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EL表达式取值的范围-动力节点

fenglingwan 2020-09-25 16:26 113 查看 https://blog.51cto.com/1488107

有些朋友对于EL表达式取值的范围不是很了解,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。

EL表达式取值

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();

}
}

以上就是动力节点java培训机构的小编针对“EL表达式取值的范围”的内容进行的回答,希望对大家有所帮助,如有疑问,请在线咨询,有专业老师随时为你服务。

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