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_jspworkCatalinalocalhost 1_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;
}