【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> 
例如：
private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
首先分析getExtensionLoader方法：很简单先查询EXTENSION_LOADERS是否存在不存在就new一个返回
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 interface!");
}
if (!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
}
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;
}再分析getAdaptiveExtension方法(获取一个扩展类)
public T getAdaptiveExtension() {
//从缓存查
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
if (createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
//查不到就创建一个
instance = createAdaptiveExtension();
cachedAdaptiveInstance.set(instance);
} catch (Throwable t) {
createAdaptiveInstanceError = t;
throw new IllegalStateException("fail to create adaptive instance: " + t.toString(), t);
}
}
}
} else {
throw new IllegalStateException("fail to create adaptive instance: " + createAdaptiveInstanceError.toString(), createAdaptiveInstanceError);
}
}
return (T) instance;
}private T createAdaptiveExtension() {
try {
//首先分析getAdaptiveExtensionClass
//其次再分析injectExtension
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e);
}
}private Class<?> getAdaptiveExtensionClass() {
//获取xtensionClasses
getExtensionClasses();
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}private Map<String, Class<?>> getExtensionClasses() {
//还是先从缓存取
Map<String, Class<?>> classes = cachedClasses.get();
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
//取不到就加载一遍
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}// synchronized in getExtensionClasses
//加载配置项 主要在这几个路径下
//META-INF/dubbo/internal/com.alibaba.dubbo.rpc.Protocol
//META-INF/dubbo/com.alibaba.dubbo.rpc.Protocol
//META-INF/services/ com.alibaba.dubbo.rpc.Protocol
private Map<String, Class<?>> loadExtensionClasses() {
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
if (defaultAnnotation != null) {
String value = defaultAnnotation.value();
if ((value = value.trim()).length() > 0) {
String[] names = NAME_SEPARATOR.split(value);
if (names.length > 1) {
throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()
+ ": " + Arrays.toString(names));
}
if (names.length == 1) {
cachedDefaultName = names[0];
}
}
}
Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
//加载配置文件 并返回
loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName());
loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName());
loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName());
loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
return extensionClasses;
}//创建这个类
private Class<?> createAdaptiveExtensionClass() {
String code = createAdaptiveExtensionClassCode();
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);
}//注入属性
private T injectExtension(T instance) {
try {
if (objectFactory != null) {
//遍历方法注入属性
for (Method method : instance.getClass().getMethods()) {
if (method.getName().startsWith("set")
&& method.getParameterTypes().length == 1
&& Modifier.isPublic(method.getModifiers())) {
/**
* Check {@link DisableInject} to see if we need auto injection for this property
*/
if (method.getAnnotation(DisableInject.class) != null) {
continue;
}
Class<?> pt = method.getParameterTypes()[0];
try {
String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
//这里是一种ioc实现
//从objectFactory中获取对象 注入到instance中
Object object = objectFactory.getExtension(pt, property);
if (object != null) {
method.invoke(instance, object);
}
} catch (Exception e) {
logger.error("fail to inject via method " + method.getName()
+ " of interface " + type.getName() + ": " + e.getMessage(), e);
}
}
}
}
} catch (Exception e) {
logger.error(e.getMessage(), e);
}
return instance;
}最终获得类如下
package com.alibaba.dubbo.rpc;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
/**
* 协议自适应类
*/
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!");
}
/**
* 服务引用
*/
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])");
// 根据extName找到具体适应类，然后调用方法
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);
}
/**
* 服务暴露
*/
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])");
// 根据extName找到具体适应类，然后调用方法
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);
}
}
获取适应类过程结束----------------------------------------------------------------------------------
上述过程中存在了ioc过程 在此分析下
ExtensionLoader初始化代码如下
private ExtensionLoader(Class<?> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}这里分析下objectFactory
若项目中配置文件 /META-INF/dubbo/internal/org.apache.dubbo.common.extension.ExtensionFactory配置的是
spring=org.apache.dubbo.config.spring.extension.SpringExtensionFactory
则上述获取属性注入的代码即如下
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.dubbo.config.spring.extension;
import org.apache.dubbo.common.extension.ExtensionFactory;
import org.apache.dubbo.common.extension.SPI;
import org.apache.dubbo.common.logger.Logger;
import org.apache.dubbo.common.logger.LoggerFactory;
import org.apache.dubbo.common.utils.ConcurrentHashSet;
import org.apache.dubbo.config.DubboShutdownHook;
import org.apache.dubbo.config.spring.util.BeanFactoryUtils;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ContextClosedEvent;
import java.util.Set;
/**
* SpringExtensionFactory
*/
public class SpringExtensionFactory implements ExtensionFactory {
private static final Logger logger = LoggerFactory.getLogger(SpringExtensionFactory.class);
private static final Set<ApplicationContext> contexts = new ConcurrentHashSet<ApplicationContext>();
private static final ApplicationListener shutdownHookListener = new ShutdownHookListener();
public static void addApplicationContext(ApplicationContext context) {
contexts.add(context);
BeanFactoryUtils.addApplicationListener(context, shutdownHookListener);
}
public static void removeApplicationContext(ApplicationContext context) {
contexts.remove(context);
}
public static Set<ApplicationContext> getContexts() {
return contexts;
}
// currently for test purpose
public static void clearContexts() {
contexts.clear();
}
@Override
@SuppressWarnings("unchecked")
public <T> T getExtension(Class<T> type, String name) {
//SPI should be get from SpiExtensionFactory
if (type.isInterface() && type.isAnnotationPresent(SPI.class)) {
return null;
}
for (ApplicationContext context : contexts) {
if (context.containsBean(name)) {
Object bean = context.getBean(name);
if (type.isInstance(bean)) {
return (T) bean;
}
}
}
logger.warn("No spring extension (bean) named:" + name + ", try to find an extension (bean) of type " + type.getName());
if (Object.class == type) {
return null;
}
for (ApplicationContext context : contexts) {
try {
return context.getBean(type);
} catch (NoUniqueBeanDefinitionException multiBeanExe) {
logger.warn("Find more than 1 spring extensions (beans) of type " + type.getName() + ", will stop auto injection. Please make sure you have specified the concrete parameter type and there's only one extension of that type.");
} catch (NoSuchBeanDefinitionException noBeanExe) {
if (logger.isDebugEnabled()) {
logger.debug("Error when get spring extension(bean) for type:" + type.getName(), noBeanExe);
}
}
}
logger.warn("No spring extension (bean) named:" + name + ", type:" + type.getName() + " found, stop get bean.");
return null;
}
private static class ShutdownHookListener implements ApplicationListener {
@Override
public void onApplicationEvent(ApplicationEvent event) {
if (event instanceof ContextClosedEvent) {
// we call it anyway since dubbo shutdown hook make sure its destroyAll() is re-entrant.
// pls. note we should not remove dubbo shutdown hook when spring framework is present, this is because
// its shutdown hook may not be installed.
DubboShutdownHook shutdownHook = DubboShutdownHook.getDubboShutdownHook();
shutdownHook.destroyAll();
}
}
}
}
即直接从spring容器中获取bean注入到属性中。
通过获取到的协议自适应类Protocol$Adaptive我们发现了getExtension方法，这就是我们分析的核心方法，根据name获取不同的实现类
public T getExtension(String name) {
if (name == null || name.length() == 0) {
throw new IllegalArgumentException("Extension name == null");
}
if ("true".equals(name)) {
return getDefaultExtension();
}
//从缓存查
Holder<Object> holder = cachedInstances.get(name);
if (holder == null) {
cachedInstances.putIfAbsent(name, new Holder<Object>());
holder = cachedInstances.get(name);
}
Object instance = holder.get();
if (instance == null) {
synchronized (holder) {
instance = holder.get();
if (instance == null) {
//创建一个Extension类
instance = createExtension(name);
holder.set(instance);
}
}
}
return (T) instance;
}private T createExtension(String name) {
//获取class类 是在加载配置文件的时候用反射生成的保存在缓存中
//key:name value:class
Class<?> clazz = getExtensionClasses().get(name);
if (clazz == null) {
throw findException(name);
}
try {
//获取实例
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
//newInstance 方式获取
EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
//ioc注入属性
injectExtension(instance);
//下面这个即为aop的实现 循环包装 从而实现一层层调用
Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (wrapperClasses != null && !wrapperClasses.isEmpty()) {
for (Class<?> wrapperClass : wrapperClasses) {
//把实例装配到wrapper类中 包装实现aop的功能
//向包装类注入属性
instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
}
}
return instance;
} catch (Throwable t) {
throw new IllegalStateException("Extension instance(name: " + name + ", class: " +
type + ") could not be instantiated: " + t.getMessage(), t);
}
}ioc模块上面已经分析过，这里关注下aop的实现。
其中cachedWrapperClasses的获取是在加载配置文件时生成的
org.apache.dubbo.common.extension.ExtensionLoader#loadDirectory
-->org.apache.dubbo.common.extension.ExtensionLoader#loadResource
-->org.apache.dubbo.common.extension.ExtensionLoader#loadClass
private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, Class<?> clazz, String name) throws NoSuchMethodException {
if (!type.isAssignableFrom(clazz)) {
throw new IllegalStateException("Error when load extension class(interface: " +
type + ", class line: " + clazz.getName() + "), class "
+ clazz.getName() + "is not subtype of interface.");
}
if (clazz.isAnnotationPresent(Adaptive.class)) {
if (cachedAdaptiveClass == null) {
cachedAdaptiveClass = clazz;
} else if (!cachedAdaptiveClass.equals(clazz)) {
throw new IllegalStateException("More than 1 adaptive class found: "
+ cachedAdaptiveClass.getClass().getName()
+ ", " + clazz.getClass().getName());
}
} else if (isWrapperClass(clazz)) {
//这里初始化了wrappers
Set<Class<?>> wrappers = cachedWrapperClasses;
if (wrappers == null) {
cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
wrappers = cachedWrapperClasses;
}
wrappers.add(clazz);
} else {
clazz.getConstructor();
if (name == null || name.length() == 0) {
name = findAnnotationName(clazz);
if (name.length() == 0) {
throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + resourceURL);
}
}
String[] names = NAME_SEPARATOR.split(name);
if (names != null && names.length > 0) {
Activate activate = clazz.getAnnotation(Activate.class);
if (activate != null) {
cachedActivates.put(names[0], activate);
} else {
// support com.alibaba.dubbo.common.extension.Activate
com.alibaba.dubbo.common.extension.Activate oldActivate = clazz.getAnnotation(com.alibaba.dubbo.common.extension.Activate.class);
if (oldActivate != null) {
cachedActivates.put(names[0], oldActivate);
}
}
for (String n : names) {
if (!cachedNames.containsKey(clazz)) {
cachedNames.put(clazz, n);
}
Class<?> c = extensionClasses.get(n);
if (c == null) {
extensionClasses.put(n, clazz);
} else if (c != clazz) {
throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
}
}
}
}
}
示例：
通过上述过程得到的Protocol$Adaptive。若程序中调用了export方法其实调用的org.apache.dubbo.rpc.protocol.dubbo.DubboProtocol#export方法。
来源：oschina
链接：https://my.oschina.net/u/3667353/blog/3148992