ExtensionLoader扩展点加载

官方文档.png
在官方文档中，提到扩展点加载机制，不妨看看是如何实现的。
扩展点加载机制，最重要的类就是ExtensionLoader。

getExtensionLoader的实现原理

在获取扩展加载器的时候，首先会判断是否有注解SPI，没有则抛异常，有SPI注解的前提下，先从缓存取，取不到，根据class类型新建一个新的扩展点加载器。

public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        if (type == null)
            throw new IllegalArgumentException("Extension type == null");
        if (!type.isInterface()) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
        }
        //是否有SPI注解
        if (!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type(" + type +
                    ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
        }
        //从缓存中加载
        ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
        if (loader == null) {
            根据类型新增一个扩展点加载器
            EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
            loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
        }
        return loader;
    }

在新增加载点扩展器的时候，会根据class类型获取一个运行期自适应的扩展类型

private ExtensionLoader(Class<?> type) {
        this.type = type;
        objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}

我们看到，如果类型不是ExtensionFactory，则从ExtensionFactory的扩展加载器中获取一个可扩展的的加载器。这里先不看ExtensionFactory，先看一下获取可扩展加载器方法。

getAdaptiveExtension获取可调整的扩展加载器

在获取可扩展加载器的时候，先从缓存获取，若缓存不存在，创建新的扩展加载器。在创建新的扩展加载器的时候，首先判断配置文件中（META-INF/dubbo/，META-INF/services/,META-INF/dubbo/internal）配置的class是否有被Adaptive注解的类，如果有，则直接返回被注解Adaptive的可扩展加载类，如果没有，则生成一个扩展类。

public T getAdaptiveExtension() {
        //先从缓存中获取
        Object instance = cachedAdaptiveInstance.get();
        if (instance == null) {
            if (createAdaptiveInstanceError == null) {
                synchronized (cachedAdaptiveInstance) {
                    instance = cachedAdaptiveInstance.get();
                    if (instance == null) {
                        try {
                            //创建一个新的可扩展加载器
                            instance = createAdaptiveExtension();
                            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;
    }

    private T createAdaptiveExtension() {
         //根据可扩展实例做扩展
        try {
            return injectExtension((T) getAdaptiveExtensionClass().newInstance());
        } catch (Exception e) {
            throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e);
        }
    }

   //获取可扩展实例
    private Class<?> getAdaptiveExtensionClass() {
      // 加载当前Extension的所有实现,如果有@Adaptive类型，则会赋值为cachedAdaptiveClass属性缓存起来
        getExtensionClasses();
        if (cachedAdaptiveClass != null) {
            //如果缓存有可扩展类，则直接返回
            return cachedAdaptiveClass;
        }
        //缓存中不存在创建一个
        return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }

 获取可扩展类 
 private Map<String, Class<?>> getExtensionClasses() {
        Map<String, Class<?>> classes = cachedClasses.get();
        if (classes == null) {
            synchronized (cachedClasses) {
                classes = cachedClasses.get();
                if (classes == null) {
                    //如果没有，则从文件中加载
                    classes = loadExtensionClasses();
                    cachedClasses.set(classes);
                }
            }
        }
        return classes;
    }

  从配置文件中加载
  private Map<String, Class<?>> loadExtensionClasses() {
        final SPI defaultAnnotation = type.getAnnotation(SPI.class);
         //忽略1w行代码
        // 文件路径： 
        loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
        //文件路径
        loadFile(extensionClasses, DUBBO_DIRECTORY);
        //文件路径 
        loadFile(extensionClasses, SERVICES_DIRECTORY);
        return extensionClasses;
    }
//根据文件名（包路径）查找所有文件，将每一行的配置读取出来，判断是否是实现该接口的类，如果接口中有注解`Adaptive`，则更新`cachedAdaptiveClass`的值。
private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
   //文件路径为基本路径+包路径
   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 {
                        //读文件
                        BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
                        //读每一行数据，判断=之前为bean的ID，后面为bean的class类型
                        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)) {
                                                //如果类不是实现该接口的实例，则启动报错
                                                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)) {
                                                //如果该类注解了Adaptive注解，则更新cachedAdaptiveClass为class。
                                                if (cachedAdaptiveClass == null) {
                                                    cachedAdaptiveClass = clazz;
                                                } else if (!cachedAdaptiveClass.equals(clazz)) {
                                                    throw new IllegalStateException("More than 1 adaptive class found: "
                                                            + cachedAdaptiveClass.getClass().getName()
                                                            + ", " + clazz.getClass().getName());
                                                }
                                            } else {
                                                try {
                                                    clazz.getConstructor(type);
                                                    Set<Class<?>> wrappers = cachedWrapperClasses;
                                                    if (wrappers == null) {
                                                        cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
                                                        wrappers = cachedWrapperClasses;
                                                    }
                                                    wrappers.add(clazz);
                                                } catch (NoSuchMethodException e) {
                                                    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) {
                                                                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);
        }

如果没有被注解Adaptive注解的，则直接创建一个新的可扩展加载类。

 private Class<?> createAdaptiveExtensionClass() {
        //创建可扩展加载器的代码。。
        String code = createAdaptiveExtensionClassCode();
        //获取类加载器
        ClassLoader classLoader = findClassLoader();
        //获取编辑器
        com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
        //将代码编译为可扩展类
        return compiler.compile(code, classLoader);
    }

在对可扩展实例做扩展，会根据代理工厂创建一个实例,在构造方法中，使用的是ExtensionFactory的扩展

private T injectExtension(T instance) {
        try {
            if (objectFactory != null) {
                for (Method method : instance.getClass().getMethods()) {
                    if (method.getName().startsWith("set")
                            && method.getParameterTypes().length == 1
                            && Modifier.isPublic(method.getModifiers())) {
                        Class<?> pt = method.getParameterTypes()[0];
                        try {
                            String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
                            //获取扩展实例
                            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);
        }
        return instance;
    } 

看一下 ExtensionFactory的定义，和实现

@SPI
public interface ExtensionFactory {
   <T> T getExtension(Class<T> type, String name);
}


@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);
   }

   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;
   }
}

public class SpiExtensionFactory implements ExtensionFactory {

   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().size() > 0) {
               return loader.getAdaptiveExtension();
           }
       }
       return null;
   }

}


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);
   }

   @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;
   }

}

很显然，在这里用到的是AdaptiveExtensionFactory，因为它被注解@Adaptive注解了，然后从ExtensionFactory中获取扩展加载类。