JSP_003_jsp原理

最新推荐文章于 2021-11-01 18:28:36 发布

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本文介绍了JSP的工作原理及运行机制，详细解释了JSP如何在第一次访问时被编译为Servlet，并在后续访问中如何直接调用其service()方法。此外，还展示了JSP编译后的Java代码示例。

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jsp原理

···············································································

jsp页面中的 out.print() == out.write()，没有任何区别

·····jsp是特殊的Servlet（这个java是一个servlet类）

当jsp页面

1）第一次被访问时

服务器会把jsp编译成java文件······· 然后再把java编译成.class···········创建该类对象·········最后调用它的service()方法

2）第二次请求同一jsp时，直接调用service()方法。

3）第三次请求同一jsp时，直接调用service()方法，依次类推......

························第一次被访问jsp时·····································································

·····························································································

在tomcat的work目录下可以找到jsp对应的.java源代码。

* 查看jsp对应java文件：

> java脚本

> html

·····························JSP翻译成的*.java源代码·······························

/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/7.0.67
* Generated at: 2015-12-16 09:26:44 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.*;
import java.util.*;

// HttpJspBase 实现了Servlet接口，所以说jsp就是Servlet
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 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 {

// 九（8）大内置对象，程序自动创建，无需程序员手动创建

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 {

// 九（8）大内置对象赋值
response.setContentType("text/html;charset=gbk");
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;

// jsp页面中的代码在这里

---------------以下jsp页面代码-----------------
out.write('\r');
out.write('\n');

// Java代码原样输出
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";

// 其他代码字符串输出
out.write("\r\n");
out.write("\r\n");
out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">\r\n");
out.write("<html>\r\n");
out.write(" <head>\r\n");
out.write(" <base href=\"");
out.print(basePath);
out.write("\">\r\n");
out.write(" \r\n");
out.write(" <title>My JSP 'index.jsp' starting page</title>\r\n");
out.write("\t<meta http-equiv=\"pragma\" content=\"no-cache\">\r\n");
out.write("\t<meta http-equiv=\"cache-control\" content=\"no-cache\">\r\n");
out.write("\t<meta http-equiv=\"expires\" content=\"0\"> \r\n");
out.write("\t<meta http-equiv=\"keywords\" content=\"keyword1,keyword2,keyword3\">\r\n");
out.write("\t<meta http-equiv=\"description\" content=\"This is my page\">\r\n");
out.write("\t\r\n");
out.write(" </head>\r\n");
out.write(" \r\n");
out.write(" <body>\r\n");
out.write(" This is my JSP page. <br>\r\n");
out.write(" This is my JSP page. <br>\r\n");
out.write(" </body>\r\n");
out.write("</html>\r\n");

-------------以上jsp页面代码-------------------

到这里结束

} 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);
}
}
}