Struts struts-config.xml的解析

Struts使用了digester进行自身配置文件的解析。
在ActionServlet类中，由下列方法完成。

由configDigester.addRuleSet(new ConfigRuleSet());

添加一个degister解析规则

    protected Digester initConfigDigester()
        throws ServletException {
        // :FIXME: Where can ServletException be thrown?
        // Do we have an existing instance?
        if (configDigester != null) {
            return (configDigester);
        }

        // Create a new Digester instance with standard capabilities
        configDigester = new Digester();
        configDigester.setNamespaceAware(true);
        configDigester.setValidating(this.isValidating());
        configDigester.setUseContextClassLoader(true);
        configDigester.addRuleSet(new ConfigRuleSet());

        for (int i = 0; i < registrations.length; i += 2) {
            URL url = this.getClass().getResource(registrations[i + 1]);

            if (url != null) {
                configDigester.register(registrations[i], url.toString());
            }
        }

        this.addRuleSets();

        // Return the completely configured Digester instance
        return (configDigester);
    }

ConfigRuleSet类中描述了如何对struts-config文件的节点进行处理。
ActionServlet类中push进digester的是ModuleConfigImpl。

规则中的addSetNext方法，将创建的各个对象调用ModuleConfigImpl的相应方法
加入到了ModuleConfigImpl中。

        digester.addRule("struts-config/action-mappings",
            new SetActionMappingClassRule());

        digester.addFactoryCreate("struts-config/action-mappings/action",
            new ActionMappingFactory(cl));
        digester.addSetProperties("struts-config/action-mappings/action");
        digester.addSetNext("struts-config/action-mappings/action",
            "addActionConfig", "org.apache.struts.config.ActionConfig");

        digester.addRule("struts-config/action-mappings/action/set-property",
            new BaseConfigSetPropertyRule());

        digester.addObjectCreate("struts-config/action-mappings/action/exception",
            "org.apache.struts.config.ExceptionConfig", "className");
        digester.addSetProperties(
            "struts-config/action-mappings/action/exception");
        digester.addSetNext("struts-config/action-mappings/action/exception",
            "addExceptionConfig", "org.apache.struts.config.ExceptionConfig");

        digester.addRule("struts-config/action-mappings/action/exception/set-property",
            new BaseConfigSetPropertyRule());

        digester.addFactoryCreate("struts-config/action-mappings/action/forward",
            new ActionForwardFactory(cl));
        digester.addSetProperties(
            "struts-config/action-mappings/action/forward");
        digester.addSetNext("struts-config/action-mappings/action/forward",
            "addForwardConfig", "org.apache.struts.config.ForwardConfig");

        digester.addRule("struts-config/action-mappings/action/forward/set-property",
            new BaseConfigSetPropertyRule());

        digester.addObjectCreate("struts-config/controller",
            "org.apache.struts.config.ControllerConfig", "className");
        digester.addSetProperties("struts-config/controller");
        digester.addSetNext("struts-config/controller", "setControllerConfig",
            "org.apache.struts.config.ControllerConfig");

        digester.addRule("struts-config/controller/set-property",
            new BaseConfigSetPropertyRule());

        digester.addRule("struts-config/form-beans",
            new SetActionFormBeanClassRule());

        digester.addFactoryCreate("struts-config/form-beans/form-bean",
            new ActionFormBeanFactory(cl));
        digester.addSetProperties("struts-config/form-beans/form-bean");
        digester.addSetNext("struts-config/form-beans/form-bean",
            "addFormBeanConfig", "org.apache.struts.config.FormBeanConfig");

        digester.addObjectCreate("struts-config/form-beans/form-bean/form-property",
            "org.apache.struts.config.FormPropertyConfig", "className");
        digester.addSetProperties(
            "struts-config/form-beans/form-bean/form-property");
        digester.addSetNext("struts-config/form-beans/form-bean/form-property",
            "addFormPropertyConfig",
            "org.apache.struts.config.FormPropertyConfig");

        digester.addRule("struts-config/form-beans/form-bean/form-property/set-property",
            new BaseConfigSetPropertyRule());

        digester.addRule("struts-config/form-beans/form-bean/set-property",
            new BaseConfigSetPropertyRule());

        digester.addObjectCreate("struts-config/global-exceptions/exception",
            "org.apache.struts.config.ExceptionConfig", "className");
        digester.addSetProperties("struts-config/global-exceptions/exception");
        digester.addSetNext("struts-config/global-exceptions/exception",
            "addExceptionConfig", "org.apache.struts.config.ExceptionConfig");

        digester.addRule("struts-config/global-exceptions/exception/set-property",
            new BaseConfigSetPropertyRule());

        digester.addRule("struts-config/global-forwards",
            new SetActionForwardClassRule());

        digester.addFactoryCreate("struts-config/global-forwards/forward",
            new GlobalForwardFactory(cl));
        digester.addSetProperties("struts-config/global-forwards/forward");
        digester.addSetNext("struts-config/global-forwards/forward",
            "addForwardConfig", "org.apache.struts.config.ForwardConfig");

        digester.addRule("struts-config/global-forwards/forward/set-property",
            new BaseConfigSetPropertyRule());

        digester.addObjectCreate("struts-config/message-resources",
            "org.apache.struts.config.MessageResourcesConfig", "className");
        digester.addSetProperties("struts-config/message-resources");
        digester.addSetNext("struts-config/message-resources",
            "addMessageResourcesConfig",
            "org.apache.struts.config.MessageResourcesConfig");

        digester.addRule("struts-config/message-resources/set-property",
            new BaseConfigSetPropertyRule());

        digester.addObjectCreate("struts-config/plug-in",
            "org.apache.struts.config.PlugInConfig");
        digester.addSetProperties("struts-config/plug-in");
        digester.addSetNext("struts-config/plug-in", "addPlugInConfig",
            "org.apache.struts.config.PlugInConfig");

        digester.addRule("struts-config/plug-in/set-property",
            new PlugInSetPropertyRule());