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？