本文通过分析 Dubbo 服务暴露过程中使用到的3个 SPI 加载的类 ExtensionFactory、ProxyFactory、Protocol 来理解 SPI 的灵活加载逻辑。
以最简单的暴露服务到 jvm 为例，下面是位于 org.apache.dubbo.config.ServiceConfig 的关键源代码
// PROTOCOL 的动态类
private static final Protocol PROTOCOL = (Protocol)ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
// PROXY_FACTORY 的动态类
private static final ProxyFactory PROXY_FACTORY = (ProxyFactory)ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();
// 暴露服务到本地 jvm 中
private void exportLocal(URL url) {
URL local = URLBuilder.from(url).setProtocol("injvm").setHost("127.0.0.1").setPort(0).build();
Exporter exporter = PROTOCOL.export(PROXY_FACTORY.getInvoker(this.ref, this.interfaceClass, local));
// ...
}
获取 AdaptiveClass
类中两个静态变量 PROXY_FACTORY 或 PROTOCOL 使用了相同的方法初始化。
- 获取指定类的 ExtensionLoader 实例
- 通过 ExtensionLoader 实例调用 getAdaptiveExtension() 方法获取指定类的可扩展类。
获取指定 type 的 ExtensionLoader 方法如下：
public static ExtensionLoader getExtensionLoader(Class type) {
if (type == null) {
throw new IllegalArgumentException("Extension type == null");
} else if (!type.isInterface()) {
throw new IllegalArgumentException("Extension type (" + type + ") is not an interface!");
} else if (!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type (" + type + ") is not an extension, because it is NOT annotated with @" + SPI.class.getSimpleName() + "!");
} else {
// 先查看是否已经生成过
ExtensionLoader loader = (ExtensionLoader)EXTENSION_LOADERS.get(type);
if (loader == null) {
// 当 type=ProxyFactory 时，首次进入这里会初始化 ProxyFactory 类的 ExtensionLoader 实例
// 并把实例都会缓存到静态变量中
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader(type));
loader = (ExtensionLoader)EXTENSION_LOADERS.get(type);
}
return loader;
}
}
关键是使用 ExtensionLoader 类的构造函数进行实例化的。
private ExtensionLoader(Class type) {
this.type = type;
// 当前 type=ProxyFactory， 它的 objectFactory 为 ExtensionFactory 类的动态类
// 当 type=ExtensionFactory 时，它的 objectFactory 为 null
this.objectFactory = type == ExtensionFactory.class ? null : (ExtensionFactory)getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension();
}
objectFactory 是用来在注入依赖的时候，获取依赖属性的对象工厂。它是 ExtensionFactory 类的 AdaptiveClass。
获取 ExtensionLoader的 AdaptiveClass，它的流程比较长，如下图：
ExtensionLoader 类加载.png
最终是在项目的 META-INF 指定文件夹中查找到 type 配置的所有实现类，获取到的 AdaptiveClass 有两种情况：
- Type 接口的实现类上有
@Adaptive注解，则直接返回有此注解的类，如 type=ExtensionFactory 的实现类AdaptiveExtensionFactory。 - Type 接口的实现类上均无
@Adaptive注解，但是实现类的方法上有此注解，则会在有注解的方法上动态生成实现类，如 type=ProxyFactory 接口均有@Adaptive方法，又@SPI("javassist")代表默认获取到的是 JavassistProxyFactory 类。
(ProxyFactory)ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension()获得的类如下：
package org.apache.dubbo.rpc;
import org.apache.dubbo.common.extension.ExtensionLoader;
public class ProxyFactory$Adaptive implements org.apache.dubbo.rpc.ProxyFactory {
public java.lang.Object getProxy(org.apache.dubbo.rpc.Invoker arg0) throws org.apache.dubbo.rpc.RpcException {
// 检查是否参数异常
if (arg0 == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument == null");
if (arg0.getUrl() == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument getUrl() == null");
org.apache.dubbo.common.URL url = arg0.getUrl();
// 首先根据 url 中指定的 proxy 名称，为空的话默认使用 javassist 类名对应的 ProxyFactory 实现类
String extName = url.getParameter("proxy", "javassist");
if(extName == null) throw new IllegalStateException("Failed to get extension (org.apache.dubbo.rpc.ProxyFactory) name from url (" + url.toString() + ") use keys([proxy])");
org.apache.dubbo.rpc.ProxyFactory extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
return extension.getProxy(arg0);
}
public java.lang.Object getProxy(org.apache.dubbo.rpc.Invoker arg0, boolean arg1) throws org.apache.dubbo.rpc.RpcException {
if (arg0 == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument == null");
if (arg0.getUrl() == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument getUrl() == null");
org.apache.dubbo.common.URL url = arg0.getUrl();
String extName = url.getParameter("proxy", "javassist");
if(extName == null) throw new IllegalStateException("Failed to get extension (org.apache.dubbo.rpc.ProxyFactory) name from url (" + url.toString() + ") use keys([proxy])");
org.apache.dubbo.rpc.ProxyFactory extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
return extension.getProxy(arg0, arg1);
}
public org.apache.dubbo.rpc.Invoker getInvoker(java.lang.Object arg0, java.lang.Class arg1, org.apache.dubbo.common.URL arg2) throws org.apache.dubbo.rpc.RpcException {
if (arg2 == null) throw new IllegalArgumentException("url == null");
org.apache.dubbo.common.URL url = arg2;
String extName = url.getParameter("proxy", "javassist");
if(extName == null) throw new IllegalStateException("Failed to get extension (org.apache.dubbo.rpc.ProxyFactory) name from url (" + url.toString() + ") use keys([proxy])");
org.apache.dubbo.rpc.ProxyFactory extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
return extension.getInvoker(arg0, arg1, arg2);
}
}
(Protocol)ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension() 获得的类如下：
package org.apache.dubbo.rpc;
import org.apache.dubbo.common.extension.ExtensionLoader;
public class Protocol$Adaptive implements org.apache.dubbo.rpc.Protocol {
// 没有 @Adaptive 注解的方法会直接抛出异常
public void destroy() {
throw new UnsupportedOperationException("The method public abstract void org.apache.dubbo.rpc.Protocol.destroy() of interface org.apache.dubbo.rpc.Protocol is not adaptive method!");
}
public int getDefaultPort() {
throw new UnsupportedOperationException("The method public abstract int org.apache.dubbo.rpc.Protocol.getDefaultPort() of interface org.apache.dubbo.rpc.Protocol is not adaptive method!");
}
// 有 Adaptive 注解的方法都会生成动态方法
public org.apache.dubbo.rpc.Exporter export(org.apache.dubbo.rpc.Invoker arg0) throws org.apache.dubbo.rpc.RpcException {
// 参数异常检查
if (arg0 == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument == null");
if (arg0.getUrl() == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument getUrl() == null");
// 这里使用的是 url 中的 protocol 作为动态获取的类名，如果 url 中没有，则使用默认的类名
org.apache.dubbo.common.URL url = arg0.getUrl();
String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );
if(extName == null) throw new IllegalStateException("Failed to get extension (org.apache.dubbo.rpc.Protocol) name from url (" + url.toString() + ") use keys([protocol])");
org.apache.dubbo.rpc.Protocol extension = (org.apache.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.Protocol.class).getExtension(extName);
return extension.export(arg0);
}
public org.apache.dubbo.rpc.Invoker refer(java.lang.Class arg0, org.apache.dubbo.common.URL arg1) throws org.apache.dubbo.rpc.RpcException {
if (arg1 == null) throw new IllegalArgumentException("url == null");
org.apache.dubbo.common.URL url = arg1;
String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );
if(extName == null) throw new IllegalStateException("Failed to get extension (org.apache.dubbo.rpc.Protocol) name from url (" + url.toString() + ") use keys([protocol])");
org.apache.dubbo.rpc.Protocol extension = (org.apache.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.Protocol.class).getExtension(extName);
return extension.refer(arg0, arg1);
}
public java.util.List getServers() {
throw new UnsupportedOperationException("The method public default java.util.List org.apache.dubbo.rpc.Protocol.getServers() of interface org.apache.dubbo.rpc.Protocol is not adaptive method!");
}
}
通过 SPI 获取到了 Type 接口的 AdaptiveClass，接下来看在使用的地方如何动态获取指定的类。
暴露服务到 jvm 本地
代码很短小，实际运行的内容却不简单
private void exportLocal(URL url) {
// 设置 local URL 属性
URL local = URLBuilder.from(url).setProtocol("injvm").setHost("127.0.0.1").setPort(0).build();
Exporter exporter = PROTOCOL.export(PROXY_FACTORY.getInvoker(this.ref, this.interfaceClass, local));
// ...
}
通过前面已经知道 PROXY_FACTORY 是动态生成ProxyFactory$Adaptive 类的实例，
PROXY_FACTORY.getInvoker(this.ref, this.interfaceClass, local) 会调用ProxyFactory$Adaptive 类的下面几行代码
org.apache.dubbo.common.URL url = arg2;
// 为 local 的 URL 中 proxy 参数为空，会去获取名为 javassist 的默认实现类
String extName = url.getParameter("proxy", "javassist");
// 这里根据运行时得到的名称加载指定类实例
org.apache.dubbo.rpc.ProxyFactory extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
// 调用类实例的 getInvoker() 方法
return extension.getInvoker(arg0, arg1, arg2);
getExtension(extName) 方法调用逻辑如下
ExtensionLoader 类加载指定类.png
PROXY_FACTORY.getInvoker(this.ref, this.interfaceClass, local) 实际调用栈如下
ProxyFactory的getInvoker.png
接着是 PROTOCOL.export(invoker) 分析过程与 PROXY_FACTORY.getInvoker(...) 一致，它的调用栈逻辑如下
Protocal的export.png