1.前言
为了能深入浅出的理解这个框架的由来，我们首先来了解一下JSP解析器将我们写的JSP代码转换成的JAVA文件的内容。
下面是一个JSP文件test.jsp
<%@ page language="java" contentType="text/html;charset=GB2312" %>
<%
out.write("<!--文件开始-->");
%>
<html>
<head>
<body>
<%= "输出"%>
</body>
</head>
</html>
经过TOMCAT转换出的JAVA文件test$jsp.java内容如下：
package org.apache.jsp;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import org.apache.jasper.runtime.*;
public class test$jsp extends HttpJspBase {
static {
}
public testOutRedir$jsp( ) {
}
private static boolean _jspx_inited = false;
public final void _jspx_init() throws org.apache.jasper.runtime.JspException {
}
public void _jspService(HttpServletRequest request, HttpServletResponse response)
throws java.io.IOException, ServletException {
JspFactory _jspxFactory = null;
PageContext pageContext = null;
HttpSession session = null;
ServletContext application = null;
ServletConfig config = null;
JspWriter out = null;
Object page = this;
String _value = null;
try {
if (_jspx_inited == false) {
synchronized (this) {
if (_jspx_inited == false) {
_jspx_init();
_jspx_inited = true;
}
}
}
_jspxFactory = JspFactory.getDefaultFactory();
response.setContentType("text/html;charset=GB2312");
pageContext = _jspxFactory.getPageContext(this, request, response,
"", true, 8192, true);
application = pageContext.getServletContext();
config = pageContext.getServletConfig();
session = pageContext.getSession();
out = pageContext.getOut();
//为了节省篇幅，我删除了解释器添加的注释
out.write("\r\n");
//上一句是由于<%@ page language="java" contentType="text/html;charset=GB2312" %>后面的换行产生的
out.write("<!--文件开始-->");
out.write("\r\n<html>\r\n<head>\r\n<body>\r\n");
out.print( "输出" );
out.write("\r\n</body>\r\n</head>\r\n</html>\r\n");
} catch (Throwable t) {
if (out != null && out.getBufferSize() != 0)
out.clearBuffer();
if (pageContext != null) pageContext.handlePageException(t);
} finally {
if (_jspxFactory != null) _jspxFactory.releasePageContext(pageContext);
}
}
}
从上面的代码中可以清晰的看到JSP内建的几个对象（out、request、response、session、pageContext、application、config、page）是怎么产生的，懂servlet的朋友一看就能明白。
下面重点理解一下out对象，它被声明为JspWriter类型，JspWriter是一个抽象类，在包javax.servlet.jsp中可以找到它的定义。
abstract public class javax.servlet.jsp.JspWriter extends java.io.Writer{
final public static int NO_BUFFER = 0;
final public static int DEFAULT_BUFFER = -1;
final public static int UNBOUNDED_BUFFER = -2;
protected int bufferSize;
protected Boolean autoFlush;
protected javax.servlet.jsp.JspWriter(int arg1, boolean arg2);
abstract public void newLine() throws IOException ;
abstract public void print(boolean arg0) throws IOException ;
abstract public void print(char arg0) throws IOException ;
abstract public void print(int arg0) throws IOException ;
abstract public void print(long arg0) throws IOException ;
abstract public void print(float arg0) throws IOException ;
abstract public void print(double arg0) throws IOException ;
abstract public void print(char[] arg0) throws IOException ;
abstract public void print(String arg0) throws IOException ;
abstract public void print(Object arg0) throws IOException ;
abstract public void println() throws IOException ;
abstract public void println(boolean arg0) throws IOException ;
abstract public void println(char arg0) throws IOException ;
abstract public void println(int arg0) throws IOException ;
abstract public void println(long arg0) throws IOException ;
abstract public void println(float arg0) throws IOException ;
abstract public void println(double arg0) throws IOException ;
abstract public void println(char[] arg0) throws IOException ;
abstract public void println(String arg0) throws IOException ;
abtract public void println(Object arg0) throws IOException ;
abstract public void clear() throws IOException ;
abstract public void clearBuffer() throws IOException ;
abstract public void flush() throws IOException ;
abstract public void close() throws IOException ;
public int getBufferSize() ;
abstract public int getRemaining() ;
public boolean isAutoFlush() ;
}
我相信当我写到这里你可能已经知道我想怎么做了。是的，来个偷天换日，继承JspWriter类，然后实现其定义的虚函数，然后把out变量替换成你自己实现的类的实例就ok了。
2.实现替换
假设
<%@ page language="java" contentType="text/html;charset=GB2312"
import="jwb.util.HtmlIntoFile,jwb.util.TempSinglet,java.io.File"%><%
JspWriter out_bak = out;
String arg1="argument1";
String filePath = "/cache/根据参数生成文件名_" + arg1 + ".html";
//首先判断文件是否已经存在，如果不存在则执行本页面，否则跳转到静态页面就OK了
File f = new File(pageContext.getServletContext().getRealPath(filePath));
if(f.exists())
{
out_bak.clear();
pageContext.forward(filePath);
System.out.println("直接转到静态页面");
return;
}
out = new HtmlIntoFile(pageContext.getServletContext().getRealPath(filePath));
out.write("<!--文件开始-->");
%>
<html>
<head>
<body>
<%= "看吧，这就是输出被重定向到文件的实现，很简单吧^_^"%>
</body>
</head>
</html>
<%
out.close();//关闭生成的静态文件
out_bak.clear();
pageContext.forward(filePath);
System.out.println("执行本页面后再转到静态页面");
return;
%>
3.更新问题
下面就讨论一下如何更新生成静态文件，其实从上面实现中你可以看到，很简单的就是将生成的静态文件删除即可，至于什么时候删除，要看你的需求了。我能想到的几种情况如下
当用来生成页面的数据更新时
如果不需要很提供时时的数据可以定时更新
永远不更新
待更新……
