jsp <%! %> 与 <% %> 区别

转自huangqiqing123.iteye.com/blog/1922014

 1 <body>
 2 <%!
 3 
 4 //1、可定义方法
 5 public String outMethod(){
 6     return "outMethod";
 7 }
 8 //2、可定义static方法
 9 public static String outStaticMethod(){
10     return "outStaticMethod";
11 }
12 //3、可定义static属性
13 public static int count = 0;
14 
15 //4、不可以使用out对象
16 %>
17 
18 <%
19 //1、不可定义方法
20 //2、不可定义static方法
21 //3、不可定义static属性
22 
23 //可以使用out对象
24 out.print(outMethod());
25 out.print("<br>");
26 out.print(outStaticMethod());
27 out.print("<br>");
28 out.print(count);
29 %>
30 </body>

 1 <body>
 2 <%!
 3 
 4 //1、可定义方法
 5 public String outMethod(){
 6     return "outMethod";
 7 }
 8 //2、可定义static方法
 9 public static String outStaticMethod(){
10     return "outStaticMethod";
11 }
12 //3、可定义static属性
13 public static int count = 0;
14 
15 //4、不可以使用out对象
16 %>
17 
18 <%
19 //1、不可定义方法
20 //2、不可定义static方法
21 //3、不可定义static属性
22 
23 //可以使用out对象
24 out.print(outMethod());
25 out.print("<br>");
26 out.print(outStaticMethod());
27 out.print("<br>");
28 out.print(count);
29 %>
30 </body>

经Tomcat编译后，生成的java文件如下：

package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;

public final class test_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {



//1、可定义方法
public String outMethod(){
    return "outMethod";
}
//2、可定义static方法
public static String outStaticMethod(){
    return "outStaticMethod";
}
//3、可定义static属性
public static int count = 0;

//4、不可以使用out对象

  private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory();

  private static java.util.List _jspx_dependants;

  private javax.el.ExpressionFactory _el_expressionfactory;
  private org.apache.AnnotationProcessor _jsp_annotationprocessor;

  public Object getDependants() {
    return _jspx_dependants;
  }

  public void _jspInit() {
    _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
    _jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName());
  }

  public void _jspDestroy() {
  }

  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 {
      response.setContentType("text/html; charset=utf-8");
      pageContext = _jspxFactory.getPageContext(this, request, response,
                  null, true, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;

      out.write("
");
      out.write("<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
");
      out.write("<html>
");
      out.write("<head>
");
      out.write("<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
");
      out.write("<title>Insert title here</title>
");
      out.write("</head>
");
      out.write("<body>
");
      out.write("
");
      out.write("
");

//1、不可定义方法
//2、不可定义static方法
//3、不可定义static属性

//可以使用out对象
out.print(outMethod());
out.print("<br>");
out.print(outStaticMethod());
out.print("<br>");
out.print(count);

      out.write("
");
      out.write("</body>
");
      out.write("</html>");
    } catch (Throwable t) {
      if (!(t instanceof SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          try { out.clearBuffer(); } catch (java.io.IOException e) {}
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
      }
    } finally {
      _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
}

在生成的java文件中，我们可以看到，<%! %>中的代码对应于java类文件的成员方法、静态方法，静态变量，而<% %>中的代码对应于java类文件的_jspService方法中的代码（_jspService对应于servlet中的service方法），这也就是为什么在<% %>中不能声明方法、静态变量的原因了（方法中是不可以再创建方法的）。