Dubbo源码分析之SPI(三) | ExtensionLoader

目录

一、概述

二、AdaptiveClassCodeGenerator

三、getExtension方法

四、getActivateExtension方法

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一、概述

上一篇文章介绍了Dubbo的SPI机制ExtensionLoader源码，分析了ExtensionLoader如何从META-INFO/dubbo等目录下
获取service provider，并了解了它的缓存机制，缓存class字节码，缓存实例化异常Exception,缓存AdaptiveExtension实体对象，
以及它的自动注入ExtensionFactory的实现原理。

https://blog.csdn.net/qq_33513250/article/details/102818893

本文接着介绍ExtensionLoader通过JavassistCompiler动态生成AdaptiveExtension代码逻辑，并通过Protocol的加载进行实例分析和学习Dubbo的自动包装Wapper功能。

二、AdaptiveClassCodeGenerator

createAdaptiveExtensionClass方法中使用AdaptiveClassCodeGenerator动态生成代码，下面是生成的Protocol$Adaptive代码。

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.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 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);
    }
}

其中destroy和getDefaultPort方法都是不支持的，会抛出异常UnsupportedOperationException。是因为接口上的Method，没有标注@Adaptive注解，只有标注@Adaptive注解的才会generateExtensionAssignment。

private String generateMethodContent(Method method) {
        Adaptive adaptiveAnnotation = method.getAnnotation(Adaptive.class);
        StringBuilder code = new StringBuilder(512);
        if (adaptiveAnnotation == null) {
            return generateUnsupported(method);
        } else {
            int urlTypeIndex = getUrlTypeIndex(method);
            
            // found parameter in URL type
            if (urlTypeIndex != -1) {
                // Null Point check
                code.append(generateUrlNullCheck(urlTypeIndex));
            } else {
                // did not find parameter in URL type
                code.append(generateUrlAssignmentIndirectly(method));
            }

            String[] value = getMethodAdaptiveValue(adaptiveAnnotation);

            boolean hasInvocation = hasInvocationArgument(method);
            
            code.append(generateInvocationArgumentNullCheck(method));
            
            code.append(generateExtNameAssignment(value, hasInvocation));
            // check extName == null?
            code.append(generateExtNameNullCheck(value));
            
            code.append(generateExtensionAssignment());

            // return statement
            code.append(generateReturnAndInvocation(method));
        }
        
        return code.toString();
    }

  refer和export方法在这里可以看到生成code的时候，会根据他们的协议去自动使用不同的协议处理器处理逻辑。

a)when the url is registry://224.5.6.7:1234/org.apache.dubbo.registry.RegistryService?application=dubbo-sample, then the protocol is RegistryProtocol

b)when the url is dubbo://224.5.6.7:1234/org.apache.dubbo.config.api.DemoService?application=dubbo-sample, thenthe protocol is DubboProtocol

c) Actually，when the {@link ExtensionLoader} init the {@link Protocol} instants,it will automatically wraps two layers, and eventually will get a ProtocolFilterWrapper or ProtocolListenerWrapper

 private String generateExtNameAssignment(String[] value, boolean hasInvocation) {
        // TODO: refactor it
        String getNameCode = null;
        for (int i = value.length - 1; i >= 0; --i) {
            if (i == value.length - 1) {
                if (null != defaultExtName) {
                    if (!"protocol".equals(value[i])) {
                        if (hasInvocation) {
                            getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                        } else {
                            getNameCode = String.format("url.getParameter(\"%s\", \"%s\")", value[i], defaultExtName);
                        }
                    } else {
                        getNameCode = String.format("( url.getProtocol() == null ? \"%s\" : url.getProtocol() )", defaultExtName);
                    }
                } else {
                    if (!"protocol".equals(value[i])) {
                        if (hasInvocation) {
                            getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                        } else {
                            getNameCode = String.format("url.getParameter(\"%s\")", value[i]);
                        }
                    } else {
                        getNameCode = "url.getProtocol()";
                    }
                }
            } else {
                if (!"protocol".equals(value[i])) {
                    if (hasInvocation) {
                        getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                    } else {
                        getNameCode = String.format("url.getParameter(\"%s\", %s)", value[i], getNameCode);
                    }
                } else {
                    getNameCode = String.format("url.getProtocol() == null ? (%s) : url.getProtocol()", getNameCode);
                }
            }
        }
        
        return String.format(CODE_EXT_NAME_ASSIGNMENT, getNameCode);
    }

三、getExtension方法

 ExtensionLoader还提供了通过name获取service provider的getExtension方法，为了保证线程安全采用了Holder对象持有name的实体类，具体创建逻辑在createExtension中。

 public T getExtension(String name) {
        if (StringUtils.isEmpty(name)) {
            throw new IllegalArgumentException("Extension name == null");
        }
        if ("true".equals(name)) {
            return getDefaultExtension();
        }
        final Holder