JSP---由服务器编译成java文件（实质上是一个servlet）---java文件进一步编译成class文件。
客户端浏览器向服务器端请求jsp页面时，实际上执行的是编译之后的class文件，该class文件通过流的形式向客户端输出信息。
JSP文件在客户端首次请求时，执行向java---class的转换，故而首次访问时，会显得有些慢。
在Tomcat下，JSP变成生成的Java文件继承自org.apache.jasper.runtime .HttpJspBase，而HttpJspBase实际上是HttpServlet的子类。
Tomcat60\lib\jasper.jar
package org.apache.jasper.runtime;
import java.io.IOException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.jsp.HttpJspPage;
import org.apache.jasper.compiler.Localizer;
public abstract class HttpJspBase extends HttpServlet
implements HttpJspPage
{
public final void init(ServletConfig config)
throws ServletException
{
super.init(config);
jspInit();
_jspInit();
}
public String getServletInfo() {
return Localizer.getMessage("jsp.engine.info");
}
public final void destroy() {
jspDestroy();
_jspDestroy();
}
public final void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
_jspService(request, response);
}
public void jspInit()
{
}
public void _jspInit()
{
}
public void jspDestroy()
{
}
protected void _jspDestroy()
{
}
public abstract void _jspService(HttpServletRequest paramHttpServletRequest, HttpServletResponse paramHttpServletResponse)
throws ServletException, IOException;
}
Tomcat下，JSP编译生成的java和class文件在Tomcat60\work\Catalina 目录下，如：
如head.jsp编译之后的内容如下（主要包含3个方法init，service，destroy）：
package org.apache.jsp.jsp.public_;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
public final class head_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent {
private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory();
private static java.util.List _jspx_dependants;
static {
_jspx_dependants = new java.util.ArrayList(1);
_jspx_dependants.add("/WEB-INF/tld/next-web.tld");
}
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("\r\n");
out.write("\r\n");
out.write("<HTML>\r\n");
out.write("\t<HEAD>\r\n");
out.write("\t<style type=\"text/css\">\r\n");
out.write("\t</style>\r\n");
out.write("\t</HEAD>\r\n");
out.write("\t<body>\r\n");
out.write("\t\t<img alt=\"\" width=\"100%\" height=\"100%\" src=\"images/pic.JPG\">\r\n");
out.write("\t</body>\r\n");
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);
}
}
}
Head.jsp编译之前内容如下：
| <%@ page contentType="text/html; charset=UTF-8" %> <%@ taglib uri="/tags/next-web" prefix="next"%> <HTML> <HEAD> <style type="text/css"> </style> </HEAD> <body> <img alt="" width="100%" height="100%" src="images/pic.JPG"> </body> </HTML> |
JSP、servlet在实际执行时，实际上都是单例模式，所以成员变量是非线程安全的。
JSP页面会编译成servet，每个servlet在容器中只有一个实例。
JSP声明<%! 声明部分 %>
JSP声明部分定义的变量和方法对应servlet中的成员变量和成员方法。