JSP 原理
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2022-03-24 23:45:40
1、jsp的由来
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1、jsp的由来
| 姓名 | 年龄 | 性别 |
|---|---|---|
| 张三 | 18 | 男 |
/*
* generated by the jasper component of apache tomcat
* version: apache tomcat/7.0.42
* generated at: 2015-12-27 10:03:19 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.*;
public final class hello_jsp extends org.apache.jasper.runtime.httpjspbase
implements org.apache.jasper.runtime.jspsourcedependent {
public void fun1() { //要特别注意，这是在中定义的方法，没有被放在service方法中
system.out.println("hello");
}
private static final javax.servlet.jsp.jspfactory _jspxfactory =
javax.servlet.jsp.jspfactory.getdefaultfactory();
private static java.util.map _jspx_dependants;
private javax.el.expressionfactory _el_expressionfactory;
private org.apache.tomcat.instancemanager _jsp_instancemanager;
public java.util.map 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 { //这是_jspservice()方法
final javax.servlet.jsp.pagecontext pagecontext; //内置对象的初始化
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("\r\n");
out.write("\r\n");
out.write("\r\n");
string s = request.getheader("user-agent"); //它的原身是string s = request.getheader("user-agent");，被直接拿过来了
out.write("\r\n");
out.write("\r\n");
out.write("\r\n");
out.write("\r\n"); //这些都是html代码，底层被包装成和servlet 一样的实现方式
out.write(" \r\n");
out.write(" my jsp 'a.jsp' starting page\r\n");
out.write(" \r\n");
out.write("\t\r\n");
out.write("\t\r\n");
out.write("\t \r\n");
out.write("\t\r\n");
out.write("\t\r\n");
out.write("\t\r\n");
out.write("\r\n");
out.write(" \r\n");
out.write(" \r\n");
out.write(" \r\n");
out.write(" \r\n");
out.write("\t\r\n");
out.write("\t 姓名\r\n");
out.write("\t 年龄\r\n");
out.write("\t 性别\r\n");
out.write("\t\r\n");
for(int i = 0; i \r\n");
out.write("\t 张三\r\n");
out.write("\t 18\r\n");
out.write("\t 男\r\n");
out.write("\t\r\n");
}
out.write("\r\n");
out.write("\r\n");
out.write("\r\n");
out.write("\r\n");
out.write("\r\n");
out.write('\r');
out.write('\n');
int a = 10; //这些是定义的变量，可以看到是放在了service方法中的
out.write("\r\n");
out.write("\r\n");
a++;
out.write("\r\n");
out.write("\r\n");
out.print(a );
out.write("\r\n");
out.write("\r\n");
out.write(" \r\n");
out.write("\r\n");
} catch (java.lang.throwable t) {
if (!(t instanceof javax.servlet.jsp.skippageexception)){
out = _jspx_out;
if (out != null && out.getbuffersize() != 0)
try { 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);
}
}
}
得出结论，jsp中虽然能直接写出html代码，但是在底层依然是被包装成 servlet 实现方式的，所以更印证了jsp是特殊的servlet 。
其次，在 和 脚本中定义的java 代码都会放在jsp 的 _jspservice() 方法中（实际上就是servlet中的service 方法），而 脚本中定义的却会放到 hello_jsp 类的成员位置的，这一点很重要，因为jsp中鼎鼎大名的九大内置对象是在_jspservice() 方法中初始化的，只有在本方法中才能够使用内置对象，所以 脚本中是不能使用内置对象的（在实际开发中，本脚本很少用到）。
5、jsp和html、servlet 的比较
● servlet：
缺点：不适合设置html响应体，需要大量的response.getwriter().print("")
优点：动态资源，可以。
● html：
缺点：html是静态页面，不能包含动态信息
优点：不用为输出html标签而发愁
● jsp：
优点：在原有html的基础上添加java脚本，构成jsp页面。
6、jsp 和 servlet 的分工
在设计中，jsp和servlet 是相互配合使用的，其分工为：
● jsp：
作为请求发起页面，例如显示表单、超链接，并将请求发给 servlet ；
作为请求结束页面，例如显示数据。
● servlet：
作为请求中处理数据的环节。