dubbo的ExtensionLoader

了解4个概念：接口，实现类，wrapper，adaptive。

扩展是接口实现类被wrap之后的对象，adaptive扩展是动态生成的类（例如Dubbo$Adaptive类）。

dubbo框架为接口指定一个同名配置文件，文件中定义了实现类（包括 wrapper）。

以dubbo-2.5.2.jar/META-INF/dubbo/internal/com.alibaba.dubbo.rpc.protocol 文件为例，

文件内容由名值对组成（扩展名=类的全限定名）。

registry=com.alibaba.dubbo.registry.integration.RegistryProtocol
filter=com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper
listener=com.alibaba.dubbo.rpc.protocol.ProtocolListenerWrapper
mock=com.alibaba.dubbo.rpc.support.MockProtocol
injvm=com.alibaba.dubbo.rpc.protocol.injvm.InjvmProtocol
dubbo=com.alibaba.dubbo.rpc.protocol.dubbo.DubboProtocol
rmi=com.alibaba.dubbo.rpc.protocol.rmi.RmiProtocol
hessian=com.alibaba.dubbo.rpc.protocol.hessian.HessianProtocol
com.alibaba.dubbo.rpc.protocol.http.HttpProtocol
com.alibaba.dubbo.rpc.protocol.webservice.WebServiceProtocol
thrift=com.alibaba.dubbo.rpc.protocol.thrift.ThriftProtocol
memcached=memcom.alibaba.dubbo.rpc.protocol.memcached.MemcachedProtocol
redis=com.alibaba.dubbo.rpc.protocol.redis.RedisProtocol

provider.xml配置：

<dubbo:protocol name="dubbo" port="20880" threadpool="fixed" threads="2" iothreads="2" queues="2"/>

name="dubbo"，threadpool="fixed"，dubbo和fixed等都是扩展名。

每个扩展对应一个ExtensionLoader对象，ExtensionLoader对象存放在EXTENSION_LOADERS中。

private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS 
                     = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>();

以com.alibaba.dubbo.rpc.Protocol接口为例，分析整个过程：

com.alibaba.dubbo.rpc.Protocol接口对应一个ExtensionLoader实例，它的type = com.alibaba.dubbo.rpc.Protocol，静态变量EXTENSION_LOADERS中对应的键值分别是：com.alibaba.dubbo.rpc.Protocol -> ExtensionLoader<com.alibaba.dubbo.rpc.Protocol>。

public class ExtensionLoader<T> {
    private static final Logger logger = LoggerFactory.getLogger(ExtensionLoader.class);
    private static final String SERVICES_DIRECTORY = "META-INF/services/";
    private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";
    private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";
    private static final Pattern NAME_SEPARATOR = Pattern.compile("\\s*[,]+\\s*");
    private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> 
                            EXTENSION_LOADERS = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>();
    private static final ConcurrentMap<Class<?>, Object> 
                            EXTENSION_INSTANCES = new ConcurrentHashMap<Class<?>, Object>();
    // 成员变量
    // ==============================
    private final Class<?> type;
    private final ExtensionFactory objectFactory;
    private final ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap<Class<?>, String>();
    private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<Map<String,Class<?>>>();
    private final Map<String, Activate> cachedActivates = new ConcurrentHashMap<String, Activate>();
    private volatile Class<?> cachedAdaptiveClass = null;
    private final ConcurrentMap<String, Holder<Object>> 
                                cachedInstances = new ConcurrentHashMap<String, Holder<Object>>();
    private String cachedDefaultName;
    private final Holder<Object> cachedAdaptiveInstance = new Holder<Object>();
    private volatile Throwable createAdaptiveInstanceError;
    private Set<Class<?>> cachedWrapperClasses;
    private Map<String, IllegalStateException> exceptions = 
                                  new ConcurrentHashMap<String, IllegalStateException>();
}

getExtension函数：返回指定名字的扩展（名字就是上图中的registry，filter等），极有可能返回一个wrapper。

以dubbo为例，该方法最终返回的是一个wrapper，ProtocolFilterWrapper或者ProtocolListenerWrapper。

//name = "dubbo"
public T getExtension(String name) {
    if (name == null || name.length() == 0)
        throw new IllegalArgumentException("Extension name == null");
    if ("true".equals(name)) {
        return getDefaultExtension();
    }
    Holder<Object> holder = cachedInstances.get(name);
    if (holder == null) {
        cachedInstances.putIfAbsent(name, new Holder<Object>());
        holder = cachedInstances.get(name);
    }
    Object instance = holder.get();
    if (instance == null) {
        synchronized (holder) {
            instance = holder.get();
            if (instance == null) {
                //创建扩展
                instance = createExtension(name);
                holder.set(instance);
            }
        }
    }
    return (T) instance;
}

createExtension函数：

@SuppressWarnings("unchecked")
private T createExtension(String name) {
    // 查找扩展对应的类
    Class<?> clazz = getExtensionClasses().get(name);
    if (clazz == null) {
        //这个函数有点奇怪，直接返回异常。
        throw findException(name);
    }
    try {
        T instance = (T) EXTENSION_INSTANCES.get(clazz);
        if (instance == null) {
            //使用反射创建对象
            EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
            instance = (T) EXTENSION_INSTANCES.get(clazz);
        }
        injectExtension(instance);
        //对原生的DubboProtocol对象进行wrap
        //set中包含ProtocolFilterWrapper, ProtocolListenerWrapper
        Set<Class<?>> wrapperClasses = cachedWrapperClasses;
        if (wrapperClasses != null && wrapperClasses.size() > 0) {
            for (Class<?> wrapperClass : wrapperClasses) {
                //instance从DubboProtocol变成ProtocolFilterWrapper，然后变成ProtocolListenerWrapper
                //反射翻译过来就是：
                //instance = new ProtocolFilterWrapper(instance);
                //instance = new ProtocolListenerWrapper(instance);
                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);
    }
}

getExtensionClasses函数：

private Map<String, Class<?>> getExtensionClasses() {
    //对应dubbo-2.5.2.jar/META-INF/dubbo/internal/com.alibaba.dubbo.rpc.protoco的文件内容
    //dubbo=com.alibaba.dubbo.rpc.protocol.dubbo.DubboProtocol
    Map<String, Class<?>> classes = cachedClasses.get();
    if (classes == null) {
        synchronized (cachedClasses) {
            classes = cachedClasses.get();
            if (classes == null) {
                classes = loadExtensionClasses();
                cachedClasses.set(classes);
            }
        }
    }
    return classes;
}

loadExtensionClasses函数：

// 此方法已经getExtensionClasses方法同步过。
private Map<String, Class<?>> loadExtensionClasses() {
    // type是com.alibaba.dubbo.rpc.Protocol
    final SPI defaultAnnotation = type.getAnnotation(SPI.class);
    if(defaultAnnotation != null) {
        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<?>>();
    //加载配置文件
    loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
    loadFile(extensionClasses, DUBBO_DIRECTORY);
    loadFile(extensionClasses, SERVICES_DIRECTORY);
    return extensionClasses;
}

loadFile函数：

private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
    // META-INF/dubbo/internal/com.alibaba.dubbo.rpc.Protocol
    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)) {
                                            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)) {
                                            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 { //一般情况走这个分支
                                            try {
                                                //type = com.alibaba.dubbo.rpc.Protocol
                                                //查找该类对应的构造函数，参数类型是type，未找到则抛出异常
                                                //能找到的都是wrapper,例如ProtocolFilterWrapper和ProtocolListenerWrapper
                                                clazz.getConstructor(type);
                                                Set<Class<?>> wrappers = cachedWrapperClasses;
                                                if (wrappers == null) {
                                                    cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
                                                    wrappers = cachedWrapperClasses;
                                                }
                                                wrappers.add(clazz);
                                            } catch (NoSuchMethodException e) {
                                                //未找到构造函数，进入该分支
                                                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);
    }
}

扩展对象的创建不是一次性创建，而是在get的时候，如果没有才创建，是按需创建。

getExtension -> createExtension -> getExtensionClasses -> loadExtensionClasses

但是，dubbo使用扩展，并不是直接通过ExtensionLoader调用getExtension，而是调用getAdaptiveExtension方法获取最终的扩展，它是一个动态生成的xxx$Adpative类，这里是 Protocol$Adpative类。

// ReferenceConfig<T>类
private static final Protocol refprotocol =
                     ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();

Protocol$Adpative类代码：

package com.alibaba.dubbo.rpc;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class Protocol$Adpative implements com.alibaba.dubbo.rpc.Protocol {
    public void destroy() {
        throw new UnsupportedOperationException(
                "method public abstract void com.alibaba.dubbo.rpc.Protocol.destroy() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
    }

    public int getDefaultPort() {
        throw new UnsupportedOperationException(
         "method public abstract int com.alibaba.dubbo.rpc.Protocol.getDefaultPort() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
    }

    public com.alibaba.dubbo.rpc.Invoker refer(java.lang.Class arg0, com.alibaba.dubbo.common.URL arg1) 
           throws java.lang.Class {
        if (arg1 == null)
            throw new IllegalArgumentException("url == null");
        com.alibaba.dubbo.common.URL url = arg1;
        String extName = (url.getProtocol() == null ? "dubbo" : url
                .getProtocol());
        if (extName == null)
            throw new IllegalStateException(
                    "Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url("
                            + url.toString() + ") use keys([protocol])");
        com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol) ExtensionLoader
                .getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class)
                .getExtension(extName);
        return extension.refer(arg0, arg1);
    }

    public com.alibaba.dubbo.rpc.Exporter export(com.alibaba.dubbo.rpc.Invoker arg0)
            throws com.alibaba.dubbo.rpc.Invoker {
        if (arg0 == null)
            throw new IllegalArgumentException(
                    "com.alibaba.dubbo.rpc.Invoker argument == null");
        if (arg0.getUrl() == null)
            throw new IllegalArgumentException(
                    "com.alibaba.dubbo.rpc.Invoker argument getUrl() == null");
        com.alibaba.dubbo.common.URL url = arg0.getUrl();
        String extName = (url.getProtocol() == null ? "dubbo" : url
                .getProtocol());
        if (extName == null)
            throw new IllegalStateException(
                    "Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url("
                            + url.toString() + ") use keys([protocol])");
        com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol) ExtensionLoader
                .getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class)
                .getExtension(extName);
        return extension.export(arg0);
    }
}

当调用Protocol$Adpative类的refer和export方法时，才会去调用getExtension方法。

// ReferenceConfig<T>类
private static final Protocol refprotocol =
           ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
private T createProxy(Map<String, String> map) {
    //省略其他代码
    invoker = refprotocol.refer(interfaceClass, urls.get(0));
}

我的问题是，为什么要这么做，感觉好绕！为了从url中获取扩展名？