String applicationConfig = "consumer-applicationContext.xml";
ApplicationContext context = new ClassPathXmlApplicationContext(applicationConfig);
UserInfoService userInfoService = (UserInfoService) context.getBean("userInfoService");
//负载均衡的处理是一个典型的懒加载模式，只有在第一次调用接口时，才进行负载均衡的处理。
System.out.println(userInfoService.sayHello("zhangsan"));

1.在进入sayHello的调用时，堆栈信息和帧代码如下


其中Constants.LOADBALANCE_KEY的值是“loadbalance”，而Constants.DEFAULT_LOADBALANCE的值是“random”，每个接口（reference）均可以灵活配置一个均衡方式，默认不配置的情况下都是random的。

不配置loadbalance时，是random的模式。

2.假如当前的loadbalance设置了leastactive时，它是如何把名字和类LeastActiveLoadBalance的实例关联起来呢？
3.首先它在com.alibaba.dubbo.rpc.cluster.loadbalance目录下实现了4种均衡方式，如下：

ConsistentHashLoadBalance:一致哈希
LeastActiveLoadBalance：最久不使用
RandomLoadBalance：随机
RoundRobinLoadBalance：顺序循环

注：上1点中的loadbalance返回的则是上述的其中一个均衡策略类实例。
4.其它dubbo实现了一套类似spi的服务加载机制如下：

private void loadFile(Map> extensionClasses, String dir) {
        String fileName = dir + type.getName();
        try {
            Enumeration urls;
            ClassLoader classLoader = findClassLoader();
            if (classLoader != null) {
                urls = classLoader.getResources(fileName);
            } else {
                urls = ClassLoader.getSystemResources(fileName);
            }
            if (urls != null) {
                while (urls.hasMoreElements()) {
                    java.net.URL url = urls.nextElement();
                    try {
                        BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
                        try {
                            String line = null;
                            while ((line = reader.readLine()) != null) {
                                final int ci = line.indexOf('#');
                                if (ci >= 0) line = line.substring(0, ci);
                                line = line.trim();
                                if (line.length() > 0) {
                                    try {
                                        String name = null;
                                        int i = line.indexOf('=');
                                        if (i > 0) {
                                            name = line.substring(0, i).trim();
                                            line = line.substring(i + 1).trim();
                                        }
                                        if (line.length() > 0) {
                                            Class clazz = Class.forName(line, true, classLoader);
                                            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 {
                                                try {
                                                    clazz.getConstructor(type);
                                                    Set> wrappers = cachedWrapperClasses;
                                                    if (wrappers == null) {
                                                        cachedWrapperClasses = new ConcurrentHashSet>();
                                                        wrappers = cachedWrapperClasses;
                                                    }
                                                    wrappers.add(clazz);
                                                } catch (NoSuchMethodException e) {
                                                    clazz.getConstructor();
                                                    if (name == null || name.length() == 0) {
                                                        name = findAnnotationName(clazz);
                                                        if (name == null || name.length() == 0) {
                                                            if (clazz.getSimpleName().length() > type.getSimpleName().length()
                                                                    && clazz.getSimpleName().endsWith(type.getSimpleName())) {
                                                                name = clazz.getSimpleName().substring(0, clazz.getSimpleName().length() - type.getSimpleName().length()).toLowerCase();
                                                            } else {
                                                                throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + url);
                                                            }
                                                        }
                                                    }
                                                    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);
                                                        }
                                                        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());
                                                            }
                                                        }
                                                    }
                                                }
                                            }
                                        }
                                    } catch (Throwable t) {
                                        IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + url + ", cause: " + t.getMessage(), t);
                                        exceptions.put(line, e);
                                    }
                                }
                            } // end of while read lines
                        } finally {
                            reader.close();
                        }
                    } catch (Throwable t) {
                        logger.error("Exception when load extension class(interface: " +
                                            type + ", class file: " + url + ") in " + url, t);
                    }
                } // end of while urls
            }
        } catch (Throwable t) {
            logger.error("Exception when load extension class(interface: " +
                    type + ", description file: " + fileName + ").", t);
        }
    }

5.每个均衡策略类均实现了select接口，通过该接口从多个可用的后端服务中返回其中一个服务。

 Invoker select(List> invokers, URL url, Invocation invocation) throws RpcException;