Dubbo ExtensionLoader分析

ExtensionLoader用法都是这样的形式

ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension()

直接看ServiceConfig的申明变量，

public class ServiceConfig<T> extends AbstractServiceConfig {    private static final long   serialVersionUID = 3033787999037024738L;    private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();        private static final ProxyFactory proxyFactory = ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();    private static final Map<String, Integer> RANDOM_PORT_MAP = new HashMap<String, Integer>();

就以ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension()为例解析，返回的Protocol 究竟是什么，

以及Exporter<?> exporter = protocol.export(invoker);发生了什么。

先看几个方法

getAdaptiveExtension

从缓存cachedAdaptiveInstance中获取，如果没有调用createAdaptiveExtension获取。

createAdaptiveExtension

调用getAdaptiveExtensionClass得到class，然后newinstance生成对象。

getAdaptiveExtensionClass

先调用一下getExtensionClasses方法，然后如果cachedAdaptiveClass不为空就用这个class，否则使用createAdaptiveExtensionClass方法

cachedAdaptiveClass的设置看下面的loadExtensionClasses方法，这个方法的调用顺序是getExtensionClasses-》getExtensionClasses-》loadExtensionClasses

getExtensionClasses

从缓存cachedClasses获取classes，如果为空则调用loadExtensionClasses方法加载

loadExtensionClasses

获取Protocol接口的SPI注解，然后将cachedDefaultName设为SPI注解的value

@SPI("dubbo")
public interface Protocol

然后在读取本地文件配置的class信息

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/";

具体根据什么条件选择，暂时没有看，我这里返回了7个，如下，但是getAdaptiveExtensionClass方法并没有使用这个返回值

其中如果这些文件中的类如果带有Adaptive注解，会把cachedAdaptiveClass设置为这个class，默认是都没有带这个注解的

createAdaptiveExtensionClass

默认情况下，第一次都会使用这个方法返回的class，这个方法使用了createAdaptiveExtensionClassCode方法生成一个string描述的对象，

code中包含了需要生成的对象的信息

createAdaptiveExtensionClassCode

完全就是用stringbuilder拼接一个对象，66666

codeBuidler.append("package " + type.getPackage().getName() + ";");
codeBuidler.append("\nimport " + ExtensionLoader.class.getName() + ";");
codeBuidler.append("\npublic class " + type.getSimpleName() + "$Adpative" + " implements " + type.getCanonicalName() + " {")

这三行等于

package com.alibaba.dubbo.rpc;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class Protocol$Adpative implements com.alibaba.dubbo.rpc.Protocol {

然后下面一大堆，不说了，结果就是

对于protocol

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.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);}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);}}

类似的对于ProxyFactory

package com.alibaba.dubbo.rpc;import com.alibaba.dubbo.common.extension.ExtensionLoader;public class ProxyFactory$Adpative implements com.alibaba.dubbo.rpc.ProxyFactory {public java.lang.Object getProxy(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.getParameter("proxy", "javassist");if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.ProxyFactory) name from url(" + url.toString() + ") use keys([proxy])");com.alibaba.dubbo.rpc.ProxyFactory extension = (com.alibaba.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.ProxyFactory.class).getExtension(extName);return extension.getProxy(arg0);}public com.alibaba.dubbo.rpc.Invoker getInvoker(java.lang.Object arg0, java.lang.Class arg1, com.alibaba.dubbo.common.URL arg2) throws java.lang.Object {if (arg2 == null) throw new IllegalArgumentException("url == null");com.alibaba.dubbo.common.URL url = arg2;String extName = url.getParameter("proxy", "javassist");if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.ProxyFactory) name from url(" + url.toString() + ") use keys([proxy])");com.alibaba.dubbo.rpc.ProxyFactory extension = (com.alibaba.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.ProxyFactory.class).getExtension(extName);return extension.getInvoker(arg0, arg1, arg2);}}

对于registryfactory

package com.alibaba.dubbo.registry;import com.alibaba.dubbo.common.extension.ExtensionLoader;public class RegistryFactory$Adpative implements com.alibaba.dubbo.registry.RegistryFactory {public com.alibaba.dubbo.registry.Registry getRegistry(com.alibaba.dubbo.common.URL arg0) {if (arg0 == null) throw new IllegalArgumentException("url == null");com.alibaba.dubbo.common.URL url = arg0;String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.registry.RegistryFactory) name from url(" + url.toString() + ") use keys([protocol])");com.alibaba.dubbo.registry.RegistryFactory extension = (com.alibaba.dubbo.registry.RegistryFactory)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.registry.RegistryFactory.class).getExtension(extName);return extension.getRegistry(arg0);}}

对于cluster

package com.alibaba.dubbo.rpc.cluster;import com.alibaba.dubbo.common.extension.ExtensionLoader;public class Cluster$Adpative implements com.alibaba.dubbo.rpc.cluster.Cluster {public com.alibaba.dubbo.rpc.Invoker join(com.alibaba.dubbo.rpc.cluster.Directory arg0) throws com.alibaba.dubbo.rpc.cluster.Directory {if (arg0 == null) throw new IllegalArgumentException("com.alibaba.dubbo.rpc.cluster.Directory argument == null");if (arg0.getUrl() == null) throw new IllegalArgumentException("com.alibaba.dubbo.rpc.cluster.Directory argument getUrl() == null");com.alibaba.dubbo.common.URL url = arg0.getUrl();String extName = url.getParameter("cluster", "failover");if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.cluster.Cluster) name from url(" + url.toString() + ") use keys([cluster])");com.alibaba.dubbo.rpc.cluster.Cluster extension = (com.alibaba.dubbo.rpc.cluster.Cluster)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.cluster.Cluster.class).getExtension(extName);return extension.join(arg0);}}

写到这里返回的Protocol是什么已经知道了，就是自己生成字节码然后反射生成的，

生成之后查看export方法发现获取了一个Protocol，然后在调用这个Protocol的export方法，

ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);

String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );

url是从arg0获取的 arg0是Invoker类型，关于Invoker请看http://blog.csdn.net/u011686226/article/details/53787477，

从Invoker的url中获取protocol类型，如果没有默认是dubbo，我这里返回的是registry。

然后调用getExtension -》createExtension，返回的的是RegistryProtocol。

RegistryProtocol的export注册服务。

可以发现所有的XXX$Adpative都是动态生成的，然后调用他的方法还会根据不同的参数生成一个XXX对象，真正的调用时这个XXX对象。