JSTL重要技术经验积累

一、循环遍历集合
<wbr><wbr>1、在jsp中引入标准函数声明<br><wbr>&lt;%@ taglib uri="</wbr></wbr></wbr>http://java.sun.com/jsp/jstl/core" prefix="c"%><wbr><wbr><wbr><wbr><wbr><wbr><br><wbr><wbr>2、若要判断集合的大小，则需要引入如下声明<br><wbr>&lt;%@ taglib prefix="fn" uri="</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>http://java.sun.com/jsp/jstl/functions" %>
<wbr><wbr>3、如何使用jstl判断集合是否为空<br><wbr>${user}为集合,user为集合名<br><wbr>&lt;c:if test="${empty user}"&gt;无信息!&lt;/c:if&gt;为空<br><wbr>&lt;c:if test="${!empty user}"&gt;其它&lt;/c:if&gt;非空<br><wbr><wbr>4、如何取得集合的大小<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>${fn:length(集合名&lt;如果是session中的集合则应该加上sessionScope.键&gt;)}<br><wbr>${fn:length(map)}<br><wbr><wbr>5、如何取得保存在session中的对象?<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>${sessionScope.键.对象属性}<br><wbr><wbr>6、varStatus显示循环变量的状态<br><wbr><wbr><wbr><wbr><wbr>例:&lt;tag:forEach var="currentFood" items="${sessionScope.foods}" varStatus="status"<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>&lt;tag:if test="${status%2==1}"&gt;<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>.....................<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>&lt;/tag:if&gt;<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>&lt;/tag:forEach&gt;<br><wbr><wbr><wbr><wbr><wbr>其中status这个变量就记录了循环变量的状态</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr><wbr>如何在遍历集合时，知道当前遍历的是第几个对象?</wbr></wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr><wbr><wbr>可以用&lt;c:out<wbr>value="状态变量名.count"&gt;</wbr></wbr></wbr></wbr></wbr></wbr></wbr>

<wbr><wbr><wbr><wbr><wbr>count：当前这次迭代从1开始的迭代计数。<br><wbr><wbr>7、如何遍历Map集合<br><wbr><wbr><wbr><wbr><wbr>&lt;c:forEach var="foodmap" items="${sessionScope.cart}" varStatus="status"&gt;<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>&lt;c:set var="subtotal" value="${foodmap.value.bean.foodPrice*foodmap.value.quantity}"&gt;&lt;/c:set&gt;<br><wbr><wbr><wbr><wbr><wbr>&lt;/c:forEach&gt;<br><wbr><wbr><wbr><wbr><wbr>遍历Map集合的值:<br><wbr><wbr><wbr><wbr><wbr>foodmap:保存session中的map<br><wbr><wbr><wbr><wbr><wbr>foodmap.value:取得map的值，即获取保存在map中的一个对象<br><wbr><wbr><wbr><wbr><wbr>要获取对象中的数据，必须用foodmap.value.quantity去点对象的属性(quantity就是对象的属性)</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

<wbr><wbr><wbr>8、遍历集合原理详解</wbr></wbr></wbr>

<wbr><wbr><wbr><wbr><wbr><wbr><wbr>在JSP的开发中，迭代是经常要使用到的操作。例如，逐行的显示查询的结果等。在早期的JSP中，通常使用Scriptlets来实现Iterator或者Enumeration对象的迭代输出。现在，通过JSTL的迭代标签可以在很大的程度上简化迭代操作。<br><wbr><wbr><wbr><wbr><wbr>JSTL所支持的迭代标签有两个，分别是&lt;c:forEach&gt;和&lt;c:forTokens&gt;。在这里介绍的是&lt;c:forEach&gt;标签。<br><wbr><wbr><wbr><wbr><wbr>简单点说，&lt;c:forEach&gt;标签的作用就是迭代输出标签内部的内容。它既可以进行固定次数的迭代输出，也可以依据集合中对象的个数来决定迭代的次数。<br><wbr><wbr><wbr><wbr><wbr>&lt;c:forEach&gt;标签的语法定义如下所示。<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>&lt;c:forEach var="name" items="expression" varStatus="name"<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>begin="expression" end="expression" step="expression"&gt;<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>body content<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>&lt;/c:forEach&gt;<br><wbr><wbr><wbr><wbr><wbr>&lt;c:forEach&gt;标签具有以下一些属性：<br><wbr><wbr><wbr><wbr><wbr>var：迭代参数的名称。在迭代体中可以使用的变量的名称，用来表示每一个迭代变量。类型为String。<br><wbr><wbr><wbr><wbr><wbr><wbr>items：要进行迭代的集合。对于它所支持的类型将在下面进行讲解。<br><wbr><wbr><wbr><wbr><wbr><wbr>varStatus：迭代变量的名称，用来表示迭代的状态，可以访问到迭代自身的信息。<br><wbr><wbr><wbr><wbr><wbr><wbr>begin：如果指定了items，那么迭代就从items[begin]开始进行迭代；如果没有指定items，那么就从begin开始迭代。它的类型为整数。<br><wbr><wbr><wbr><wbr><wbr><wbr>end：如果指定了items，那么就在items[end]结束迭代；如果没有指定items，那么就在end结束迭代。它的类型也为整数。<br><wbr><wbr><wbr><wbr><wbr><wbr>step：迭代的步长。<br><wbr><wbr><wbr><wbr><wbr>&lt;c:forEach&gt;标签的items属性支持Java平台所提供的所有标准集合类型。此外，您可以使用该操作来迭代数组（包括基本类型数组）中的元素。它所支持的集合类型以及迭代的元素如下所示：<br><wbr><wbr><wbr><wbr><wbr><wbr>java.util.Collection：调用iterator()来获得的元素。<br><wbr><wbr><wbr><wbr><wbr><wbr>java.util.Map：通过java.util.Map.Entry所获得的实例。<br><wbr><wbr><wbr><wbr><wbr><wbr>java.util.Iterator：迭代器元素。<br><wbr><wbr><wbr><wbr><wbr><wbr>java.util.Enumeration：枚举元素。<br><wbr><wbr><wbr><wbr><wbr><wbr>Object实例数组：数组元素。<br><wbr><wbr><wbr><wbr><wbr><wbr>基本类型值数组：经过包装的数组元素。<br><wbr><wbr><wbr><wbr><wbr><wbr>用逗号定界的String：分割后的子字符串。<br><wbr><wbr><wbr><wbr><wbr><wbr>javax.servlet.jsp.jstl.sql.Result：SQL查询所获得的行。<br><wbr><wbr><wbr><wbr><wbr><wbr>不论是对整数还是对集合进行迭代,&lt;c:forEach&gt;的varStatus属性所起的作用相同。和var属性一样，varStatus用于创建限定了作用域的变量（改变量只在当前标签体内起作用）。不过，由varStatus属性命名的变量并不存储当前索引值或当前元素，而是赋予javax.servlet.jsp.jstl.core.LoopTagStatus类的实例。该类包含了一系列的特性，它们描述了迭代的当前状态，如下这些属性的含义如下所示：<br><wbr><wbr><wbr><wbr><wbr><wbr>current：当前这次迭代的（集合中的）项。<br><wbr><wbr><wbr><wbr><wbr><wbr>index：当前这次迭代从0开始的迭代索引。<br><wbr><wbr><wbr><wbr><wbr><wbr>count：当前这次迭代从1开始的迭代计数。<br><wbr><wbr><wbr><wbr><wbr><wbr>first：用来表明当前这轮迭代是否为第一次迭代，该属性为boolean类型。<br><wbr><wbr><wbr><wbr><wbr><wbr>last：用来表明当前这轮迭代是否为最后一次迭代，该属性为boolean类型。<br><wbr><wbr><wbr><wbr><wbr><wbr>begin：begin属性的值。<br><wbr><wbr><wbr><wbr><wbr><wbr>end：end属性的值<br><wbr><wbr><wbr><wbr><wbr><wbr>step：step属性的值<br><wbr><wbr><wbr><wbr><wbr>下面就来看两个基本的例子，第一个例子是依次输出集合内的元素。<br><wbr><wbr><wbr><wbr><wbr>&lt;c:forEach var="item" items="${contents}" varStatus="status"&gt;<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>$status.count：${item}<br><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>