Question

使用JSP / bean操作让我的脚步受到影响。我收到了在我的输出页面上第14行处理JSP页面/foo/output.jsp的异常。 java文件编译好了。

的Input.jsp

<p>First Number: <input type="text" name="first" value="" /></p>
<p>Second Number: <input type="text" name="second" value="" /></p>
<p>Action: <select name="compu">
                <option value="1">+</option>
                <option value="2">-</option>
                <option value="3">*</option>
                <option value="4">/</option>
            </select></p>

<input type="submit" />
</form>

output.jsp的

<%@ page contentType="text/html; charset=utf-8" language="java" import="java.util.*" errorPage="" %>

<jsp:useBean id="foo" class="stuff.DerInput" scope="page" />
<jsp:setProperty name="foo" property="*" />
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
</head>

<body>
<jsp:getProperty name="foo" property="calculation" />
<p>The result: <%= foo.getCalculation() %></p>

</body>
</html>

DerInput.Java

package stuff;

public class DerInput {
    int first, second, compu, theresult;
    String notation;

    public void setFirst( int value )
    {
        first = value;
    }

    public void setSecond( int value )
    {
        second = value;
    }

    public void setCompu( int value )
    {
        compu = value;
    }

    public String getCalculation() {

        switch (compu) {
            case 1:
                theresult = first + second;
                notation = "+";
                break;
            case 2:
                theresult = first - second;
                notation = "-";
                break;
            case 3:
                theresult = first * second;
                notation = "*";
                break;
            case 4:
                theresult = first / second;
                notation = "/";
                break;
        }

        return first + " " + notation + " " + second + " = " + theresult;
    }
}

Answer 1

看这里，

<jsp:getProperty name="foo" property="calculation" />
<p>The result: <%= foo.getCalculation() %></p>

您希望页面范围内的bean ${foo}也可以在 scriptlet 范围<% foo %>中使用。这不是真的。它们不共享相同的变量范围。这只会导致NullPointerException来电getCalculation()，因为<% foo %>为null。

改为使用EL。

<p>The result: ${foo.calculation}</p>

（注意：<jsp:getProperty>行在这里是多余的，所以我将其删除了）

使用bean表单输出

1 个答案: