检查JSP中的反射是否存在变量

Question

我在JSP boolean isPresent = false中定义了一个变量，然后在包含的JSP中编写代码来检查该变量是否存在，但得到NoSuchFieldException。

Class thisClass = this.getClass();
try {
    Field field = thisClass.getDeclaredField("isPresent");
}
catch (Exception ex) {
    //ToDo
}

不知道怎么了

Answer 1

如果您使用:

<%
    boolean isPresent = false;
%>

变量在从JSP生成的servlet的_jspService方法中定义，如下所示:

  public void _jspService(HttpServletRequest request, HttpServletResponse response)
        throws java.io.IOException, ServletException {
    PageContext pageContext = null;
    HttpSession session = null;
    ServletContext application = null;
    ServletConfig config = null;
    JspWriter out = null;
    Object page = this;
    JspWriter _jspx_out = null;
    PageContext _jspx_page_context = null;

    try {
      /// more code
    boolean isPresent = false;

但是如果你把变量的声明改成这样:

 <%!
     boolean isPresent = false;
 %>

它会工作!!

Servlet已生成:

public final class testing_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {
    boolean isPresent = false;

正如你现在看到的变量是作用域类而不是方法作用域