DUBBO SPI部分源码浅析

    private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();

这里先上核心代码。
在分析dubbo源码时,我们发现每个config类里面都有这段代码。
首先,protocal类带有spi注解,我们可以确认,默认使用的DUBBO-PROTOCAL作为默认扩展点。

/*
 * Copyright 1999-2011 Alibaba Group.
 *  
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *  
 *      http://www.apache.org/licenses/LICENSE-2.0
 *  
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.dubbo.rpc;

import com.alibaba.dubbo.common.URL;
import com.alibaba.dubbo.common.extension.Adaptive;
import com.alibaba.dubbo.common.extension.SPI;

/**
 * Protocol. (API/SPI, Singleton, ThreadSafe)
 *
 * @author william.liangf
 */
@SPI("dubbo")
public interface Protocol {

    /**
     * 获取缺省端口,当用户没有配置端口时使用。
     *
     * @return 缺省端口
     */
    int getDefaultPort();

    /**
     * 暴露远程服务:
* 1. 协议在接收请求时,应记录请求来源方地址信息:RpcContext.getContext().setRemoteAddress();
* 2. export()必须是幂等的,也就是暴露同一个URL的Invoker两次,和暴露一次没有区别。
* 3. export()传入的Invoker由框架实现并传入,协议不需要关心。
* * @param 服务的类型 * @param invoker 服务的执行体 * @return exporter 暴露服务的引用,用于取消暴露 * @throws RpcException 当暴露服务出错时抛出,比如端口已占用 */
@Adaptive Exporter export(Invoker invoker) throws RpcException; /** * 引用远程服务:
* 1. 当用户调用refer()所返回的Invoker对象的invoke()方法时,协议需相应执行同URL远端export()传入的Invoker对象的invoke()方法。
* 2. refer()返回的Invoker由协议实现,协议通常需要在此Invoker中发送远程请求。
* 3. 当url中有设置check=false时,连接失败不能抛出异常,并内部自动恢复。
* * @param 服务的类型 * @param type 服务的类型 * @param url 远程服务的URL地址 * @return invoker 服务的本地代理 * @throws RpcException 当连接服务提供方失败时抛出 */
@Adaptive Invoker refer(Class type, URL url) throws RpcException; /** * 释放协议:
* 1. 取消该协议所有已经暴露和引用的服务。
* 2. 释放协议所占用的所有资源,比如连接和端口。
* 3. 协议在释放后,依然能暴露和引用新的服务。
*/
void destroy(); }

首先通过类加载器获取一个Adaptive实例,先去缓存里面找,看有没有这个实例,如果没有,使用synchronized同步再次获取(为什么要再次获取?可能上一个进程已经在同步代码内set进去了,而这里状态还没有实时更新);如果没有,新创建一个Adaptive实例,放入缓存。

 @SuppressWarnings("unchecked")
    public T getAdaptiveExtension() {
        Object instance = cachedAdaptiveInstance.get();
        if (instance == null) {
            if (createAdaptiveInstanceError == null) {
                synchronized (cachedAdaptiveInstance) {
                    instance = cachedAdaptiveInstance.get();
                    if (instance == null) {
                        try {
                            instance = createAdaptiveExtension();
                            cachedAdaptiveInstance.set(instance);
                        } catch (Throwable t) {
                            createAdaptiveInstanceError = t;
                            throw new IllegalStateException("fail to create adaptive instance: " + t.toString(), t);
                        }
                    }
                }
            } else {
                throw new IllegalStateException("fail to create adaptive instance: " + createAdaptiveInstanceError.toString(), createAdaptiveInstanceError);
            }
        }

        return (T) instance;
    }

创建代码如下,这里通过1反射获取扩展点如果有直接返回,否则通过扩展点类,2compile出类文件。
在1过程中,不断将扩展点的所有参数的类型,参数的名称,通过扩展点工厂中去拿,有值就注入该扩展点中。

 try {
            return injectExtension((T) getAdaptiveExtensionClass().newInstance());
        } catch (Exception e) {
            throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
        }

最后,他的缓存部分我们可以看到在扩展点加载类ExtensionLoader中可以看到有一个Holder实例维护的缓存类

    private final Holder cachedAdaptiveInstance = new Holder(); 
  

这个缓存类中的Object用volatile修饰,是一个共享变量。

动态类实现

通过类compile出来的类文件如下:可以看出只有标记了Adaptive注解的会在运行时决定扩展点的实现

package com.alibaba.dubbo.rpc;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class Protocol$Adpative implementscom.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(co
m.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 getextension(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(co
m.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
return extension.refer(arg0, arg1);
}
}

下面分析获取扩展点:

 /**
     * 返回指定名字的扩展。如果指定名字的扩展不存在,则抛异常 {@link IllegalStateException}.
     *
     * @param name
     * @return
     */
    @SuppressWarnings("unchecked")
    public T getExtension(String name) {
        if (name == null || name.length() == 0)
            throw new IllegalArgumentException("Extension name == null");
        if ("true".equals(name)) {
            return getDefaultExtension();
        }
        Holder holder = cachedInstances.get(name);
        if (holder == null) {
            cachedInstances.putIfAbsent(name, new Holder());
            holder = cachedInstances.get(name);
        }
        Object instance = holder.get();
        if (instance == null) {
            synchronized (holder) {
                instance = holder.get();
                if (instance == null) {
                    instance = createExtension(name);
                    holder.set(instance);
                }
            }
        }
        return (T) instance;
    } 
  

再看createExtension:

@SuppressWarnings("unchecked")
    private T createExtension(String name) {
        Class clazz = getExtensionClasses().get(name);
        if (clazz == null) {
            throw findException(name);
        }
        try {
            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> 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);
        }
    }

未完待续……

你可能感兴趣的:(java,分布式)