dubbo之Protocol获取适应扩展过程分析

开篇

 这篇文章尝试分析Protocol的getAdaptiveExtension过程，和dubbo之ExtensionFactory获取适应扩展过程分析不一样的过程在于ExtensionFactory的扩展类是定义的AdaptiveExtensionFactory，而Protocol的适应扩展是动态生成的。

 所以重点关注下Protocol$Adaptive的动态生成过程，文中会有动态生成后的源码。

引用例子

说明：

• ExtensionLoader的使用分两步走，先getExtensionLoader后getAdaptiveExtension。
• getExtensionLoader返回对应类的ExtensionLoader。
• getAdaptiveExtension返回对应类的AdaptiveExtension。
• Protocol会返回Protocol$Adaptive的扩展类.

public class ServiceConfig<T> extends AbstractServiceConfig {
    private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
}

ExtensionLoader介绍

• 核心部分可以参考dubbo之ExtensionFactory获取适应扩展过程分析的ExtensionLoader介绍。
• Protocol对应的ExtensionLoader的objectFactory为AdaptiveExtensionFactory。
• 非ExtensionFactory的ExtensionLoader的objectFactory都是AdaptiveExtensionFactory。

public class ExtensionLoader<T> {

    private static final Logger logger = LoggerFactory.getLogger(ExtensionLoader.class);
    private static final String SERVICES_DIRECTORY = "META-INF/services/";
    private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";
    private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";
    private static final Pattern NAME_SEPARATOR = Pattern.compile("\\s*[,]+\\s*");
    private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<>();
    private static final ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap<>();
    private final Class<?> type;
    private final ExtensionFactory objectFactory;
    private final ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap<>();
    private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<>();
    private final Map<String, Object> cachedActivates = new ConcurrentHashMap<>();
    private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<>();
    private final Holder<Object> cachedAdaptiveInstance = new Holder<>();
    private volatile Class<?> cachedAdaptiveClass = null;
    private String cachedDefaultName;
    private volatile Throwable createAdaptiveInstanceError;
    private Set<Class<?>> cachedWrapperClasses;
    private Map<String, IllegalStateException> exceptions = new ConcurrentHashMap<>();

    private ExtensionLoader(Class<?> type) {
        this.type = type;
        objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
    }

    @SuppressWarnings("unchecked")
    public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        if (type == null) {
            throw new IllegalArgumentException("Extension type == null");
        }
        if (!type.isInterface()) {
            throw new IllegalArgumentException("Extension type (" + type + ") is not an interface!");
        }
        if (!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type (" + type +
                    ") is not an extension, because it is NOT annotated with @" + SPI.class.getSimpleName() + "!");
        }

        ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
        if (loader == null) {
            EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
            loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
        }
        return loader;
    }
}

createAdaptiveExtension

• 按照getAdaptiveExtension => createAdaptiveExtension => getAdaptiveExtensionClass => createAdaptiveExtensionClass流程创建AdaptiveExtensionClass。
• new AdaptiveClassCodeGenerator(type, cachedDefaultName).generate()负责生成Protocol$Adaptive的源码。

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 {
                        instance = createAdaptiveExtension();
                        cachedAdaptiveInstance.set(instance);
                    } catch (Throwable t) {
                        createAdaptiveInstanceError = t;
                        throw new IllegalStateException("Failed to create adaptive instance: " + t.toString(), t);
                    }
                }
            }
        }

        return (T) instance;
    }


    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();
    }

    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);
    }
}

AdaptiveClassCodeGenerator

说明：

• generate()方法是生成目标类的主干方法，其主要分为如下几个步骤：
• 生成package信息。
• 生成import信息，只导入了ExtensionLoader类，其余的类都通过全限定名的方式来使用。
• 生成类声明信息。
• 生成各个方法的实现。

public class AdaptiveClassCodeGenerator {

    public AdaptiveClassCodeGenerator(Class<?> type, String defaultExtName) {
        this.type = type;
        this.defaultExtName = defaultExtName;
    }

    public String generate() {
        // no need to generate adaptive class since there's no adaptive method found.
        if (!hasAdaptiveMethod()) {
            throw new IllegalStateException("No adaptive method exist on extension " + type.getName() + ", refuse to create the adaptive class!");
        }

        StringBuilder code = new StringBuilder();
        // 生成package信息
        code.append(generatePackageInfo());
        // 生成import信息，这里只导入了ExtensionLoader类，其余的类都通过全限定名的方式来使用
        code.append(generateImports());
        // 生成类声明信息
        code.append(generateClassDeclaration());
        // 为各个方法生成实现方法信息
        Method[] methods = type.getMethods();
        for (Method method : methods) {
            code.append(generateMethod(method));
        }
        code.append("}");

        if (logger.isDebugEnabled()) {
            logger.debug(code.toString());
        }
        return code.toString();
    }
}

Protocol$Adaptive

说明：

• Protocol本身通过SPI注解。
• 通过@Adaptive注解的方法在AdaptiveClassCodeGenerator的generate()重新生成源码。
• 没有通过@Adaptive注解的方法的方法体内会抛异常。
• 在Protocol$Adaptive的export和refer方法通过ExtensionLoader.getExtensionLoader().getExtension()动态获取扩展。
• 根据URL动态获取扩展名，String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol())。

package org.apache.dubbo.rpc;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.extension.Adaptive;
import org.apache.dubbo.common.extension.SPI;

@SPI("dubbo")
public interface Protocol {

    int getDefaultPort();

    @Adaptive
    <T> Exporter<T> export(Invoker<T> invoker) throws RpcException;

    @Adaptive
    <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException;

    void destroy();
}

package org.apache.dubbo.rpc;
import org.apache.dubbo.common.extension.ExtensionLoader;

public class Protocol$Adaptive implements org.apache.dubbo.rpc.Protocol {

    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!");
    }

    public org.apache.dubbo.rpc.Exporter export(org.apache.dubbo.rpc.Invoker arg0) 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.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);
    }
}