Struts2中EL表达式的取值范围问题

Struts2中EL表达式的取值范围问题
     在Struts2中对request进行了装饰,增强了getAttribute()方法,改变了EL该方法的查找范围,具体为,查找request域,不存在,查找值栈,不存在,查找ContextMap,还是不存在,则返回null。
     因为${对象名},会使用findAttribute的查找,其顺序是page域->request域->session域->application域原理如下:
public class PageContext{

    //在page域中根据name获取value
    public Object getAttribute(String name){
        return "找到了返回对象" | "没找到返回null";
    }

    //从四个域中逐个搜索,只要在其中一个找到,就不再继续寻找
    public Object findAttribute(String name){
        Object value = null;
        //搜page域
        value = this.getAttribute(name);
        if(value != null){
            return value;
        }
        //搜request域
        value = this.getRequest().getAttribute(name);
        if(value != null){
            return value;
        }
        //搜session域
        value = this.getSession().getAttribute(name);
        if(value != null){
            return value;
        }
        //搜application域
        value = this.getServletContext().getAttribute(name);
        if(value != null){
            return value;
        }
        return value;
    }
}
 

PS:由上述原因,Struts2对request进行了装饰,那么访问顺序变为page域->request域->值栈->ContextMap->session->application。

如果还想深究源码,可以参考ServletConfigInterceptor拦截器
public class ServletConfigInterceptor extends AbstractInterceptor implements StrutsStatics {
    private static final long serialVersionUID = 605261777858676638L;
    public String intercept(ActionInvocation invocation) throws Exception {
        final Object action = invocation.getAction();
        final ActionContext context = invocation.getInvocationContext();

        if (action instanceof ServletRequestAware) {
            HttpServletRequest request = (HttpServletRequest) context.get(HTTP_REQUEST);
            ((ServletRequestAware) action).setServletRequest(request);
        }

        if (action instanceof ServletResponseAware) {
            HttpServletResponse response = (HttpServletResponse) context.get(HTTP_RESPONSE);
            ((ServletResponseAware) action).setServletResponse(response);
        }

        if (action instanceof ParameterAware) {
            ((ParameterAware) action).setParameters((Map)context.getParameters());
        }

        if (action instanceof ApplicationAware) {
            ((ApplicationAware) action).setApplication(context.getApplication());
        }

        if (action instanceof SessionAware) {
            ((SessionAware) action).setSession(context.getSession());
        }

        if (action instanceof RequestAware) {
            ((RequestAware) action).setRequest((Map) context.get("request"));
        }

        if (action instanceof PrincipalAware) {
            HttpServletRequest request = (HttpServletRequest) context.get(HTTP_REQUEST);
            if(request != null) {
                // We are in servtlet environment, so principal information resides in HttpServletRequest
                ((PrincipalAware) action).setPrincipalProxy(new ServletPrincipalProxy(request));
            }
        }
        if (action instanceof ServletContextAware) {
            ServletContext servletContext = (ServletContext) context.get(SERVLET_CONTEXT);
            ((ServletContextAware) action).setServletContext(servletContext);
        }
        return invocation.invoke();
    }
}


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