【Java Web总结】JSP页面实现类详解
JSP页面实现类详解
JSP从结构上看与HTML页面类似，但是JSP实际上是作为Servlet运行的
当JSP页面被访问时，Web容器容器解析JSP文件并转为相应的Java文件
该文件声明了一个Servlet类，称为页面的实现类
接着
Web容器编译该类并将其装进内存
然后与其他Servlet一样执行并输出结果发送至客户端
下面以todayDate.jsp为例：
<%@ page contentType="text/html;charset = UTF-8" pageEncoding="UTF-8"%>
<%@ page import="java.time.LocalDate" %>
<%! LocalDate date = null; %>
<html><head><title>当前日期</title></head>
<body>
<%
date = LocalDate.now(); // 创建一个LocalDate对象
%>
今天的日期是：<%=date.toString() %>
</body>
</html>
在页面转换阶段，Web容器会将该文件转换为todayDate_jsp.java的类文件
该文件是JSP文件的实现类
下面观察该实现类todayDate_jsp.java：
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/9.0.21
* Generated at: 2019-09-11 14:36:55 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.temp;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import java.time.LocalDate;
public final class todayDate_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent,
org.apache.jasper.runtime.JspSourceImports {
LocalDate date = null;
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 = new java.util.HashSet<>();
_jspx_imports_classes.add("java.time.LocalDate");
}
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 {
if (!javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
final java.lang.String _jspx_method = request.getMethod();
if ("OPTIONS".equals(_jspx_method)) {
response.setHeader("Allow","GET, HEAD, POST, OPTIONS");
return;
}
if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method)) {
response.setHeader("Allow","GET, HEAD, POST, OPTIONS");
response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSP 只允许 GET、POST 或 HEAD。Jasper 还允许 OPTIONS");
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;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");
out.write("当前日期 \r\n");
out.write("\r\n");
out.write(" ");
date = LocalDate.now(); // 创建一个LocalDate对象
out.write(" \r\n");
out.write("今天的日期是：");
out.print(date.toString() );
out.write("\r\n");
out.write("\r\n");
out.write("");
} 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);
}
}
}
页面实现类继承了HttpJspBase类，同时实现JspSourseDependent和JspSourseImports接口
HttpJspBase类实现了HttpJspPage接口，该接口又继承JspPage接口，该接口又继承Servlet接口
因此，JSP页面实现类实现了这三个接口的所有方法
从上面的实现类中可以看到
实现了jspInit()、jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)、jspDestroy()三个方法
这三个方法分别对应Servlet的init()、service()、和destroy()方法
称为JSP页面的生命周期方法
关于JSP页面的生命周期详解，可参考如下的技术文章：
Java Web总结】JSP页面的生命周期详解
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