Dubbo源码解析(二)---Extension 机制

Dear 丶 2022-01-15 17:49 245阅读 0赞

## 一 什么是Extension 机制 ##

**Dubbo**的类加载机制是模仿**jdk**的**spi**加载机制；

**Jdk**的**SPI**扩展加载机制：约定是当服务的提供者每增加一个接口的实现类时，需要在**jar**包的**META-INF/service/**目录下同时创建一个以服务接口命名的具体实现类，该文件里面就是保存的实现该接口的具体实现类。而当外部程序装配这个模块的时候，就能通过该jar包META-INF/services/里的配置文件找到具体的实现类名，并装载实例化，完成模块的注入。 基于这样一个约定就能很好的找到服务接口的实现类，而不需要再代码里制定。jdk提供服务实现查找的一个工具类：java.util.ServiceLoader。

比如：

jdbc:在jdbc4以前，开发人员还需要基于Class.forName("xxx")的方式来装载驱动。jdbc4也基于spi的机制来发现驱动提供商了，可以通过META-INF/services/java.sql.Driver文件里指定实现类的方式来暴露驱动提供者如下图：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjIzMjk5_size_16_color_FFFFFF_t_70][]

但是原始的jdk扩展点加载机制有些缺陷：

·JDK标准的SPI会一次性实例化扩展点所有实现，如果有扩展实现初始化很耗时，但如果没用上也加载，会很浪费资源。

·如果扩展点加载失败，连扩展点的名称都拿不到了。比如：JDK标准的ScriptEngine，通过getName();获取脚本类型的名称，但如果RubyScriptEngine因为所依赖的jruby.jar不存在，导致RubyScriptEngine类加载失败，这个失败原因被吃掉了，和ruby对应不起来，当用户执行ruby脚本时，会报不支持ruby，而不是真正失败的原因。

·Dubbo增加了对扩展点IoC和AOP的支持，一个扩展点可以直接setter注入其它扩展点。

理解Dubbo的SPI机制之前的几个概念：

·扩展点:Dubbo作为一个非常灵活的框架，并不会强制所有用户必须使用Dubbo框架里自带的某些架构，比如注册中心的话，dubbo提供了zookeeper和redis，而开发者可以根据自己的需要使用自定的注册中心，针对这种可灵活替换的技术我们就称之为扩展点技术，类似扩展点在Dubbo中有很多，比如Protocol，Filter，LoadBlance，Cluster等等；

·Wrapper：Dubbo在加载某个接口的扩展类时候，如果发现某个实现中有一个拷贝构造函数，那么该接口实现是就是该接口的包装类，此时dubbo会在真正的实现类上包装一层wrapper，比如ProtocolFilterWrapper中含有Protocol引用，及构造函数，所以加载filter配置时会返回wrapper类；即这个时候从ExtensionLoader中返回的实际扩展类是被Wrapper包装的接口实现类。

**@SPI 注解** ：可以认为是定义默认实现类；  
比如 Protocol 接口中，定义默认协议时 dubbo；  
@SPI("dubbo")  
public interface Protocol \{\}

**@Adaptive 注解：**该注解打在接口方法上；调 ExtensionLoader.getAdaptiveExtension()获取设配类，会先通过前面的过程生成 java 的源代码，在通过编译器编译成 class 加载。但是 Compiler 的实现策略选择也是通过 通过编译器编译成 class 文件那岂不是要死循环下去了吗？

此时分析 ExtensionLoader.getAdaptiveExtension()函数，对于有实现类上去打了注解 @Adaptive的dubbo spi扩展机制，它获取设配类不在通过前面过程生成设配类java源代码， 而是在读取扩展文件的时候遇到实现类打了注解@Adaptive 就把这个类作为设配类缓存在 ExtensionLoader 中，调用是直接返回。

·扩展点自动激活：可以理解为条件激活，比如Filter接口有很多扩展点实现类，当想简化配置用到哪些过滤器时，可以@Activate自动激活，或者配置为@Activate(“xxx”)条件配置激活。

## **二 ExtensionLoader**过程详解 ##

在**Dubbo**的扩展点加载机制中，**ExtensionLoader**是整个**SPI**加载的核心，而在**Dubbo**中**ExtensionLoader**的调用一般如下：

private static final ExtensionLoader<Container> loader = ExtensionLoader.getExtensionLoader(Container.class);

Container:

@SPI("spring")
    public interface Container {
        
        /**
         * start.
         */
        void start();
        
        /**
         * stop.
         */
        void stop();
    
    }

基于以下启动类的流程进行ExtensionLoader过程讲解(这种加载机制dubbo用的很多，后面的proctol也用到，为了条理清晰点，从启动开始梳理)：

public class DemoProvider {
    
        public static void main(String[] args) {
            com.alibaba.dubbo.container.Main.main(args);
        }
    
    }

main启动类：

/*
     * Copyright 1999-2011 Alibaba Group.
     *  
     * Licensed under the Apache License, Version 2.0 (the "License");
     * you may not use this file except in compliance with the License.
     * You may obtain a copy of the License at
     *  
     *      http://www.apache.org/licenses/LICENSE-2.0
     *  
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */
    package com.alibaba.dubbo.container;
    
    import java.text.SimpleDateFormat;
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.Date;
    import java.util.List;
    
    import com.alibaba.dubbo.common.Constants;
    import com.alibaba.dubbo.common.extension.ExtensionLoader;
    import com.alibaba.dubbo.common.logger.Logger;
    import com.alibaba.dubbo.common.logger.LoggerFactory;
    import com.alibaba.dubbo.common.utils.ConfigUtils;
    
    /**
     * Main. (API, Static, ThreadSafe)
     * 
     * @author william.liangf
     */
    public class Main {
    
        public static final String CONTAINER_KEY = "dubbo.container";
    
        public static final String SHUTDOWN_HOOK_KEY = "dubbo.shutdown.hook";
        
        private static final Logger logger = LoggerFactory.getLogger(Main.class);
        
        //获取容器类的扩展点加载器
        private static final ExtensionLoader<Container> loader = ExtensionLoader.getExtensionLoader(Container.class);
        
        private static volatile boolean running = true;
    
        public static void main(String[] args) {
            try {
                if (args == null || args.length == 0) {
                    String config = ConfigUtils.getProperty(CONTAINER_KEY, loader.getDefaultExtensionName());
                    args = Constants.COMMA_SPLIT_PATTERN.split(config);
                }
                
                final List<Container> containers = new ArrayList<Container>();
                for (int i = 0; i < args.length; i ++) {
                    containers.add(loader.getExtension(args[i]));
                }
                logger.info("Use container type(" + Arrays.toString(args) + ") to run dubbo serivce.");
                
                if ("true".equals(System.getProperty(SHUTDOWN_HOOK_KEY))) {
    	            Runtime.getRuntime().addShutdownHook(new Thread() {
    	                public void run() {
    	                    for (Container container : containers) {
    	                        try {
    	                            container.stop();
    	                            logger.info("Dubbo " + container.getClass().getSimpleName() + " stopped!");
    	                        } catch (Throwable t) {
    	                            logger.error(t.getMessage(), t);
    	                        }
    	                        synchronized (Main.class) {
    	                            running = false;
    	                            Main.class.notify();
    	                        }
    	                    }
    	                }
    	            });
                }
                
                for (Container container : containers) {
                    container.start();
                    logger.info("Dubbo " + container.getClass().getSimpleName() + " started!");
                }
                System.out.println(new SimpleDateFormat("[yyyy-MM-dd HH:mm:ss]").format(new Date()) + " Dubbo service server started!");
            } catch (RuntimeException e) {
                e.printStackTrace();
                logger.error(e.getMessage(), e);
                System.exit(1);
            }
            synchronized (Main.class) {
                while (running) {
                    try {
                        Main.class.wait();
                    } catch (Throwable e) {
                    }
                }
            }
        }
        
    }

### 1. 获取扩展点加载器的方法 ###

private static final ExtensionLoader<Container> loader = ExtensionLoader.getExtensionLoader(Container.class);

private static <T> boolean withExtensionAnnotation(Class<T> type) {
     
            return type.isAnnotationPresent(SPI.class);
        }
        
        @SuppressWarnings("unchecked")
        public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        	//1.判断该class是否为空
            if (type == null)
                throw new IllegalArgumentException("Extension type == null");
            //2.判断该class是否为接口类型
            if(!type.isInterface()) {
                throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
            }
            //3.判断该class是否是@SPI注解类型
            
            if(!withExtensionAnnotation(type)) {
                throw new IllegalArgumentException("Extension type(" + type + 
                        ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
            }
            //创建一个class对应的扩展点加载程序
            ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
           //如果该loader为null，则创建一个class对应的extensionloader加载器放入EXTENSION_LOADERS中，
           // 并且返回loader
            if (loader == null) {
                EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
                
                loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
            }
            return loader;
        }
    
        private ExtensionLoader(Class<?> type) {
            this.type = type;
            // EXTENSION_LOADERS 静态的线程安全map,每个class类对应一个extensionloader加载器
            objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
        }

getAdaptiveExtension()::

@SuppressWarnings("unchecked")
        public T getAdaptiveExtension() {
        	//获取一个自适应缓存对象
            Object instance = cachedAdaptiveInstance.get();
            if (instance == null) {
                if(createAdaptiveInstanceError == null) {
                    synchronized (cachedAdaptiveInstance) {
                        instance = cachedAdaptiveInstance.get();
                        if (instance == null) {
                            try {
                            	//没有就自行创建一个AdaptiveExtension对象
                                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;
        }

createAdaptiveExtension():

@SuppressWarnings("unchecked")
        private T createAdaptiveExtension() {
            try {
                return injectExtension((T) getAdaptiveExtensionClass().newInstance());
            } catch (Exception e) {
                throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
            }
        }
        
        private Class<?> getAdaptiveExtensionClass() {
            getExtensionClasses();
            if (cachedAdaptiveClass != null) {
                return cachedAdaptiveClass;
            }
            return cachedAdaptiveClass = createAdaptiveExtensionClass();
        }

getExtensionClasses()

private Map<String, Class<?>> getExtensionClasses() {
            Map<String, Class<?>> classes = cachedClasses.get();
            if (classes == null) {
                synchronized (cachedClasses) {
                    classes = cachedClasses.get();
                    if (classes == null) {
                        classes = loadExtensionClasses();
                        cachedClasses.set(classes);
                    }
                }
            }
            return classes;
    	}

injectExtension():

//为生成的 instance 注入变量；
    //其目标是搜索所有 set 开头，同时只有一个入参的函数，执行该函数，对变量进行注入  
    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) : "";
                                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;
        }

### 2.加载指定文件里的key-value值 例如：spring=com.alibaba.dubbo.container.spring.SpringContainer ###

**loadExtensionClasses()**

// 此方法已经getExtensionClasses方法同步过。
        private Map<String, Class<?>> loadExtensionClasses() {
        	//1.获取注解为@SPI的类
            final SPI defaultAnnotation = type.getAnnotation(SPI.class);
            if(defaultAnnotation != null) {
            	//2.获取@SPI("")中的值
                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META-INF/dubbo/internal/下的文件内容
            loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
            //加载META-INF/dubbo/下的文件内容
            loadFile(extensionClasses, DUBBO_DIRECTORY);
            //加载META-INF/services/下的文件内容
            loadFile(extensionClasses, SERVICES_DIRECTORY);
            return extensionClasses;
        }

**loadFile():**

private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
            String fileName = dir + type.getName();
            try {
            	//Enumeration（枚举）接口的作用和Iterator类似，
            	//只提供了遍历Vector和HashTable类型集合元素的功能，不支持元素的移除操作
                Enumeration<java.net.URL> urls;
                //创建一个ExtensionLoader加载器
                ClassLoader classLoader = findClassLoader();
                if (classLoader != null) {
                    urls = classLoader.getResources(fileName);
                } else {
                    urls = ClassLoader.getSystemResources(fileName);
                }
                if (urls != null) {
                	//遍历Enumeration
                    while (urls.hasMoreElements()) {
                        java.net.URL url = urls.nextElement();
                        try {
                      //读取url文件中的内容
                            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);
                                                //判定此 Class 对象所表示的类或接口与指定的 Class 参数所表示的类或接口是否相同，
                                                //或是否是其超类或超接口。如果是则返回 true；否则返回 false。
                                                //如果该 Class 表示一个基本类型，且指定的 Class 参数正是该 Class 对象，则该方法返回 true；否则返回 false。 
                                                if (! type.isAssignableFrom(clazz)) {
                                                    throw new IllegalStateException("Error when load extension class(interface: " +
                                                            type + ", class line: " + clazz.getName() + "), class " 
                                                            + clazz.getName() + "is not subtype of interface.");
                                                }
                                                //isAnnotationPresent判断是否有@Adative
                                                if (clazz.isAnnotationPresent(Adaptive.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) {
                                                    	//获得构造器（Constructor）对象并调用其newInstance()方法创建对象
                                                        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它有两个设置过滤条件的字段，group，value 都是字符数组。用来指定这个扩展类在什么条件下激活
                                                            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);
            }
        }

extensionClasses的结果为：

\{spring=class com.alibaba.dubbo.container.spring.SpringContainer, jetty=class com.alibaba.dubbo.container.jetty.JettyContainer, log4j=class com.alibaba.dubbo.container.log4j.Log4jContainer\}

### 3.获取dubbo.properties的属性值 ###

if (args == null || args.length == 0) {
                	//获取加载器中指定为dubbo.container的值
                    String config = ConfigUtils.getProperty(CONTAINER_KEY, loader.getDefaultExtensionName());
                    //将参数进行格式处理
                    args = Constants.COMMA_SPLIT_PATTERN.split(config);
                }

@SuppressWarnings({ "unchecked", "rawtypes" })
        public static String getProperty(String key, String defaultValue) {
           //先从系统参数中寻找是否有dubbo.container的key
            String value = System.getProperty(key);
            if (value != null && value.length() > 0) {
                return value;
            }
            //否则从自定义的properties中获取
            Properties properties = getProperties();
            return replaceProperty(properties.getProperty(key, defaultValue), (Map)properties);
        }

getProperties()：

public static Properties getProperties() {
            if (PROPERTIES == null) {
                synchronized (ConfigUtils.class) {
                    if (PROPERTIES == null) {
                    	//默认在项目路径下去寻找dubbo.properties.file的进程变量值
                        String path = System.getProperty(Constants.DUBBO_PROPERTIES_KEY);
                        if (path == null || path.length() == 0) {
                        	//寻找dubbo.properties.file的环境变量值
                            path = System.getenv(Constants.DUBBO_PROPERTIES_KEY);
                            if (path == null || path.length() == 0) {
                            	//最后获取默认的dubbo.properties里的路径
                                path = Constants.DEFAULT_DUBBO_PROPERTIES;
                            }
                        }
                        //加载该路径下的属性值
                        PROPERTIES = ConfigUtils.loadProperties(path, false, true);
                    }
                }
            }
            return PROPERTIES;
        }

loadProperties():

public static Properties loadProperties(String fileName, boolean allowMultiFile, boolean optional) {
            Properties properties = new Properties();
            if (fileName.startsWith("/")) {
                try {
                    FileInputStream input = new FileInputStream(fileName);
                    try {
                        properties.load(input);
                    } finally {
                        input.close();
                    }
                } catch (Throwable e) {
                    logger.warn("Failed to load " + fileName + " file from " + fileName + "(ingore this file): " + e.getMessage(), e);
                }
                return properties;
            }
            
            List<java.net.URL> list = new ArrayList<java.net.URL>();
            try {
                Enumeration<java.net.URL> urls = ClassHelper.getClassLoader().getResources(fileName);
                list = new ArrayList<java.net.URL>();
                while (urls.hasMoreElements()) {
                    list.add(urls.nextElement());
                }
            } catch (Throwable t) {
                logger.warn("Fail to load " + fileName + " file: " + t.getMessage(), t);
            }
            
            if(list.size() == 0) {
                if (! optional) {
                    logger.warn("No " + fileName + " found on the class path.");
                }
                return properties;
            }
            
            if(! allowMultiFile) {
                if (list.size() > 1) {
                    String errMsg = String.format("only 1 %s file is expected, but %d dubbo.properties files found on class path: %s",
                            fileName, list.size(), list.toString());
                    logger.warn(errMsg);
                    // throw new IllegalStateException(errMsg); // see http://code.alibabatech.com/jira/browse/DUBBO-133
                }
    
                // fall back to use method getResourceAsStream
                try {
                    properties.load(ClassHelper.getClassLoader().getResourceAsStream(fileName));
                } catch (Throwable e) {
                    logger.warn("Failed to load " + fileName + " file from " + fileName + "(ingore this file): " + e.getMessage(), e);
                }
                return properties;
            }
            
            logger.info("load " + fileName + " properties file from " + list);
            
            for(java.net.URL url : list) {
                try {
                    Properties p = new Properties();
                    InputStream input = url.openStream();
                    if (input != null) {
                        try {
                            p.load(input);
                            properties.putAll(p);
                        } finally {
                            try {
                                input.close();
                            } catch (Throwable t) {}
                        }
                    }
                } catch (Throwable e) {
                    logger.warn("Fail to load " + fileName + " file from " + url + "(ingore this file): " + e.getMessage(), e);
                }
            }
            
            return properties;
        }

此时properties的结果为：

\{dubbo.container=log4j,spring,

dubbo.service.loadbalance=roundrobin,

dubbo.registry.address=zookeeper://127.0.0.1:2181,

dubbo.application.owner=william,

dubbo.application.name=demo-provider,

dubbo.protocol.port=20880,

dubbo.protocol.name=dubbo\}

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjIzMjk5_size_16_color_FFFFFF_t_70 1][]

### 4.添加dubbo.properties中指定的dubbo.container ###

final List<Container> containers = new ArrayList<Container>();
                for (int i = 0; i < args.length; i ++) {
                    containers.add(loader.getExtension(args[i]));
                }

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, (T) clazz.newInstance());
                    instance = (T) EXTENSION_INSTANCES.get(clazz);
                }
                injectExtension(instance);
                Set<Class<?>> wrapperClasses = cachedWrapperClasses;
                if (wrapperClasses != null && wrapperClasses.size() > 0) {
                    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);
            }
        }

Class<?> clazz = getExtensionClasses().get(name);获取对应key的class，

\{spring=class com.alibaba.dubbo.container.spring.SpringContainer, jetty=class com.alibaba.dubbo.container.jetty.JettyContainer, log4j=class com.alibaba.dubbo.container.log4j.Log4jContainer\}

测试：如果我在后面添加一个logback的容器，dubbo.container=log4j,spring, logback，此时运行报错

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjIzMjk5_size_16_color_FFFFFF_t_70 2][]

原因找不到该为logback的容器。

## 小结： ##

由于从消费者启动容器开始梳理源码，有些细节性的还没涉及到，比如 扩展点的 Wrapper 类，@Adaptive的注解在dubbo中具体的作用，在后续的源码解析流程中会细说。

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjIzMjk5_size_16_color_FFFFFF_t_70]: /images/20220115/4a06d45dcb9d4395b26b3b9f0c5298a5.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjIzMjk5_size_16_color_FFFFFF_t_70 1]: /images/20220115/3ece6de774194b4a93cfa62acfa915e2.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMjIzMjk5_size_16_color_FFFFFF_t_70 2]: /images/20220115/aa5c4679bd184b8f81892054adae80f9.png