第四章 dubbo内核之aop源码解析

        ExtensionLoader<Protocol> loader = ExtensionLoader.getExtensionLoader(Protocol.class);
        final Protocol dubboProtocol = loader.getExtension("dubbo");
        final Protocol adaptiveExtension = loader.getAdaptiveExtension();

第一行代码在2.2 dubbo-spi源码解析中讲过，本节来看第二行代码。

一、获取一个ExtensionLoader

第一行代码后获得的loader：

• Class<?> type = interface com.alibaba.dubbo.rpc.Protocol
• ExtensionFactory objectFactory = AdaptiveExtensionFactory（适配类）
  • factories = [SpringExtensionFactory实例, SpiExtensionFactory实例]

二、getExtension("dubbo")

调用层级：

ExtensionLoader<T>.getExtension()
--createExtension(String name)
----getExtensionClasses().get(name)//获取扩展类
------loadExtensionClasses()
--------loadFile(Map<String, Class<?>> extensionClasses, String dir)
----injectExtension(instance);//ioc
----wrapper包装;//aop

createExtension(String name)，该方法源码如下：

    private T createExtension(String name) {
        /** 从cachedClasses缓存中获取所有的实现类map,之后通过name获取到对应的实现类的Class对象 */
        Class<?> clazz = getExtensionClasses().get(name);
        if (clazz == null) {
            throw findException(name);
        }
        try {
            /** 从EXTENSION_INSTANCES缓存中获取对应的实现类的Class对象,如果没有,直接创建,之后放入缓存 */
            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);
15             Set<Class<?>> wrapperClasses = cachedWrapperClasses;
16             if (wrapperClasses != null && wrapperClasses.size() > 0) {
17                 for (Class<?> wrapperClass : wrapperClasses) {
18                     instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
19                 }
20             }
            return instance;
        } catch (Throwable t) {
            throw new IllegalStateException("Extension instance(name: " + name + ", class: " + type
                                            + ")  could not be instantiated: " + t.getMessage(),
                t);
        }
    }

这里，先给出META-INF/dubbo/internal/com.alibaba.dubbo.rpc.Protocol内容：

registry=com.alibaba.dubbo.registry.integration.RegistryProtocol
dubbo=com.alibaba.dubbo.rpc.protocol.dubbo.DubboProtocol
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
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=com.alibaba.dubbo.rpc.protocol.memcached.MemcachedProtocol
redis=com.alibaba.dubbo.rpc.protocol.redis.RedisProtocol

com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper和com.alibaba.dubbo.rpc.protocol.ProtocolListenerWrapper，这两个类不含有@Adaptive注解且具有含有Protocol的单参构造器，符合这样条件的会被列入AOP增强类。放置在loader的私有属性cachedWrapperClasses中。

此时的loader：

• Class<?> type = interface com.alibaba.dubbo.rpc.Protocol
• ExtensionFactory objectFactory = AdaptiveExtensionFactory（适配类）
  • factories = [SpringExtensionFactory实例, SpiExtensionFactory实例]
• cachedWrapperClasses = [class ProtocolListenerWrapper, class ProtocolFilterWrapper]

再来看createExtension(String name)中的红色部分，就是今天的重点AOP。如上所讲，我在cachedWrapperClasses中缓存了两个AOP增强类：class ProtocolListenerWrapper和class ProtocolFilterWrapper。

首先是获取ProtocolListenerWrapper的单参构造器，然后创建ProtocolListenerWrapper实例，最后完成对ProtocolListenerWrapper实例进行属性注入，注意此时的instance=ProtocolListenerWrapper实例，而不再是之前的DubboProtocol实例了。之后使用ProtocolFilterWrapper以同样的方式进行包装，只是此时ProtocolFilterWrapper包装的是ProtocolListenerWrapper实例，也就是类似于这样的关系：

instance = ProtocolFilterWrapper实例 {
      protocol = ProtocolListenerWrapper实例 {
          protocol = DubboProtocol实例
      }  
}    

来看一下ProtocolListenerWrapper源码：

package com.alibaba.dubbo.rpc.protocol;

import com.alibaba.dubbo.common.Constants;
import com.alibaba.dubbo.common.URL;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
import com.alibaba.dubbo.rpc.Exporter;
import com.alibaba.dubbo.rpc.ExporterListener;
import com.alibaba.dubbo.rpc.Invoker;
import com.alibaba.dubbo.rpc.InvokerListener;
import com.alibaba.dubbo.rpc.Protocol;
import com.alibaba.dubbo.rpc.RpcException;
import com.alibaba.dubbo.rpc.listener.ListenerExporterWrapper;
import com.alibaba.dubbo.rpc.listener.ListenerInvokerWrapper;

import java.util.Collections;

public class ProtocolListenerWrapper implements Protocol {
    private final Protocol protocol;

    public ProtocolListenerWrapper(Protocol protocol) {
        if (protocol == null) {
            throw new IllegalArgumentException("protocol == null");
        }
        this.protocol = protocol;
    }

    public int getDefaultPort() {
        return protocol.getDefaultPort();
    }

    public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
        if (Constants.REGISTRY_PROTOCOL.equals(invoker.getUrl().getProtocol())) {
            return protocol.export(invoker);
        }
        return new ListenerExporterWrapper<T>(protocol.export(invoker),
                Collections.unmodifiableList(ExtensionLoader.getExtensionLoader(ExporterListener.class)
                        .getActivateExtension(invoker.getUrl(), Constants.EXPORTER_LISTENER_KEY)));
    }

    public <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException {
        if (Constants.REGISTRY_PROTOCOL.equals(url.getProtocol())) {
            return protocol.refer(type, url);
        }
        return new ListenerInvokerWrapper<T>(protocol.refer(type, url),
                Collections.unmodifiableList(
                        ExtensionLoader.getExtensionLoader(InvokerListener.class)
                                .getActivateExtension(url, Constants.INVOKER_LISTENER_KEY)));
    }

    public void destroy() {
        protocol.destroy();
    }
}

这里的方法不做讲解，等到了服务提供者暴露服务和服务消费者引用服务的时候再做讲解。

ProtocolFilterWrapper源码如下：

package com.alibaba.dubbo.rpc.protocol;

import com.alibaba.dubbo.common.Constants;
import com.alibaba.dubbo.common.URL;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
import com.alibaba.dubbo.rpc.Exporter;
import com.alibaba.dubbo.rpc.Filter;
import com.alibaba.dubbo.rpc.Invocation;
import com.alibaba.dubbo.rpc.Invoker;
import com.alibaba.dubbo.rpc.Protocol;
import com.alibaba.dubbo.rpc.Result;
import com.alibaba.dubbo.rpc.RpcException;

import java.util.List;

public class ProtocolFilterWrapper implements Protocol {
    private final Protocol protocol;

    public ProtocolFilterWrapper(Protocol protocol) {
        if (protocol == null) {
            throw new IllegalArgumentException("protocol == null");
        }
        this.protocol = protocol;
    }

    private static <T> Invoker<T> buildInvokerChain(final Invoker<T> invoker, String key, String group) {
        Invoker<T> last = invoker;
        List<Filter> filters = ExtensionLoader.getExtensionLoader(Filter.class).getActivateExtension(invoker.getUrl(), key, group);
        if (filters.size() > 0) {
            for (int i = filters.size() - 1; i >= 0; i--) {
                final Filter filter = filters.get(i);
                final Invoker<T> next = last;
                last = new Invoker<T>() {

                    public Class<T> getInterface() {
                        return invoker.getInterface();
                    }

                    public URL getUrl() {
                        return invoker.getUrl();
                    }

                    public boolean isAvailable() {
                        return invoker.isAvailable();
                    }

                    public Result invoke(Invocation invocation) throws RpcException {
                        return filter.invoke(next, invocation);
                    }

                    public void destroy() {
                        invoker.destroy();
                    }

                    @Override
                    public String toString() {
                        return invoker.toString();
                    }
                };
            }
        }
        return last;
    }

    public int getDefaultPort() {
        return protocol.getDefaultPort();
    }

    public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
        if (Constants.REGISTRY_PROTOCOL.equals(invoker.getUrl().getProtocol())) {
            return protocol.export(invoker);
        }
        return protocol.export(buildInvokerChain(invoker, Constants.SERVICE_FILTER_KEY, Constants.PROVIDER));
    }

    public <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException {
        if (Constants.REGISTRY_PROTOCOL.equals(url.getProtocol())) {
            return protocol.refer(type, url);
        }
        return buildInvokerChain(protocol.refer(type, url), Constants.REFERENCE_FILTER_KEY, Constants.CONSUMER);
    }

    public void destroy() {
        protocol.destroy();
    }
}

这里的方法不做讲解，等到了服务提供者暴露服务和服务消费者引用服务的时候再做讲解。

最后返回的instance是ProtocolFilterWrapper对象，也就是说final Protocol dubboProtocol = loader.getExtension("dubbo");这句代码最后的dubboProtocol是ProtocolFilterWrapper实例。

至此，aop结束。