2019-04-14 EL（Expression Language）

EL全名为Expression langusage

它是JSTL 1.0为方便存取数据所定义的语言

<!-- 7. EL 的运算符 -->
    ${param.score > 60 ? "及格" : "不及格" }
    <br>
    
    <% 
        List<String> names = new ArrayList<String>();
        names.add("abc");
        request.setAttribute("names", names);
    %>
    <!-- empty 可以作用于一个集合, 若该集合不存在或集合中没有元素, 其结果都为 true -->
    names is empty: ${empty requestScope.names }
    <br>
    
    <!-- 6. 其他隐含对象: pageContext 等(cookie, header, initParam 只需了解.) -->
    pageContext: pageContext 即为 PageContext 类型, 但只能读取属性就可以一直的 . 下去。 
    <br>
    contextPath: ${pageContext.request.contextPath }
    
    <br>
    sessionId: ${pageContext.session.id }
    
    <br>
    sessionAttributeNames: ${pageContext.session.attributeNames }
    
    <br>
    
    
    initParam: ${initParam.initName }
    <br>
    
    Accept-Language: ${header["Accept-Language"] }
    <br>

    JSESSIONID: ${cookie.JSESSIONID.name } -- ${cookie.JSESSIONID.value }
    <br>

    <!-- 5. 与输入有关的隐含对象: param, paramValues -->
    score: ${param.score }
    <%-- 
    <%= request.getParameter("score") %>
    --%>
    <br>
    names: ${paramValues.name[0].class.name }
    <%-- 
    <%= 
        request.getParameterValues("name")[0].getClass().getName()
    %>
    --%>
    <br>
    
    <!-- 4. 隐含对象之与范围相关的: pageScope, requestScope, sessionScope, applicationScope -->
    time: ${applicationScope.time.time }
    <%-- 
    <%= application.getAttribute("time") %>
    --%>
    <br>
    
    <!-- 3. EL 可以进行自动的类型转换 -->
    score: ${param.score + 11}
    <br>
    score: <%= request.getParameter("score") + 11 %>
    <br>
    
    <!-- 2. EL 中的隐含对象 -->
    <% 
        Customer cust2 = new Customer();
        cust2.setAge(28);
        request.setAttribute("customer", cust2);
    %>
    
    age: ${customer.age } 

    <br>
    <!-- 1. EL 的 . 或 [] 运算符 -->
    age: ${sessionScope.customer["age"] }
    
    <%-- 
        Customer customer = (Customer)session.getAttribute("customer");
        out.print(customer.getAge());
    --%>
    
    <% 
        Customer customer = new Customer();
        customer.setName("ATGUIGU");    
    
        session.setAttribute("com.atguigu.customer", customer);
    %>
    
    <br>
    <!--  
        如果域对象中的属性名带有特殊字符, 则使用 [] 运算符会很方便. 
    -->
    name: ${sessionScope["com.atguigu.customer"].name }