spi（service provider interface），服務提供者介面，目的是定義一些介面規範，讓一些廠商去實現介面，在程式執行時按照一定的規則選擇使用哪個廠商提供的介面服務實現。最常見的例子就是連線資料庫的驅動Driver介面，有mysql，oracle的驅動實現。
這篇文章講的是spi機制在dubbo擴充套件服務中的使用

dubbo的擴充套件實現類放在圖中的路徑裡面

其中檔案名錶示一個可擴充套件點的全類名或者wrapper的全類名，一個可擴充套件點就是我們說的服務介面，跟資料庫驅動的Driver介面概念差不多。檔案的內容就是實現了這個擴充套件點介面的擴充套件實現類資訊。以com.alibaba.dubbo.remoting.Transporter
為例，它 的內容如下

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

主要就是配置各個擴充套件服務實現類的簡稱與全類名。dubbo簡單點來說就是根據url引數裡面的簡稱來指定選用哪一個擴充套件服務實現，下面原始碼分析時細講。

然後再看一下Transporter原始碼

@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註解表示這個類是一個可擴充套件服務介面，預設載入netty簡稱的擴充套件實現，dubbo根據這個註解來判斷類是否是一個可擴充套件服務介面詳細說明在這–>

瞭解@Adaptive註解，需要先簡單說一下介面卡類，介面卡類的作用是根據配置，選擇並例項化一個擴充套件服務介面實現，同時呼叫例項的對應方法，用來提供服務處理。
方法上的@Adaptive註解表示這個方法需要被介面卡類[代理實現]代理實現，類級別的@Adaptive表示這個類就是我們自定義的介面卡類[自定義實現]

說完上面一些概念，可以總原始碼上面進行切入分析spi工作原理。
首先是找一個需要使用到Transporter服務的一個切入點，dubbo獲取介面卡類都是統一呼叫Extendsionloader例項的getAdaptiveExtension方法的，我們可以看一下下面哪些地方使用了這個方法

選擇看Transporters原始碼，找到呼叫獲取Transporter服務的地點

/**
 * 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);
    }
//切入點->這裡選擇獲取一個擴充套件服務介面卡類例項
    public static Transporter getTransporter() {
        return ExtensionLoader.getExtensionLoader(Transporter.class).getAdaptiveExtension();
    }

}

接下來看看ExtensionLoader.getExtensionLoader(Transporter.class)方法

    public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        if (type == null)
            throw new IllegalArgumentException("Extension type == null");
         //判斷是不是介面
        if (!type.isInterface()) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
        }
        //根據SPI註解判斷介面是不是擴充套件服務介面
        if (!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type(" + type +
                    ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
        }
//先從EXTENSION_LOADERS快取獲取這個服務介面的loader，獲取不到則新建一個快取並返回
        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);
    }

獲取到Extensionloader後，執行getAdaptiveExtension()方法，進入看這個方法，這個方法的目的就是返回一個擴充套件服務介面的介面卡例項。

    @SuppressWarnings("unchecked")
    public T getAdaptiveExtension() {
    //嘗試獲取當前擴充套件服務介面介面卡例項
        Object instance = cachedAdaptiveInstance.get();
        if (instance == null) {
            if (createAdaptiveInstanceError == null) {
            //基於double check新建一個例項
                synchronized (cachedAdaptiveInstance) {
                    instance = cachedAdaptiveInstance.get();
                    if (instance == null) {
                        try {
                        //新建一個例項
                            instance = createAdaptiveExtension();
                            //快取例項
                            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;
    }

接著進入建立例項的關鍵方法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()，這個方法就是獲取到介面卡Class資訊，然後呼叫newInstance()建立一個介面卡例項，然後作為引數傳入injectExtension（）中，injectExtension作用就是給介面卡類裡面的屬性注入bean值
程式碼如下

    private T injectExtension(T instance) {
        try {
            if (objectFactory != null) {
            //遍歷方法
                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哪個屬性
                            String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
                            //從dubbo context獲取這個屬性值
                            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);
        }
        //返回介面卡例項
        return instance;
    }

理解的 injectExtension方法後，再回頭去看getAdaptiveExtensionClass()方法

    private Class<?> getAdaptiveExtensionClass() {
    //載入META-INF中配置的擴充套件實現類的class
        getExtensionClasses();
        if (cachedAdaptiveClass != null) {
            return cachedAdaptiveClass;
        }
        //建立並返回介面卡類
        return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }

進入getExtensionClasses方法看看是怎麼載入擴充套件實現類的型別資訊的

private Map<String, Class<?>> getExtensionClasses() {
//同樣，先嚐試從快取取，取不到就進行載入
        Map<String, Class<?>> classes = cachedClasses.get();
        if (classes == null) {
            synchronized (cachedClasses) {
                classes = cachedClasses.get();
                if (classes == null) {
                //載入擴充套件器類
                    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) {
        //獲取註解上預設配置值
            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<?>>();
        //載入檔案配置中內容，warpper類，擴充套件服務實現類，自定義介面卡類
        loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
        loadFile(extensionClasses, DUBBO_DIRECTORY);
        loadFile(extensionClasses, SERVICES_DIRECTORY);
        return extensionClasses;
    }

loadFile方法中獲取了extensionClasses【擴充套件實現類class】,同時也載入包裝類【如果存在】，下面原始碼

 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)) {
                                            //不是擴充套件服務介面實現類
                                                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)) {
                                            //是自定義的介面卡類
                                                if (cachedAdaptiveClass == null) {
                                                    cachedAdaptiveClass = clazz;
                                                } else if (!cachedAdaptiveClass.equals(clazz)) {
                                                //只允許一個自定義介面卡類
                                                    throw new IllegalStateException("More than 1 adaptive class found: "
                                                            + cachedAdaptiveClass.getClass().getName()
                                                            + ", " + clazz.getClass().getName());
                                                }
                                            } else {
                                                try {
                                                //判斷載入的類有沒有引數是服務介面型別的構造器，例如：*wrapper(Protocol protocol)就是Protocol服務介面的包裝類，可以有多個包裝類
                                                    clazz.getConstructor(type);
                                                    Set<Class<?>> wrappers = cachedWrapperClasses;
                                                    if (wrappers == null) {
                                                        cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
                                                        wrappers = cachedWrapperClasses;
                                                    }
                                                    wrappers.add(clazz);
                                                } catch (NoSuchMethodException e) {
                                                //沒有這個構造器，那就是一般的服務介面實現類
                                                    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) {
                                                            //存放載入的實現類
                                                                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);
        }
    }

然後再回到createAdaptiveExtensionClass()方法，目的就是利用-javassist建立一個動態代理介面卡類。

    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);
    }

最後，看一下程式碼裡面生成的介面卡類是怎麼樣子的，以擴充套件介面Transporter為例。

在以下程式碼處斷點獲取code的值，就是我們動態生成的介面卡類

    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);
    }

獲取到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，最後獲取服務例項，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);根據extName來獲取例項，最終在下面的地方建立例項

    @SuppressWarnings("unchecked")
    private T createExtension(String name) {
    //根據loadFile獲取的class獲取對應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);
            //檢視是否有包裝類，有的話，建立包裝類作為返回
            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);
        }
    }

總結一下關鍵方法呼叫鏈：