[网络协议]Dubbo源码（3）- 扩展点机制

dubbo扩展点 ExtensionLoader

dubbo扩展点加载流程

1. 第一步，创建对应的扩展加载类

private static final ProxyFactory PROXY_FACTORY = ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();

2. 调用getAdaptiveExtension() 进行SPI加载

public class ExtensionLoader<T> {
    public T getAdaptiveExtension() {
        Object instance = cachedAdaptiveInstance.get();
        if (instance == null) {
            if (createAdaptiveInstanceError != null) {
                throw new IllegalStateException("Failed to create adaptive instance: " +
                        createAdaptiveInstanceError.toString(),
                        createAdaptiveInstanceError);
            }

            synchronized (cachedAdaptiveInstance) {
                instance = cachedAdaptiveInstance.get();
                if (instance == null) {
                    try {
                        // 创建
                        instance = createAdaptiveExtension();
                        cachedAdaptiveInstance.set(instance);
                    } catch (Throwable t) {
                        createAdaptiveInstanceError = t;
                        throw new IllegalStateException("Failed to create adaptive instance: " + t.toString(), t);
                    }
                }
            }
        }

        return (T) instance;
    }
}

创建AdaptiveExtension

• getAdaptiveExtensionClass()
• newInstance()
• injectExtension() 注入

public class ExtensionLoader<T> {
    
    @SuppressWarnings("unchecked")
    private T createAdaptiveExtension() {
        try {
            return injectExtension((T) getAdaptiveExtensionClass().newInstance());
        } catch (Exception e) {
            throw new IllegalStateException("Can't create adaptive extension " + type + ", cause: " + e.getMessage(), e);
        }
    }
    
    private Class<?> getAdaptiveExtensionClass() {
        getExtensionClasses();
        if (cachedAdaptiveClass != null) {
            return cachedAdaptiveClass;
        }
        return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }

    /**
     *  生成 代理代码code, 使用compiler编译并加载入jvm
     * @return
     */
    private Class<?> createAdaptiveExtensionClass() {
        String code = new AdaptiveClassCodeGenerator(type, cachedDefaultName).generate();
        ClassLoader classLoader = findClassLoader();
        org.apache.dubbo.common.compiler.Compiler compiler =
                ExtensionLoader.getExtensionLoader(org.apache.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
        return compiler.compile(code, classLoader);
    }
    private Map<String, Class<?>> getExtensionClasses() {
        Map<String, Class<?>> classes = cachedClasses.get();
        if (classes == null) {
            synchronized (cachedClasses) {
                classes = cachedClasses.get();
                if (classes == null) {
                    classes = loadExtensionClasses();
                    cachedClasses.set(classes);
                }
            }
        }
        return classes;
    }

    /**
     * 去加载类
     * @return
     */
    private Map<String, Class<?>> loadExtensionClasses() {
        // 缓存默认的扩展类的名称
        cacheDefaultExtensionName();

        Map<String, Class<?>> extensionClasses = new HashMap<>();

        // 加载策略，此处会去加载META-INFO/dubbo/internal/* 、 META-INFO/dubbo/*   META-INFO/services/*  对应包名类名的文件
        for (LoadingStrategy strategy : strategies) {
            loadDirectory(extensionClasses, strategy.directory(), type.getName(), strategy.preferExtensionClassLoader(),
                    strategy.overridden(), strategy.excludedPackages());
            loadDirectory(extensionClasses, strategy.directory(), type.getName().replace("org.apache", "com.alibaba"),
                    strategy.preferExtensionClassLoader(), strategy.overridden(), strategy.excludedPackages());
        }

        return extensionClasses;
    }
}

简要知识

1. 指定名称的扩展点

 ExtensionLoader.getExtensionLoader(ProxyFactory.class).getExtension("name");

制定名称的扩展点
META-INF/dubbo/internal下
2. 自适应扩展点

 ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();

标识位扩展点
@Adaptive

// 默认是random这个扩展
@SPI("random") 

可以声明类级别和方法级别

• 如果修饰在方法级别，那么会使用javasisit动态创建一个代理类
• 如果修饰类，那么直接返回背修饰的这个类

    private Class<?> getAdaptiveExtensionClass() {
        getExtensionClasses();
        if (cachedAdaptiveClass != null) {
            return cachedAdaptiveClass;
        }
        return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }

如果存在自适应的扩展点类，那么就直接返回，否则，进行动态创建字节码，然后返回
3. 激活扩展点
类似于spring中的conditionOnBean
@Activate

自定义代理处理类

1. 自定义代理工厂类

@Slf4j
public class MyJavassistProxyFactory extends AbstractProxyFactory {

    @Override
    @SuppressWarnings("unchecked")
    public <T> T getProxy(Invoker<T> invoker, Class<?>[] interfaces) {
        log.info("provider getProxy：【{}】", interfaces.getClass().getName());
        return (T) Proxy.getProxy(interfaces).newInstance(new InvokerInvocationHandler(invoker));
    }

    @Override
    public <T> Invoker<T> getInvoker(T proxy, Class<T> type, URL url) {
        log.info("provider getInvoker：【{}】", type.getName());
        // TODO Wrapper cannot handle this scenario correctly: the classname contains '$'
        final Wrapper wrapper = Wrapper.getWrapper(proxy.getClass().getName().indexOf('$') < 0 ? proxy.getClass() : type);
        return new AbstractProxyInvoker<T>(proxy, type, url) {
            @Override
            protected Object doInvoke(T proxy, String methodName,
                                      Class<?>[] parameterTypes,
                                      Object[] arguments) throws Throwable {
                log.info("provider doInvoke：【{}】-【{}】", type.getName(), methodName);
                return wrapper.invokeMethod(proxy, methodName, parameterTypes, arguments);
            }
        };
    }
}

2. 在resources 添加文件夹META-INF/dubbo/internal
   创建文件org.apache.dubbo.rpc.ProxyFactory ，内容如下

myjavassist=com.example.dubbo.MyJavassistProxyFactory

3. 修改yml，设置代理类的key为myjavassist

dubbo:  
  provider:
    tag: tag1
    proxy: myjavassist

4. 完成，启动后日志中就会有如下结果

  provider getInvoker：【com.example.dubbo.UserDubboService】
  provider getInvoker：【com.example.dubbo.RoleDubboService】

调用时的日志

provider doInvoke：【com.example.dubbo.UserDubboService】-【getUserName】