EL表达式

1.基本格式：${}
2.4种取值范围：pageScope，requestScope，sessionScope，applicationScope，只能适用于这4中范围

注意：在struts2中将数据放在OnglValueStack root对象下，但是可以通过EL表达式获取其内值，是因为在struts2中进行了封装EL表达式，添加如果在request中获取不好，就去ValueStack中查找

public class StrutsRequestWrapper extends HttpServletRequestWrapper {
public Object getAttribute(String key) {
        if (key == null) {
            throw new NullPointerException("You must specify a key value");
        }

        if (disableRequestAttributeValueStackLookup || key.startsWith("javax.servlet")) {
            // don't bother with the standard javax.servlet attributes, we can short-circuit this
            // see WW-953 and the forums post linked in that issue for more info
            return super.getAttribute(key);
        }

        ActionContext ctx = ActionContext.getContext();
        Object attribute = super.getAttribute(key);

        if (ctx != null && attribute == null) {
            boolean alreadyIn = isTrue((Boolean) ctx.get(REQUEST_WRAPPER_GET_ATTRIBUTE));

            // note: we don't let # come through or else a request for
            // #attr.foo or #request.foo could cause an endless loop
            if (!alreadyIn && !key.contains("#")) {
                try {
                    // If not found, then try the ValueStack
                    ctx.put(REQUEST_WRAPPER_GET_ATTRIBUTE, Boolean.TRUE);
                    ValueStack stack = ctx.getValueStack();
                    if (stack != null) {
                        attribute = stack.findValue(key);
                    }
                } finally {
                    ctx.put(REQUEST_WRAPPER_GET_ATTRIBUTE, Boolean.FALSE);
                }
            }
        }
        return attribute;
    }
}

 

${requestScope.user.name}
3.基本运算：.和[] .用于取属性[]用于取Array，List，Map，Set
${requestScope.map['apple']},${requestScope.list[1]}
4.{}里允许的运算：算术运算，关系运算，逻辑运算，empty/not empty空判断
${1+1}
${user.age<18}
${(user.age>18)&&(user.sex=='男')}
${empty user}判断user对象是否为空
5.EL的11个隐含对象
pageContext,param和paramValues,header和headerValues，cookie,initParam和4种取值范围对象
*pageContext可获取jsp的request,response,out,session,config,servletContext等对象
${pageContext.session.uesr}
header
${header.host}
*cookie
${cookie.key}