层层递进Struts1（四）之预加载ActionServlet

Struts的执行相当于分为两个阶段，预加载阶段和执行阶段，预加载阶段是指在Tomcat启动之时就开始执 行的内容，而此时我们并未真正进入跳转逻辑，这篇博客我们来分析一下预加载阶段。

配置文件

还记得web.xml中关于Struts的Servlet是如何配置的吗？

  <servlet-

name>action</servlet-name>
  <servlet-class>org.apache.struts.action.ActionServlet</servlet-class>
  <init-param>
    <param-name>config</param-name>
    <param-value>/WEB-INF/struts-config.xml</param-value>
  </init-param>
  <init-param>
    <param-name>debug</param-name>
    <param-value>2</param-value>
  </init-param>
  <init-param>
    <param-name>detail</param-name>
    <param-value>2</param-value>
  </init-param>
  <load-on-startup>2</load-on-startup>
</servlet>
<!-- Standard Action Servlet Mapping -->
<servlet-mapping>
  <servlet-name>action</servlet-name>
  <url-pattern>*.do</url-pattern>
</servlet-mapping>

其中<load-on-startup>2</load-on-startup>指的就是在 Tomcat启动之时即执行，实际只要数字大于0，意思就是Tomcat启动即执行，为了进入调试，我们把这里改为0 。

<load-on-startup>0</load-on-startup>  

主函数init()

经过调 试发现，当执行完此函数，Struts的预加载阶段即结束，所以我们主要来看这个函数即可。

/**
 * <p>Initialize this servlet.  Most of the processing has been factored into
 * support methods so that you can override particular functionality at a
 * fairly granular level.</p>
 *
 * @exception ServletException if we cannot configure ourselves correctly
 */
public void init() throws ServletException {  

    // Wraps the entire initialization in a try/catch to better handle
    // unexpected exceptions and errors to provide better feedback
    // to the developer
    try {
        initInternal();
        initOther();
        initServlet();  

        getServletContext().setAttribute(Globals.ACTION_SERVLET_KEY, this);
        initModuleConfigFactory();
        // Initialize modules as needed
        ModuleConfig moduleConfig = initModuleConfig("", config);
        initModuleMessageResources(moduleConfig);
        initModuleDataSources(moduleConfig);
        initModulePlugIns(moduleConfig);
        moduleConfig.freeze();  

        Enumeration names = getServletConfig().getInitParameterNames();
        while (names.hasMoreElements()) {
            String name = (String) names.nextElement();
            if (!name.startsWith("config/")) {
                continue;
            }
            String prefix = name.substring(6);
            moduleConfig = initModuleConfig
                (prefix, getServletConfig().getInitParameter(name));
            initModuleMessageResources(moduleConfig);
            initModuleDataSources(moduleConfig);
            initModulePlugIns(moduleConfig);
            moduleConfig.freeze();
        }  

        this.initModulePrefixes(this.getServletContext());  

        this.destroyConfigDigester();
    } catch (UnavailableException ex) {
        throw ex;
    } catch (Throwable t) {  

        // The follow error message is not retrieved from internal message
        // resources as they may not have been able to have been
        // initialized
        log.error("Unable to initialize Struts ActionServlet due to an "
            + "unexpected exception or error thrown, so marking the "
            + "servlet as unavailable.  Most likely, this is due to an "
            + "incorrect or missing library dependency.", t);
        throw new UnavailableException(t.getMessage());
    }
}

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