JSP的原理
Posted yusiming
tags:
篇首语:本文由小常识网(cha138.com)小编为大家整理,主要介绍了JSP的原理相关的知识,希望对你有一定的参考价值。
JSP其实就是Servlet,在运行时,JSP将由JSP编译器进行转换,将JSP转换为Java代码,由JSP所创建的类都实现了Servlet接口,所以JSP就是Servlet,它也有声明声明方法,其实请求该JSP页面实际上也是在调用该JSP转换过了的Servlet的service方法,我们到tomcat目录下查看对应的Servlet代码
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>第一个web程序</title>
</head>
<body>
Hello,World!
</body>
</html>
该JSP的名字是index.jsp,我们看到它对应的Servlet的名字是index_jsp.java,由tomcat生成的源码如下:
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/8.0.52
* Generated at: 2018-10-09 11:42:02 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,
org.apache.jasper.runtime.JspSourceImports {
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 static final java.util.Set<java.lang.String> _jspx_imports_packages;
private static final java.util.Set<java.lang.String> _jspx_imports_classes;
static {
_jspx_imports_packages = new java.util.HashSet<>();
_jspx_imports_packages.add("javax.servlet");
_jspx_imports_packages.add("javax.servlet.http");
_jspx_imports_packages.add("javax.servlet.jsp");
_jspx_imports_classes = null;
}
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 java.util.Set<java.lang.String> getPackageImports() {
return _jspx_imports_packages;
}
public java.util.Set<java.lang.String> getClassImports() {
return _jspx_imports_classes;
}
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 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("
");
out.write("
");
out.write("<html>
");
out.write(" <head>
");
out.write(" <title>第一个web程序</title>
");
out.write(" </head>
");
out.write(" <body>
");
out.write(" Hello,World!
");
out.write(" </body>
");
out.write("</html>
");
} 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);
}
}
}
这个类继承了org.apache.jasper.runtime.HttpJspBase,而该抽象类继承了HTTPServlet,而且类中还有一个 _jspService方法,参数的类型是HttpRequest和HttpResponse,可以猜测在HttpJspBase类中,将在声明周期方法service中调用这个 _jspService方法,再来看方法的具体实现,方法首先获取了请求方式,如果请求方式不是GET或者POST或者HEAD,那么客户端将收到一个错误状态码,"JSPs only permit GET POST or HEAD",可见JSP只允许GET、POST和HEAD的请求方式,我们一开始在JSP页面中写的HTML文本,都使用了JspWriter的write方法发送到客户端了,我们使用IDE中在JSP页面的HTML文本部分打一个断点,然后浏览器请求该jsp,我们发现断点被命中了,并且调试器的栈堆将显示代码停留在_jspService方法中,这也证明了JSP其实就是Servlet,
以上是关于JSP的原理的主要内容,如果未能解决你的问题,请参考以下文章