JSP的生命周期

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1、编译阶段：
servlet容器编译servlet源文件，生成servlet类。
观察一个JSP页面在第一次访问的时候会由servlet容器会生成.java文件，最终编译成.class字节码文件，如果打开.java文件查看，就是一个servlet。
2、初始化阶段：
加载与JSP对应的servlet类，创建其实例，并调用它的初始化方法。
3、执行阶段：
调用与JSP对应的servlet实例的服务方法。
4、销毁阶段：
调用与JSP对应的servlet实例的销毁方法，然后销毁servlet实例。

如下图所示的是servlet的生命周期和JSP非常的相似：

下面来测试一下JSP实现这些生命周期，在left.jsp页面如下写法：

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html><head><title>life.jsp</title></head><body><%!   private int initVar=0;  private int serviceVar=0;  private int destroyVar=0;%>  <%!  public void jspInit(){    initVar++;    System.out.println("jspInit(): JSP被初始化了"+initVar+"次");  }  public void jspDestroy(){    destroyVar++;    System.out.println("jspDestroy(): JSP被销毁了"+destroyVar+"次");  }%><%  serviceVar++;  System.out.println("_jspService(): JSP共响应了"+serviceVar+"次请求");  String content1="初始化次数 : "+initVar;  String content2="响应客户请求次数 : "+serviceVar;  String content3="销毁次数 : "+destroyVar;%><p><%=content1 %></p><p><%=content2 %></p><p><%=content3 %></p></body></html>

效果如下：

然后打开工作空间的生成的java文件：“\workspace_web_test\.metadata\.plugins\org.eclipse.wst.server.core\tmp0\work\Catalina\localhost\webtest1\org\apache\jsp\life_jsp.java”

会发现如下结构：

/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/7.0.68 * Generated at: 2016-12-14 13:18:33 UTC * Note: The last modified time of this file was set to *       the last modified time of the source file after *       generation to assist with modification tracking. */package org.apache.jsp;import javax.servlet.*;import javax.servlet.http.*;import javax.servlet.jsp.*;public final class life_jsp extends org.apache.jasper.runtime.HttpJspBase    implements org.apache.jasper.runtime.JspSourceDependent {   private int initVar=0;  private int serviceVar=0;  private int destroyVar=0;  public void jspInit(){    initVar++;    System.out.println("jspInit(): JSP被初始化了"+initVar+"次");  }  public void jspDestroy(){    destroyVar++;    System.out.println("jspDestroy(): JSP被销毁了"+destroyVar+"次");  }  private static final javax.servlet.jsp.JspFactory _jspxFactory =          javax.servlet.jsp.JspFactory.getDefaultFactory();  private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;  private volatile javax.el.ExpressionFactory _el_expressionfactory;  private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;  public java.util.Map<java.lang.String,java.lang.Long> getDependants() {    return _jspx_dependants;  }  public javax.el.ExpressionFactory _jsp_getExpressionFactory() {    if (_el_expressionfactory == null) {      synchronized (this) {        if (_el_expressionfactory == null) {          _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();        }      }    }    return _el_expressionfactory;  }  public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() {    if (_jsp_instancemanager == null) {      synchronized (this) {        if (_jsp_instancemanager == null) {          _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());        }      }    }    return _jsp_instancemanager;  }  public void _jspInit() {  }  public void _jspDestroy() {  }  public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)        throws java.io.IOException, javax.servlet.ServletException {    final javax.servlet.jsp.PageContext pageContext;    javax.servlet.http.HttpSession session = null;    final javax.servlet.ServletContext application;    final javax.servlet.ServletConfig config;    javax.servlet.jsp.JspWriter out = null;    final java.lang.Object page = this;    javax.servlet.jsp.JspWriter _jspx_out = null;    javax.servlet.jsp.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("<!DOCTYPE html PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\" \"http://www.w3.org/TR/html4/loose.dtd\">\r\n");      out.write("<html>\r\n");      out.write("<head>\r\n");      out.write("<title>life.jsp</title>\r\n");      out.write("</head>\r\n");      out.write("<body>\r\n");      out.write("\r\n");      out.write("\r\n");      out.write("  \r\n");      out.write("\r\n");      out.write("\r\n");  serviceVar++;  System.out.println("_jspService(): JSP共响应了"+serviceVar+"次请求");  String content1="初始化次数 : "+initVar;  String content2="响应客户请求次数 : "+serviceVar;  String content3="销毁次数 : "+destroyVar;      out.write("\r\n");      out.write("<p>");      out.print(content1 );      out.write("</p>\r\n");      out.write("<p>");      out.print(content2 );      out.write("</p>\r\n");      out.write("<p>");      out.print(content3 );      out.write("</p>\r\n");      out.write("\r\n");      out.write("</body>\r\n");      out.write("</html>");    } catch (java.lang.Throwable t) {      if (!(t instanceof javax.servlet.jsp.SkipPageException)){        out = _jspx_out;        if (out != null && out.getBufferSize() != 0)          try {            if (response.isCommitted()) {              out.flush();            } else {              out.clearBuffer();            }          } catch (java.io.IOException e) {}        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);        else throw new ServletException(t);      }    } finally {      _jspxFactory.releasePageContext(_jspx_page_context);    }  }}

参考：

http://www.runoob.com/jsp/jsp-life-cycle.html

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