Dubbo源码分析（二）dubbo SPI扩展点加载之扩展点自适应

上一篇介绍了dubbo SPI 扩展点的四大特性中的自动包装，接下来再介绍其他的特性。

扩展点自适应

还是先看官方文档http://dubbo.apache.org/zh-cn/docs/dev/SPI.html。可以看到

1. ExtensionLoader 注入的依赖扩展点是一个 Adaptive 实例，直到扩展点方法执行时才决定调用是一个扩展点实现
2. Dubbo 使用 URL 对象（包含了Key-Value）传递配置信息。扩展点方法调用会有URL参数（或是参数有URL成员）这样依赖的扩展点也可以从URL拿到配置信息，所有的扩展点自己定好配置的Key后，配置信息从URL上从最外层传入。URL在配置传递上即是一条总线。

通俗的讲就是，dubbo SPI可以在方法执行时动态选择扩展点实现类，原理就是通过方法中的url参数拿到配置信息，再根据拿到的配置信息选择扩展点实现类。dubbo在实现过程中通过javassist动态编译java类，实现了这个功能。接下来还是以Protocol接口为例说明。

@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();

}

其中export和refer两个方法有@Adaptive注解，表明这两个方法执行的时候会动态选择Protocol扩展实现类。
接下来我们看看dubbo源码中是怎样使用的。

我们可以看到dubbo在SeviceConfig中使用了getAdaptiveExtension()方法，接下来我们看看他是怎么实现的

  private Class<?> createAdaptiveExtensionClass() {
  		//  动态拼接类的字符串
        String code = createAdaptiveExtensionClassCode();
        // 拿到classloader
        ClassLoader classLoader = findClassLoader();
        // 拿到动态编译类
        org.apache.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(org.apache.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
        // 根据第一步拿到的字符串动态编译成java类，这里的原理时使用了javassist动态编译
        return compiler.compile(code, classLoader);
    }

接下来在看看他的动态类代码时如何生成的

	// 这方法用字符串拼接除了一个java类的代码
  private String createAdaptiveExtensionClassCode() {
        StringBuilder codeBuilder = new StringBuilder();
        Method[] methods = type.getMethods();
        boolean hasAdaptiveAnnotation = false;
        // 是否有Adaptive注解
        for (Method m : methods) {
            if (m.isAnnotationPresent(Adaptive.class)) {
                hasAdaptiveAnnotation = true;
                break;
            }
        }
        // no need to generate adaptive class since there's no adaptive method found.
        if (!hasAdaptiveAnnotation) {
            throw new IllegalStateException("No adaptive method on extension " + type.getName() + ", refuse to create the adaptive class!");
        }

		// package很熟悉，这个类所处的报名首先要拼接
        codeBuilder.append("package ").append(type.getPackage().getName()).append(";");
        // 然后import 其他的类
        codeBuilder.append("\nimport ").append(ExtensionLoader.class.getName()).append(";");
        // 动态代理的类名称为接口名+$Adaptive
        codeBuilder.append("\npublic class ").append(type.getSimpleName()).append("$Adaptive").append(" implements ").append(type.getCanonicalName()).append(" {");

		// 定义logger日志对象
        codeBuilder.append("\nprivate static final org.apache.dubbo.common.logger.Logger logger = org.apache.dubbo.common.logger.LoggerFactory.getLogger(ExtensionLoader.class);");
        codeBuilder.append("\nprivate java.util.concurrent.atomic.AtomicInteger count = new java.util.concurrent.atomic.AtomicInteger(0);\n");

	// 编译每一个接口的方法，动态生成方法的代码
        for (Method method : methods) {
            Class<?> rt = method.getReturnType();
            Class<?>[] pts = method.getParameterTypes();
            Class<?>[] ets = method.getExceptionTypes();

            Adaptive adaptiveAnnotation = method.getAnnotation(Adaptive.class);
            StringBuilder code = new StringBuilder(512);
            // 如果方法上没有Adaptive注解，表明这个方法不支持动态选择，则生成的动态类，此方法要抛出异常
            if (adaptiveAnnotation == null) {
                code.append("throw new UnsupportedOperationException(\"method ")
                        .append(method.toString()).append(" of interface ")
                        .append(type.getName()).append(" is not adaptive method!\");");
            } else {
                int urlTypeIndex = -1;
                // 找到方法中的URL参数 
                for (int i = 0; i < pts.length; ++i) {
                    if (pts[i].equals(URL.class)) {
                        urlTypeIndex = i;
                        break;
                    }
                }
                // 如果方法中有url参数，则赋值给局部变量
                if (urlTypeIndex != -1) {
                    // Null Point check
                    String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"url == null\");",
                            urlTypeIndex);
                    code.append(s);

                    s = String.format("\n%s url = arg%d;", URL.class.getName(), urlTypeIndex);
                    code.append(s);
                }
                // 如果方法中没有Url参数，则判断方法中的参数是否有get方法可以获得url对象
                else {
                    String attribMethod = null;

                    // find URL getter method
                    LBL_PTS:
                    for (int i = 0; i < pts.length; ++i) {
                        Method[] ms = pts[i].getMethods();
                        for (Method m : ms) {
                            String name = m.getName();
                            if ((name.startsWith("get") || name.length() > 3)
                                    && Modifier.isPublic(m.getModifiers())
                                    && !Modifier.isStatic(m.getModifiers())
                                    && m.getParameterTypes().length == 0
                                    && m.getReturnType() == URL.class) {
                                urlTypeIndex = i;
                                attribMethod = name;
                                break LBL_PTS;
                            }
                        }
                    }
                    // 如果实在拿不到url对象，则抛出异常
                    if (attribMethod == null) {
                        throw new IllegalStateException("fail to create adaptive class for interface " + type.getName()
                                + ": not found url parameter or url attribute in parameters of method " + method.getName());
                    }

                    // 如果从参数的get方法中可以找到url对象，则将url赋值给局部变量
                    String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"%s argument == null\");",
                            urlTypeIndex, pts[urlTypeIndex].getName());
                    code.append(s);
                    s = String.format("\nif (arg%d.%s() == null) throw new IllegalArgumentException(\"%s argument %s() == null\");",
                            urlTypeIndex, attribMethod, pts[urlTypeIndex].getName(), attribMethod);
                    code.append(s);

                    s = String.format("%s url = arg%d.%s();", URL.class.getName(), urlTypeIndex, attribMethod);
                    code.append(s);
                }
				// 获取方法Adaptive注解的值
                String[] value = adaptiveAnnotation.value();
                // value is not set, use the value generated from class name as the key
                if (value.length == 0) {
                    String splitName = StringUtils.camelToSplitName(type.getSimpleName(), ".");
                    value = new String[]{splitName};
                }

				
                boolean hasInvocation = false;
                // 方法的参数中是否有Invocation对象
                for (int i = 0; i < pts.length; ++i) {
                    if (("org.apache.dubbo.rpc.Invocation").equals(pts[i].getName())) {
                        // Null Point check
                        String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"invocation == null\");", i);
                        code.append(s);
                        s = String.format("\nString methodName = arg%d.getMethodName();", i);
                        code.append(s);
                        hasInvocation = true;
                        break;
                    }
                }

                String defaultExtName = cachedDefaultName;
                String getNameCode = null;
                // 接下来是根据注解的参数和方法中是否有Invocation类来从URL中获取参数
                // 主要用了url的三个获取参数的方法，url.getMethodParameter,url.getParameter,url.getProtocol
                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);
                        }
                    }
                }
                code.append("\nString extName = ").append(getNameCode).append(";");
                // check extName == null?
                String s = String.format("\nif(extName == null) " +
                                "throw new IllegalStateException(\"Fail to get extension(%s) name from url(\" + url.toString() + \") use keys(%s)\");",
                        type.getName(), Arrays.toString(value));
                code.append(s);
			// 从上面我们拿到了url中的变量，这个时候我们就要用ExtensionLoader.getExtensionLoader(%s.class)
			// .getExtension(extName)获取到扩展点实例
                code.append(String.format("\n%s extension = null;\n try {\nextension = (%,
                        type.getName(), ExtensionLoader.class.getSimpleName(), type.getName()));
                code.append(String.format("if (count.incrementAndGet() == 1) {\nlogger.warn(\"Failed to find extension named \" + extName + \" for type %s, will use default extension %s instead.\", e);\n}\n",
                        type.getName(), defaultExtName));
                code.append(String.format("extension = (%s)%s.getExtensionLoader(%s.class).getExtension(\"%s\");\n}",
                        type.getName(), ExtensionLoader.class.getSimpleName(), type.getName(), defaultExtName));

                // return statement
                if (!rt.equals(void.class)) {
                    code.append("\nreturn ");
                }

				// 调用扩展点实例的对应的方法
                s = String.format("extension.%s(", method.getName());
                code.append(s);
                for (int i = 0; i < pts.length; i++) {
                    if (i != 0) {
                        code.append(", ");
                    }
                    code.append("arg").append(i);
                }
                code.append(");");
            }

            codeBuilder.append("\npublic ").append(rt.getCanonicalName()).append(" ").append(method.getName()).append("(");
            for (int i = 0; i < pts.length; i++) {
                if (i > 0) {
                    codeBuilder.append(", ");
                }
                codeBuilder.append(pts[i].getCanonicalName());
                codeBuilder.append(" ");
                codeBuilder.append("arg").append(i);
            }
            codeBuilder.append(")");
            if (ets.length > 0) {
                codeBuilder.append(" throws ");
                for (int i = 0; i < ets.length; i++) {
                    if (i > 0) {
                        codeBuilder.append(", ");
                    }
                    codeBuilder.append(ets[i].getCanonicalName());
                }
            }
            codeBuilder.append(" {");
            codeBuilder.append(code.toString());
            codeBuilder.append("\n}");
        }
        codeBuilder.append("\n}");
        if (logger.isDebugEnabled()) {
            logger.debug(codeBuilder.toString());
        }
        return codeBuilder.toString();
    }

上面一堆代码可能看起来很复杂，接下我们可以看一下Protocol最后的动态类的代码是什么就很好理解了。

import com.alibaba.dubbo.common.extension.ExtensionLoader;

public class Protocol$Adaptive implements com.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.RpcException {
        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(com.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 com.alibaba.dubbo.rpc.RpcException {
        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 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(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
        return extension.refer(arg0, arg1);
    }
}

我们再来看一下另一个接口

@SPI("javassist")
public interface ProxyFactory {

    /**
     * create proxy.
     *
     * @param invoker
     * @return proxy
     */
    @Adaptive({Constants.PROXY_KEY})
    <T> T getProxy(Invoker<T> invoker) throws RpcException;

    /**
     * create invoker.
     *
     * @param 
     * @param proxy
     * @param type
     * @param url
     * @return invoker
     */
    @Adaptive({Constants.PROXY_KEY})
    <T> Invoker<T> getInvoker(T proxy, Class<T> type, URL url) throws RpcException;

}

生成的动态扩展类

import com.alibaba.dubbo.common.extension.ExtensionLoader;

public class ProxyFactory$Adaptive implements com.alibaba.dubbo.rpc.ProxyFactory {
    public java.lang.Object getProxy(com.alibaba.dubbo.rpc.Invoker arg0) throws com.alibaba.dubbo.rpc.RpcException {
        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.getParameter("proxy", "javassist");
        if (extName == null)
            throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.ProxyFactory) name from url(" + url.toString() + ") use keys([proxy])");
        com.alibaba.dubbo.rpc.ProxyFactory extension = (com.alibaba.dubbo.rpc.ProxyFactory) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.ProxyFactory.class).getExtension(extName);
        return extension.getProxy(arg0);
    }

    public com.alibaba.dubbo.rpc.Invoker getInvoker(java.lang.Object arg0, java.lang.Class arg1, com.alibaba.dubbo.common.URL arg2) throws com.alibaba.dubbo.rpc.RpcException {
        if (arg2 == null) throw new IllegalArgumentException("url == null");
        com.alibaba.dubbo.common.URL url = arg2;
        String extName = url.getParameter("proxy", "javassist");
        if (extName == null)
            throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.ProxyFactory) name from url(" + url.toString() + ") use keys([proxy])");
        com.alibaba.dubbo.rpc.ProxyFactory extension = (com.alibaba.dubbo.rpc.ProxyFactory) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.ProxyFactory.class).getExtension(extName);
        return extension.getInvoker(arg0, arg1, arg2);
    }
}