Driver接口概念差不多。文件的内容就是实现了这个扩展点接口的扩展实现类信息。以com.alibaba.dubbo.remoting.Transporter
为例，它 的内容如下

netty=com.alibaba.dubbo.remoting.transport.netty.NettyTransporter
netty4=com.alibaba.dubbo.remoting.transport.netty4.NettyTransporter
mina=com.alibaba.dubbo.remoting.transport.mina.MinaTransporter
grizzly=com.alibaba.dubbo.remoting.transport.grizzly.GrizzlyTransporter

主要就是配置各个扩展服务实现类的简称与全类名。dubbo简单点来说就是根据url参数里面的简称来指定选用哪一个扩展服务实现，下面源码分析时细讲。

然后再看一下Transporter源码

@SPI("netty")
public interface Transporter {
    @Adaptive({Constants.SERVER_KEY, Constants.TRANSPORTER_KEY})
    Server bind(URL url, ChannelHandler handler) throws RemotingException;

    @Adaptive({Constants.CLIENT_KEY, Constants.TRANSPORTER_KEY})
    Client connect(URL url, ChannelHandler handler) throws RemotingException;

}

@SPI注解表示这个类是一个可扩展服务接口，默认加载netty简称的扩展实现，dubbo根据这个注解来判断类是否是一个可扩展服务接口详细说明在这–>

了解@Adaptive注解，需要先简单说一下适配器类，适配器类的作用是根据配置，选择并实例化一个扩展服务接口实现，同时调用实例的对应方法，用来提供服务处理。
方法上的@Adaptive注解表示这个方法需要被适配器类[代理实现]代理实现，类级别的@Adaptive表示这个类就是我们自定义的适配器类[自定义实现]

说完上面一些概念，可以总源码上面进行切入分析spi工作原理。
首先是找一个需要使用到Transporter服务的一个切入点，dubbo获取适配器类都是统一调用Extendsionloader实例的getAdaptiveExtension方法的，我们可以看一下下面哪些地方使用了这个方法

选择看Transporters源码，找到调用获取Transporter服务的地点

/**
 * Transporter facade. (API, Static, ThreadSafe)
 */
public class Transporters {

    static {
        // check duplicate jar package
        Version.checkDuplicate(Transporters.class);
        Version.checkDuplicate(RemotingException.class);
    }

    private Transporters() {
    }

    public static Server bind(String url, ChannelHandler... handler) throws RemotingException {
        return bind(URL.valueOf(url), handler);
    }

    public static Server bind(URL url, ChannelHandler... handlers) throws RemotingException {
        if (url == null) {
            throw new IllegalArgumentException("url == null");
        }
        if (handlers == null || handlers.length == 0) {
            throw new IllegalArgumentException("handlers == null");
        }
        ChannelHandler handler;
        if (handlers.length == 1) {
            handler = handlers[0];
        } else {
            handler = new ChannelHandlerDispatcher(handlers);
        }
        return getTransporter().bind(url, handler);
    }

    public static Client connect(String url, ChannelHandler... handler) throws RemotingException {
        return connect(URL.valueOf(url), handler);
    }

    public static Client connect(URL url, ChannelHandler... handlers) throws RemotingException {
        if (url == null) {
            throw new IllegalArgumentException("url == null");
        }
        ChannelHandler handler;
        if (handlers == null || handlers.length == 0) {
            handler = new ChannelHandlerAdapter();
        } else if (handlers.length == 1) {
            handler = handlers[0];
        } else {
            handler = new ChannelHandlerDispatcher(handlers);
        }
        return getTransporter().connect(url, handler);
    }
//切入点->这里选择获取一个扩展服务适配器类实例
    public static Transporter getTransporter() {
        return ExtensionLoader.getExtensionLoader(Transporter.class).getAdaptiveExtension();
    }

}

接下来看看ExtensionLoader.getExtensionLoader(Transporter.class)方法

    public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        if (type == null)
            throw new IllegalArgumentException("Extension type == null");
         //判断是不是接口
        if (!type.isInterface()) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
        }
        //根据SPI注解判断接口是不是扩展服务接口
        if (!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type(" + type +
                    ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
        }
//先从EXTENSION_LOADERS缓存获取这个服务接口的loader，获取不到则新建一个缓存并返回
        ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
        if (loader == null) {
            EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
            loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
        }
        return loader;
    }


   private static <T> boolean withExtensionAnnotation(Class<T> type) {
        return type.isAnnotationPresent(SPI.class);
    }

获取到Extensionloader后，执行getAdaptiveExtension()方法，进入看这个方法，这个方法的目的就是返回一个扩展服务接口的适配器实例。

    @SuppressWarnings("unchecked")
    public T getAdaptiveExtension() {
    //尝试获取当前扩展服务接口适配器实例
        Object instance = cachedAdaptiveInstance.get();
        if (instance == null) {
            if (createAdaptiveInstanceError == null) {
            //基于double check新建一个实例
                synchronized (cachedAdaptiveInstance) {
                    instance = cachedAdaptiveInstance.get();
                    if (instance == null) {
                        try {
                        //新建一个实例
                            instance = createAdaptiveExtension();
                            //缓存实例
                            cachedAdaptiveInstance.set(instance);
                        } catch (Throwable t) {
                            createAdaptiveInstanceError = t;
                            throw new IllegalStateException("fail to create adaptive instance: " + t.toString(), t);
                        }
                    }
                }
            } else {
                throw new IllegalStateException("fail to create adaptive instance: " + createAdaptiveInstanceError.toString(), createAdaptiveInstanceError);
            }
        }

        return (T) instance;
    }

接着进入创建实例的关键方法createAdaptiveExtension();

    private T createAdaptiveExtension() {
        try {
            return injectExtension((T) getAdaptiveExtensionClass().newInstance());
        } catch (Exception e) {
            throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e);
        }
    }

上面的方法，先分析一下getAdaptiveExtensionClass()，这个方法就是获取到适配器Class信息，然后调用newInstance()创建一个适配器实例，然后作为参数传入injectExtension（）中，injectExtension作用就是给适配器类里面的属性注入bean值
代码如下

    private T injectExtension(T instance) {
        try {
            if (objectFactory != null) {
            //遍历方法
                for (Method method : instance.getClass().getMethods()) {
                //找到set方法
                    if (method.getName().startsWith("set")
                            && method.getParameterTypes().length == 1
                            && Modifier.isPublic(method.getModifiers())) {
                        Class<?> pt = method.getParameterTypes()[0];
                        try {
                        //找到set哪个属性
                            String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
                            //从dubbo context获取这个属性值
                            Object object = objectFactory.getExtension(pt, property);
                            if (object != null) {
                                method.invoke(instance, object);
                            }
                        } catch (Exception e) {
                            logger.error("fail to inject via method " + method.getName()
                                    + " of interface " + type.getName() + ": " + e.getMessage(), e);
                        }
                    }
                }
            }
        } catch (Exception e) {
            logger.error(e.getMessage(), e);
        }
        //返回适配器实例
        return instance;
    }

理解的 injectExtension方法后，再回头去看getAdaptiveExtensionClass()方法

    private Class<?> getAdaptiveExtensionClass() {
    //加载META-INF中配置的扩展实现类的class
        getExtensionClasses();
        if (cachedAdaptiveClass != null) {
            return cachedAdaptiveClass;
        }
        //创建并返回适配器类
        return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }

进入getExtensionClasses方法看看是怎么加载扩展实现类的类型信息的

private Map<String, Class<?>> getExtensionClasses() {
//同样，先尝试从缓存取，取不到就进行加载
        Map<String, Class<?>> classes = cachedClasses.get();
        if (classes == null) {
            synchronized (cachedClasses) {
                classes = cachedClasses.get();
                if (classes == null) {
                //加载扩展器类
                    classes = loadExtensionClasses();
                    cachedClasses.set(classes);
                }
            }
        }
        return classes;
    }

    // synchronized in getExtensionClasses
    private Map<String, Class<?>> loadExtensionClasses() {
        final SPI defaultAnnotation = type.getAnnotation(SPI.class);
        if (defaultAnnotation != null) {
        //获取注解上默认配置值
            String value = defaultAnnotation.value();
            if (value != null && (value = value.trim()).length() > 0) {
                String[] names = NAME_SEPARATOR.split(value);
                if (names.length > 1) {
                    throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()
                            + ": " + Arrays.toString(names));
                }
                if (names.length == 1) cachedDefaultName = names[0];
            }
        }

        Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
        //加载文件配置中内容，warpper类，扩展服务实现类，自定义适配器类
        loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
        loadFile(extensionClasses, DUBBO_DIRECTORY);
        loadFile(extensionClasses, SERVICES_DIRECTORY);
        return extensionClasses;
    }

loadFile方法中获取了extensionClasses【扩展实现类class】,同时也加载包装类【如果存在】，下面源码

 private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
        String fileName = dir + type.getName();
        try {
            Enumeration<java.net.URL> urls;
            ClassLoader classLoader = findClassLoader();
            if (classLoader != null) {
                urls = classLoader.getResources(fileName);
            } else {
                urls = ClassLoader.getSystemResources(fileName);
            }
            if (urls != null) {
                while (urls.hasMoreElements()) {
                    java.net.URL url = urls.nextElement();
                    try {
                        BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
                        try {
                            String line = null;
                            while ((line = reader.readLine()) != null) {
                                final int ci = line.indexOf('#');
                                if (ci >= 0) line = line.substring(0, ci);
                                line = line.trim();
                                if (line.length() > 0) {
                                    try {
                                        String name = null;
                                        int i = line.indexOf('=');
                                        if (i > 0) {
                                            name = line.substring(0, i).trim();
                                            line = line.substring(i + 1).trim();
                                        }
                                        if (line.length() > 0) {
                                            Class<?> clazz = Class.forName(line, true, classLoader);
                                            if (!type.isAssignableFrom(clazz)) {
                                            //不是扩展服务接口实现类
                                                throw new IllegalStateException("Error when load extension class(interface: " +
                                                        type + ", class line: " + clazz.getName() + "), class "
                                                        + clazz.getName() + "is not subtype of interface.");
                                            }
                                            if (clazz.isAnnotationPresent(Adaptive.class)) {
                                            //是自定义的适配器类
                                                if (cachedAdaptiveClass == null) {
                                                    cachedAdaptiveClass = clazz;
                                                } else if (!cachedAdaptiveClass.equals(clazz)) {
                                                //只允许一个自定义适配器类
                                                    throw new IllegalStateException("More than 1 adaptive class found: "
                                                            + cachedAdaptiveClass.getClass().getName()
                                                            + ", " + clazz.getClass().getName());
                                                }
                                            } else {
                                                try {
                                                //判断加载的类有没有参数是服务接口类型的构造器，例如：*wrapper(Protocol protocol)就是Protocol服务接口的包装类，可以有多个包装类
                                                    clazz.getConstructor(type);
                                                    Set<Class<?>> wrappers = cachedWrapperClasses;
                                                    if (wrappers == null) {
                                                        cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
                                                        wrappers = cachedWrapperClasses;
                                                    }
                                                    wrappers.add(clazz);
                                                } catch (NoSuchMethodException e) {
                                                //没有这个构造器，那就是一般的服务接口实现类
                                                    clazz.getConstructor();
                                                    if (name == null || name.length() == 0) {
                                                        name = findAnnotationName(clazz);
                                                        if (name == null || name.length() == 0) {
                                                            if (clazz.getSimpleName().length() > type.getSimpleName().length()
                                                                    && clazz.getSimpleName().endsWith(type.getSimpleName())) {
                                                                name = clazz.getSimpleName().substring(0, clazz.getSimpleName().length() - type.getSimpleName().length()).toLowerCase();
                                                            } else {
                                                                throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + url);
                                                            }
                                                        }
                                                    }
                                                    String[] names = NAME_SEPARATOR.split(name);
                                                    if (names != null && names.length > 0) {
                                                        Activate activate = clazz.getAnnotation(Activate.class);
                                                        if (activate != null) {
                                                            cachedActivates.put(names[0], activate);
                                                        }
                                                        for (String n : names) {
                                                            if (!cachedNames.containsKey(clazz)) {
                                                                cachedNames.put(clazz, n);
                                                            }
                                                            Class<?> c = extensionClasses.get(n);
                                                            if (c == null) {
                                                            //存放加载的实现类
                                                                extensionClasses.put(n, clazz);
                                                            } else if (c != clazz) {
                                                                throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
                                                            }
                                                        }
                                                    }
                                                }
                                            }
                                        }
                                    } catch (Throwable t) {
                                        IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + url + ", cause: " + t.getMessage(), t);
                                        exceptions.put(line, e);
                                    }
                                }
                            } // end of while read lines
                        } finally {
                            reader.close();
                        }
                    } catch (Throwable t) {
                        logger.error("Exception when load extension class(interface: " +
                                type + ", class file: " + url + ") in " + url, t);
                    }
                } // end of while urls
            }
        } catch (Throwable t) {
            logger.error("Exception when load extension class(interface: " +
                    type + ", description file: " + fileName + ").", t);
        }
    }

然后再回到createAdaptiveExtensionClass()方法，目的就是利用-javassist创建一个动态代理适配器类。

    private Class<?> createAdaptiveExtensionClass() {
        String code = createAdaptiveExtensionClassCode();
        ClassLoader classLoader = findClassLoader();
        com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
        return compiler.compile(code, classLoader);
    }

最后，看一下代码里面生成的适配器类是怎么样子的，以扩展接口Transporter为例。

在以下代码处断点获取code的值，就是我们动态生成的适配器类

    private Class<?> createAdaptiveExtensionClass() {
        String code = createAdaptiveExtensionClassCode();
        ClassLoader classLoader = findClassLoader();
        com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
        return compiler.compile(code, classLoader);
    }

获取到code值

import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class Transporter$Adaptive implements com.alibaba.dubbo.remoting.Transporter {
    public com.alibaba.dubbo.remoting.Client connect(com.alibaba.dubbo.common.URL arg0, com.alibaba.dubbo.remoting.ChannelHandler arg1) throws com.alibaba.dubbo.remoting.RemotingException {
        if (arg0 == null) throw new IllegalArgumentException("url == null");
        com.alibaba.dubbo.common.URL url = arg0;
        String extName = url.getParameter("client", url.getParameter("transporter", "netty"));
        if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.remoting.Transporter) name from url(" + url.toString() + ") use keys([client, transporter])");
        com.alibaba.dubbo.remoting.Transporter extension = (com.alibaba.dubbo.remoting.Transporter) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.remoting.Transporter.class).getExtension(extName);
        return extension.connect(arg0, arg1);
    }
    public com.alibaba.dubbo.remoting.Server bind(com.alibaba.dubbo.common.URL arg0, com.alibaba.dubbo.remoting.ChannelHandler arg1) throws com.alibaba.dubbo.remoting.RemotingException {
        if (arg0 == null) throw new IllegalArgumentException("url == null");
        com.alibaba.dubbo.common.URL url = arg0;
        String extName = url.getParameter("server", url.getParameter("transporter", "netty"));
        if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.remoting.Transporter) name from url(" + url.toString() + ") use keys([server, transporter])");
        com.alibaba.dubbo.remoting.Transporter extension = (com.alibaba.dubbo.remoting.Transporter)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.remoting.Transporter.class).getExtension(extName);
        return extension.bind(arg0, arg1);
    }
}

可以看出，最后获取服务实例，是在以下地方

 com.alibaba.dubbo.remoting.Transporter extension = (com.alibaba.dubbo.remoting.Transporter) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.remoting.Transporter.class).getExtension(extName);

getExtension(extName);根据extName来获取实例，最终在下面的地方创建实例

    @SuppressWarnings("unchecked")
    private T createExtension(String name) {
    //根据loadFile获取的class获取对应class
        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, (T) clazz.newInstance());
                instance = (T) EXTENSION_INSTANCES.get(clazz);
            }
            injectExtension(instance);
            //查看是否有包装类，有的话，创建包装类作为返回
            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);
        }
    }

总结一下关键方法调用链：