05_dubbo_aop

【对这行代码进行源码分析】

ExtensionLoader<Protocol> loader = ExtensionLoader.getExtensionLoader(Protocol.class);
final Protocol dubboProtocol = loader.getExtension("dubbo");  //要分析的源码
final Protocol adaptiveExtension = loader.getAdaptiveExtension();

【对上面第二行代码进行源码分析】

先看下第一行代码执行后的ExtensionLoader实例化的结果

其中

* type =interface com.alibaba.dubbo.rpc.Protocol

* ExtensionFactory objectFactory = AdaptiveExtensionFactory（适配类）

* AdaptiveExtensionFactory成员变量factories = [ SpringExtensionFactory实例，SpiExtensionFactory实例 ]

【 getExtension(String name)调用流程 】

ExtensionLoader<Protocol> loader = ExtensionLoader.getExtensionLoader(Protocol.class);
final Protocol dubboProtocol = loader.getExtension("dubbo");
--createExtension(name);
----getExtensionClasses() //获取扩展类
------loadExtensionClasses()
--------loadFile(Map<String, Class<?>> extensionClasses, String dir)
//ioc属性注入
----injectExtension(instance);
//aop+ioc属性注入  aop获得一个wrapper装饰类
----instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance)); 

【 createExtension(String name)源码 】

private T createExtension(String name) {
    /**
     * 从Holder<Map<String, Class<?>>>cachedClasses缓存中获取所有的实现类map，
     * 然后通过name获取到对应的实现类（此时只是一个对象实例，还没有属性注入）
     */
    Class<?> clazz = getExtensionClasses().get(name);
    if (clazz == null) {
        throw findException(name);
    }
    try {
        /**
         * 从 ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES 类缓存中获取到对应的实现类的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);
        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);
    }
}

回头看下META-INF/dubbo/internal/com.alibaba.dubbo.rpc.Protocol文件内容

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注解且含有Protocol的单参数构造方法，符合这2个条件的类会被列入AOP增强类，会被防止在ExtensionLoader的实例缓存Set<Class<?>> cachedWrapperClasses;中。

此时的loader：

* type =interface com.alibaba.dubbo.rpc.Protocol

* ExtensionFactory objectFactory = AdaptiveExtensionFactory（适配类）

* AdaptiveExtensionFactory成员变量factories = [ SpringExtensionFactory实例，SpiExtensionFactory实例 ]

* cachedWrapperClasses = [ class ProtocolListenerWrapper, class ProtocolFilterWrapper ]

再来看createExtension(String name )中的aop红色部分代码，即dubbo的aop部分。

Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (wrapperClasses != null && wrapperClasses.size() > 0) {
    for (Class<?> wrapperClass : wrapperClasses) {
        instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
    }
}

在cachedWrapperClasses中缓存两个AOP增强类：ProtocolListenerWrapper和ProtocolFilterWrapper。

首先获取ProtocolListenerWrapper的单参数构造方法，然后创建ProtocolWrapper实例，最后完成对ProtocolListenerWrapper实例的属性注入，注意此时的instance = ProtocolListenerWrapper实例，而不是之前的DubboProtocol实例，之后使用ProtocolFilterWrapper以同样的方式进行包装，只是此时的ProtocolFilterWrapper包装的是ProtocolListenerWrapper实例。

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

【 ProtocolListenerWrapper源码 】

/**
 * ListenerProtocol
 *
 * @author william.liangf
 */
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();
    }

}

【 ProtocolFilterWrapper源码 】

/**
 * ListenerProtocol
 *
 * @author william.liangf
 */
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();
    }

}

【结果】

最后返回的instance是ProtocolFilterWrapper对象，

final Protocol dubboProtocol = loader.getExtension("dubbo");

这句代码最后的dubboProtocol是ProtocolFilterWrapper实例。

【参考文章】

https://www.cnblogs.com/java-zhao/p/7478136.html