我在 Linux上运行Tomcat 6.0.18. 我有一个使用这样的bean的JSP: jsp:useBean id="helper" type="com.example.SomeType" scope="request"/ 该页面使用表达式语言引用helper的属性,如下所示: !-- This works properly, b
我有一个使用这样的bean的JSP:
<jsp:useBean id="helper" type="com.example.SomeType" scope="request"/>
该页面使用表达式语言引用helper的属性,如下所示:
<!-- This works properly, but could fail silently if the bean name is incorrect. --> <div><p>Here's some stuff: ${helper.stuff}</div>
在一些重构中,我错过了名称助手的出现,我注意到如果名称助手写得不正确,不会引发错误.不在屏幕上,而不在我的日志文件中.在输出中没有为表达式语言片段生成任何内容:
<!-- Wrong name! "foo" should be "helper" but no error is observed (other than missing ouput)! --> <div><p>Here's some stuff: ${foo.stuff}</div>
现在,如果我使用以下具有帮助程序名称错误的JSP语法,则会引发错误(我的自定义错误页面显示,我在日志文件中看到异常):
<!-- Wrong name, but an error is raised. --> <div><p>Here's some stuff: <jsp:getProperty name="foo" property="stuff"/></div>
在这种情况下,日志记录此条目:
SEVERE: requestURI: /some.jsp servletName: jsp statusCode: 500 org.apache.jasper.JasperException: Attempted a bean operation on a null object.
为了完整性,当bean名称正确时,jsp:getProperty语法正常工作:
<!-- Works properly, protects me from an incorrect name, but is more verbose than EL. --> <div><p>Here's some stuff: <jsp:getProperty name="helper" property="stuff"/></div>
当我写${foo.stuff}时,为什么我没有看到错误?在这种情况下是否有一些控制错误报告的配置选项?
此行为将在 Expression Language Specification Version 2.1的第1.6节中介绍.To evaluate expr-a[expr-b]:
If value-a is null:
- If expr-a[expr-b] is the last property being resolved:
- If the expression is a value expression and
ValueExpression.getValue(context) was
called to initiate this expression
evaluation, return null.- Otherwise, throw PropertyNotFoundException. trying to
de-reference null for an lvalue- Otherwise, return null.
(EL统一.和[]运算符)