JSTL 变量实际上是属性，默认情况下范围在页面上下文级别.
因此，如果您需要访问 scriptlet 中的 JSTL 变量值，您可以通过调用 getAttribute() 方法在适当范围的对象上(通常是 pageContext 和请求).

resp = resp + (String)pageContext.getAttribute("test");

完整代码

但是为什么我会想到这个例外.

一个 JSP scriptlet 用于包含任何代码片段这对页面中使用的脚本语言有效.scriptlet 的语法如下:

当脚本语言设置为Java时，一个scriptlet被转换成Java编程语言的语句片段，并插入到JSP页面的servlet的服务方法中.

在 scriptlet 中，您可以编写 Java 代码，而 ${test} 可以用非 Java 代码编写.

不相关

• 如何避免 JSP 文件中的 Java 代码?

The following code causes an error:

<c:set var="test" value="test1"/>
<%
    String resp = "abc"; 
    resp = resp + ${test};  //in this line I got an  Exception.
    out.println(resp);
%>

Why can't I use the expression language "${test}" in the scriptlet?

JSTL variables are actually attributes, and by default are scoped at the page context level.
As a result, if you need to access a JSTL variable value in a scriptlet, you can do so by calling the getAttribute() method on the appropriately scoped object (usually pageContext and request).

resp = resp + (String)pageContext.getAttribute("test"); 

Full code

 <c:set var="test" value="test1"/>
 <%
    String resp = "abc"; 
    resp = resp + (String)pageContext.getAttribute("test");   //No exception.
    out.println(resp);
  %>  

But why that exception come to me.

A JSP scriptlet is used to contain any code fragment that is valid for the scripting language used in a page. The syntax for a scriptlet is as follows:

<%
   scripting-language-statements
%>

When the scripting language is set to Java, a scriptlet is transformed into a Java programming language statement fragment and is inserted into the service method of the JSP page’s servlet.

In scriptlets you can write Java code and ${test} in not Java code.

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Not related

• How to avoid Java Code in JSP-Files?