jsp的执行流程

1.在tomcat的web.xml中, 配置了一个JspServlet, 访问路径是*.jsp. 所有以.jsp结尾的路径都归JspServlet负责. JspServlet帮忙将所有的jsp文件转换为java代码, java代码编译成class, 运行后信息就被输出到了浏览器.

2. 当客户端访问*.jsp的路径时, 被JspServlet拦下来, 将jsp文件转换为java代码, 然后编译为class文件, 执行, 页面就看到效果了.具体代码见本节最后

3.分析jsp的执行流程, 需要查看服务器的work目录, 当jsp被访问时, 会在当前目录下, 生成对应的java文件和class文件.

访问路径:C:UsersAdministrator.IntelliJIdea2018.3system omcatTomcat_7_0_79_jspworkCatalinalocalhost1_hello_jsp_war_explodedorgapachejsp

4. jsp的执行流程文字描述:

> 客户端访问jsp资源: http://localhost:8080/项目/index.jsp

> 被JspServlet匹配成功, 执行JspServlet

> JspServlet被称之为JSP引擎,JspServlet会查找index.jsp资源文件, 将jsp文件翻译为java文件, 命名为index_jsp.java

> index_jsp.java定义了一个类, 叫index_jsp, 继承了HttpJspBase类, HttpJspBase有继承了HttpServlet. 所以说, jsp本质上就是一个Servlet.

> 在index_jsp.java中, 并没有直接定义service方法, 而是重写了父类中的_jspService方法, 这个方法会被父类中的service方法自动调用.

> 其实_jspService方法中就是将jsp文件中所有的内容转换成了java输出, 结构是out.write();

> index_jsp.java被编译为index_jsp.class, 运行后, 页面就看到效果了.

5.常见疑问

1.为什么称jsp转化的java代码为servlet，并没有直接继承HttpServlet啊？

答：由代码可知，HttpJspBase继承了HttpServlet，而error500_jsp.java由继承了HttpJspBase，所以说error500_jsp.java是一个servlet

2.error500_jsp.java中并没有定义service方法，那是如何进行调用的呢？

答:由代码可知，error500_jsp.java的父类 (public abstract class HttpJspBase extends HttpServlet implements HttpJspPage)

存在service，init，destroy等方法，还有一个抽象方法_jspService，无方法体，service方法无法调用本类的抽象方法_jspService，而

其子类error500_jsp.java正好重写了父类的_jspService()，这就导致父类调用_jspService()方法时，会跑到子类error500_jsp.java中去调用该重写的方法。

有关文件代码

tomcat中的web.xml中关于jsp的设置

当客户端访问*.jsp的路径时, 被JspServlet拦下来, 将jsp文件转换为java代码, 然后编译为class文件, 执行, 页面就看到效果了

<servlet>
    <servlet-name>jsp</servlet-name>
    <servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
    <init-param>
        <param-name>fork</param-name>
        <param-value>false</param-value>
    </init-param>
    <init-param>
        <param-name>xpoweredBy</param-name>
        <param-value>false</param-value>
    </init-param>
    <load-on-startup>3</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>jsp</servlet-name>
    <url-pattern>*.jsp</url-pattern>
    <url-pattern>*.jspx</url-pattern>
</servlet-mapping>

通过JspServlet将jsp转化成的servlet

error500.jsp--->error500_jsp.java

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.79
 * Generated at: 2019-05-09 07:08:40 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 error500_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {

  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");
      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("<h1>500</h1>");
    } 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);
    }
  }
}

HttpJspBase的相关代码

error500_jsp extends org.apache.jasper.runtime.HttpJspBase

由代码可知，HttpJspBase继承了HttpServlet，而error500_jsp.java由继承了HttpJspBase，所以说error500_jsp.java是一个servlet

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
{
  private static final long serialVersionUID = 1L;

  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;
}