1. 要了解Dubbo是如何解析标签的，首先要清楚一点就是Spring如何处理自定义标签的，因为Dubbo的标签可以算是Spring自定义标签的一种情况；
2. Spring通过两个接口来解析自定义的标签：NamespaceHandler和BeanDefinitionParser接口；NamespaceHandler负责namespace的处理，而BeanDefinitionParser负责Bean的解析；
3. 我们自定义标签的时候，可以实现NamespaceHandler接口，但更多的是继承NamespaceHandler的抽象类：NamespaceHandlerSupport，然后在classpath的META-INF目录下编写一个spring.handlers文件，在该文件中定义namespace的URL和namespace处理类的映射。
比如Dubbo的spring.handlers文件内容(/是转义)：
http\://code.alibabatech.com/schema/dubbo=com.alibaba.dubbo.config.spring.schema.DubboNamespaceHandler
4. spring框架初始化时会加载所有classpath的spring.handlers文件，把namespace的URL和namespace处理类映射到Map中，Spring在解析的时候，遇到一些自定义的标签的时候，会在这个Map中查找namespace处理类，使用这个使用这个自定义的处理类来进行标签的解析工作；
5. 同样，Dubbo框架的namespace的处理类是DubboNamespaceHandler，也是实现了NamespaceHandlerSupport接口来处理命名空间，然后在这个类的初始化的时候给所有标签都注册了各自的解析器；而Dubbo的解析类DubboBeanDefinitionParser同样也是实现了BeanDefinitionParser接口；
DubboNamespaceHandler 源代码如此下：
/**
* DubboNamespaceHandler
*
* @author william.liangf
* @export
*/
public class DubboNamespaceHandler extends NamespaceHandlerSupport { static {
Version.checkDuplicate(DubboNamespaceHandler.class);
} public void init() {
registerBeanDefinitionParser("application", new DubboBeanDefinitionParser(ApplicationConfig.class, true));
registerBeanDefinitionParser("module", new DubboBeanDefinitionParser(ModuleConfig.class, true));
registerBeanDefinitionParser("registry", new DubboBeanDefinitionParser(RegistryConfig.class, true));
registerBeanDefinitionParser("monitor", new DubboBeanDefinitionParser(MonitorConfig.class, true));
registerBeanDefinitionParser("provider", new DubboBeanDefinitionParser(ProviderConfig.class, true));
registerBeanDefinitionParser("consumer", new DubboBeanDefinitionParser(ConsumerConfig.class, true));
registerBeanDefinitionParser("protocol", new DubboBeanDefinitionParser(ProtocolConfig.class, true));
registerBeanDefinitionParser("service", new DubboBeanDefinitionParser(ServiceBean.class, true));
registerBeanDefinitionParser("reference", new DubboBeanDefinitionParser(ReferenceBean.class, false));
registerBeanDefinitionParser("annotation", new DubboBeanDefinitionParser(AnnotationBean.class, true));
} }
可以看到，Dubbo有10个标签，每个标签会解析到对应的实体上，每个实体中的属性对应标签中的属性，比如ApplicationConfig类：
/**
* ApplicationConfig
*
* @author william.liangf
* @export
*/
public class ApplicationConfig extends AbstractConfig { private static final long serialVersionUID = 5508512956753757169L; // 应用名称
private String name; // 模块版本
private String version; // 应用负责人
private String owner; // 组织名(BU或部门)
private String organization; // 分层
private String architecture; // 环境，如：dev/test/run
private String environment; // Java代码编译器
private String compiler; // 日志输出方式
private String logger; // 注册中心
private List<RegistryConfig> registries; // 服务监控
private MonitorConfig monitor; // 是否为缺省
private Boolean isDefault; public ApplicationConfig() {
} public ApplicationConfig(String name) {
setName(name);
} @Parameter(key = Constants.APPLICATION_KEY, required = true)
public String getName() {
return name;
} public void setName(String name) {
checkName("name", name);
this.name = name;
if (id == null || id.length() == 0) {
id = name;
}
} @Parameter(key = "application.version")
public String getVersion() {
return version;
} public void setVersion(String version) {
this.version = version;
} public String getOwner() {
return owner;
} public void setOwner(String owner) {
checkMultiName("owner", owner);
this.owner = owner;
} public String getOrganization() {
return organization;
} public void setOrganization(String organization) {
checkName("organization", organization);
this.organization = organization;
} public String getArchitecture() {
return architecture;
} public void setArchitecture(String architecture) {
checkName("architecture", architecture);
this.architecture = architecture;
} public String getEnvironment() {
return environment;
} public void setEnvironment(String environment) {
checkName("environment", environment);
if(environment != null) {
if (! ("develop".equals(environment) || "test".equals(environment) || "product".equals(environment))) {
throw new IllegalStateException("Unsupported environment: " + environment + ", only support develop/test/product, default is product.");
}
}
this.environment = environment;
} public RegistryConfig getRegistry() {
return registries == null || registries.size() == 0 ? null : registries.get(0);
} public void setRegistry(RegistryConfig registry) {
List<RegistryConfig> registries = new ArrayList<RegistryConfig>(1);
registries.add(registry);
this.registries = registries;
} public List<RegistryConfig> getRegistries() {
return registries;
} @SuppressWarnings({ "unchecked" })
public void setRegistries(List<? extends RegistryConfig> registries) {
this.registries = (List<RegistryConfig>)registries;
} public MonitorConfig getMonitor() {
return monitor;
} public void setMonitor(MonitorConfig monitor) {
this.monitor = monitor;
} public void setMonitor(String monitor) {
this.monitor = new MonitorConfig(monitor);
} public String getCompiler() {
return compiler;
} public void setCompiler(String compiler) {
this.compiler = compiler;
AdaptiveCompiler.setDefaultCompiler(compiler);
} public String getLogger() {
return logger;
} public void setLogger(String logger) {
this.logger = logger;
LoggerFactory.setLoggerAdapter(logger);
} public Boolean isDefault() {
return isDefault;
} public void setDefault(Boolean isDefault) {
this.isDefault = isDefault;
} }
Dubbo版本：2.8.4.5