JSP的hello world及其原理
Hello World
首先，根据 http://my.oschina.net/letiantian/blog/366964的介绍创建项目HelloJSP。
在 web/目录下创建文件index.jsp：
index.jsp内容如下：
<%--
Document : index
Created on : Jan 13, 2015, 11:24:35 AM
Author : letian
--%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>JSP Page</title>
</head>
<body>
<h1>Hello World!</h1>
<%
int sum=0;
for(int i=1; i<=10; i++) {
sum += i;
out.println(i + "<br/>");
}
%>
<h1><%=sum %></h1>
</body>
</html>
如果想在访问 http://localhost:8080/HelloJSP/时候优先显示 index.jsp的内容，可以这样做：
在tomcat安装目录下的 conf目录下，打开 web.xml，找到：
<welcome-file-list>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list> 调整顺序即可。
上面的是全局配置，也可以针对项目来配置。在HelloJSP项目的 web/WEB-INF目录下创建文件 web.xml：
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
version="3.1">
<session-config>
<session-timeout>
30
</session-timeout>
</session-config>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
</welcome-file-list>
</web-app>
原理
JSP首先会被转换为JAVA文件，对于HelloJSP项目中的index.jsp，我们可以在Tomcat安装目录下的 work/Catalina/localhost/HelloJSP/org/apache/jsp 目录中找到index_jsp.java（还有一个文件是index_jsp.class），其内容如下：
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.0.15
* Generated at: 2015-01-13 03:50:41 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 index_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 javax.el.ExpressionFactory _el_expressionfactory;
private org.apache.tomcat.InstanceManager _jsp_instancemanager;
public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
return _jspx_dependants;
}
public void _jspInit() {
_el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
_jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
}
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 java.lang.String _jspx_method = request.getMethod();
if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD");
return;
}
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("\n");
out.write("\n");
out.write("\n");
out.write("<!DOCTYPE html>\n");
out.write("<html>\n");
out.write(" <head>\n");
out.write(" <meta http-equiv=\"Content-Type\" content=\"text/html; charset=UTF-8\">\n");
out.write(" <title>JSP Page</title>\n");
out.write(" </head>\n");
out.write(" <body>\n");
out.write(" <h1>Hello World!</h1>\n");
out.write(" ");
int sum=0;
for(int i=1; i<=10; i++) {
sum += i;
out.println(i + "<br/>");
}
out.write("\n");
out.write(" \n");
out.write(" <h1>");
out.print(sum );
out.write("</h1>\n");
out.write(" \n");
out.write(" </body>\n");
out.write("</html>\n");
} 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);
}
}
}