java – 为什么我们在jsp而不是System.out.println()中写出out.p

当JSP变成代码时,它(在理论上,与Tomcat实际上)通过两个步骤：JSP – > servlet源代码,然后是servlet源代码 – >类.整个页面放在一个方法中,其中Tomcat看起来像这样：

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");
        pageContext = _jspxFactory.getPageContext(this,request,response,"qdforumerror.jsp",true,65536,true);
        _jspx_page_context = pageContext;
        application = pageContext.getServletContext();
        config = pageContext.getServletConfig();
        session = pageContext.getSession();
        out = pageContext.getOut();
        _jspx_out = out;

        /* =============================================
           ...your <% ... %> JSP code here,with
           any markup outside those tags converted into
           out.print("..."); statments...
           =============================================
        */
    }
    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);
    }
}

您可以看到,out是该方法中的变量,类型为JspWriter(而不是OutputStream与System.out类似).

(旁注：您在<％！...％>标签中包含的代码,而不是正常的<％...％>标记中的代码不会放在方法中;它会放在生成的servlet类的其他位置. )