帶有jsp：useBean的Javabean。它們如何工作？我不明白
我在index.jsp和Servlet之間又做了一個jsp文件：
<jsp:useBean scope='session'/><jsp:setProperty name='binop' property='*'/>.
這做到了“魔術”。
解決方法我必須使用Java bean從jsp文件中獲取2個數字和一個運算。提交數字后，將它們從該Java Bean帶到servlet并返回它們的結果。問題是Javabean字段永遠不會用文本框中寫的數字來完成。所以，我有了index.jsp的主體：
<html><head> <meta http-equiv='Content-Type' content='text/html; charset=UTF-8'> <title>JSP Page</title> <link rel='stylesheet' type='text/css' href='http://www.4tl426be.cn/wenda/style.css' /></head><body> <h1>Calculator</h1> <FORM METHOD='POST' action='Controller'>N1: <input type ='text' name='nr1' value='0'>op: <input type ='text' name='op' value='+'>N2: <input type ='text' name='nr2' value='0'><INPUT TYPE='submit' NAME='actiune' VALUE='Calculate'/> </FORM> <jsp:useBean scope='session'/> <jsp:setProperty name='binOp' property='*'/></body>
servlet的processRequest方法Controller.java放在程序包servlet中：
protected void processRequest(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException { response.setContentType('text/html;charset=UTF-8'); PrintWriter out = response.getWriter(); HttpSession session = request.getSession(true); BinaryOperation binOp = (BinaryOperation) session.getAttribute('binOp'); try {if (!binOp.isComplete()) { System.out.println(binOp.getNr1() + binOp.getNr2() + binOp.getOp()); response.sendRedirect('index.jsp');} else { out.println('<html>'); out.println('<head>'); out.println('<title>Servlet Controller</title>'); out.println('</head>'); out.println('<body>'); out.println('<h1>Bean in controller ' + binOp.getNr1() + '__' + binOp.getOp() + '__' + binOp.getNr2() + '</h1>'); out.println(binOp.toString()); out.println('</body>'); out.println('</html>');} } finally {out.close(); }}
然后將bean BinaryOperation放入程序包bean中：
package beans;public class BinaryOperation {private String nr1;private String op;private String nr2;public void setNr1(String nr1) { this.nr1 = nr1;}public void setOp(String op) { this.op = op;}public void setNr2(String nr2) { this.nr2 = nr2;}public String getNr1() { return nr1;}public String getOp() { return op;}public String getNr2() { return nr2;}public boolean isComplete() { return !(((nr1 == null) || (nr1.length() == 0)) || ((op == null) || (op.length() == 0)) || ((nr2 == null) || (nr2.length() == 0)));}}
在Apache日志中，我有if語句的下一個輸出（請參閱servlet-System.out.println（binOp.getNr1（）+binOp.getNr2（）+ binOp.getOp（））;）：nullnullnull
我的錯誤在哪里？
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