JSP源码、改写Servlet为JSP、查看转译成为Servlet的文件、JSP字符编码设置
概述
在Servlet中编写HTML太麻烦了，应该使用JSP。JSP中可以直接编写HTML，使用指示、声明、脚本（scriptlet）等元素来堆砌各种功能，但JSP最后还是会被容器转译为Servlet源代码、自动编译为.class文件、载入.class文件，然后生成Servlet对象。
在第一次请求JSP时，容器会进行转译、编译与加载的操作，所以第一次请求JSP页面会慢很多。
源码分析
若使用Tomcat 7或Glassfish v3作为Web容器，最后由容器转译后的Servlet类是继承自HttpJspBase类，而HttpJspBase又继承了HttpServlet。
下面是HttpJspBase类的源码，这个类的源码可以在Tomcat的lib目录下的 jasper.jar\org\apache\jasper\runtime 这个文件夹找到。
package org.apache.jasper.runtime;
import java.io.IOException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.jsp.HttpJspPage;
import org.apache.jasper.compiler.Localizer;
public abstract class HttpJspBase extends HttpServlet implements HttpJspPage {
private static final long serialVersionUID = 1L;
protected HttpJspBase() {
}
public final void init(ServletConfig config) throws ServletException {
super.init(config);
this.jspInit();
this._jspInit();
}
public String getServletInfo() {
return Localizer.getMessage("jsp.engine.info");
}
public final void destroy() {
this.jspDestroy();
this._jspDestroy();
}
public final void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this._jspService(request, response);
}
public void jspInit() {}
public void _jspInit() {}
public void jspDestroy() {}
protected void _jspDestroy() {}
public abstract void _jspService(HttpServletRequest var1, HttpServletResponse var2) throws ServletException, IOException;
}
观察HttpJspBase的源代码，可以看到，Servlet的init()方法中调用了jspInit()与_jspInit()；Servlet的destroy()方法中调用了jspDestroy()与_jspDestroy()方法；service()方法中调用了_jspService()方法。
_jspInit()、_jspDestroy()与_jspService()方法名称上有个下划线，表示这些方法是由容器转译时维护，不应该重新定义这些方法。如果想要做些JSP初始化或收尾动作，则应定义jspInit()或jspDestroy()方法。
观察HttpJspBase的源代码，还可以看到，它继承了HttpServlet。但没有重写其doGet、doPost方法；只重写了HttpServlet参数为HttpServletRequest和HttpServletResponse的service方法，然后调用自己定义的抽象方法_jspService()。再观察jsp转译后的servlet，它的_jspService()方法内部并没有判断get、post方法，所以jsp既可以使用post也可以使用get来发出请求。
Servlet 改写为 JSP
将要被改写的Servlet源码：
package com.test;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
@WebServlet("/books")
public class ListBookmark extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
out.println("<!DOCTYPE html>");
out.println("<html>");
out.println("<head>");
out.println("<meta charset=\"UTF-8\">");
out.println("<title>觀看線上書籤</title>");
out.println("</head>");
out.println("<body>");
out.println("<table style=\"text-align: left; width: 100%;\" border=\"0\" >");
out.println(" <tbody>");
out.println("<tr>");
out.println("<td style=\"background-color: rgb(51, 255, 255);\">书名</td>");
out.println("<td style=\"background-color: rgb(51, 255, 255);\">分类</td>");
out.println("</tr>");
Map<String, String> bookMap = new HashMap<String, String>();
bookMap.put("三国演义", "小说");
bookMap.put("资治通鉴", "历史");
bookMap.put("窦娥冤", "戏剧");
Set<Map.Entry<String, String>> entries = bookMap.entrySet();
for (Map.Entry<String, String> entry : entries) {
out.println("<tr>");
out.println("<td>" + entry.getKey() + "</td>");
out.println("<td>" + entry.getValue() + "</td>");
out.println("</tr>");
}
out.println("</tbody>");
out.println("</table>");
out.println("</body>");
out.println("</html>");
out.close();
}
}
手动改写好的JSP源码：
<%@page contentType="text/html; charset=GBK" pageEncoding="UTF-8" %>
<%@page import="javax.servlet.*, java.io.PrintWriter, java.util.HashMap, java.util.Map, java.util.Set" %>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>在线书籍</title>
</head>
<body>
<table style="text-align:left; width:100%;" border="0">
<tbody>
<tr>
<td style="">书名</td>
<td style="">分类</td>
</tr>
<%
Map<String, String> bookMap = new HashMap<String, String>();
bookMap.put("三国演义", "小说");
bookMap.put("资治通鉴", "历史");
bookMap.put("窦娥冤", "戏剧");
Set<Map.Entry<String, String>> entries = bookMap.entrySet();
for (Map.Entry<String, String> entry : entries) {
%>
<tr>
<td><%= entry.getKey() %></td>
<td><%= entry.getValue()%></td>
</tr>
<%
}
%>
</tbody>
</table>
</body>
</html>
运行结果完全一致。
index_jsp.java源文件：
在IDEA中，JSP转译为Servlet后的源代码和进一步编译后的字节码文件可以在 target/tomcat/work/Tomcat/localhost/_/org/apache/jsp/ 这个文件夹下找到。
/*
* Generated by the Jasper component of Apache Tomcat
* Version: Apache Tomcat/7.0.47
* Generated at: 2018-12-05 16:02:28 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 javax.servlet.*;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
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 javax.el.ExpressionFactory _el_expressionfactory;
private org.apache.tomcat.InstanceManager _jsp_instancemanager;
public java.util.Map<java.lang.String,java.lang.Long> 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 {
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=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;
out.write("\n");
out.write("\n");
out.write("<!DOCTYPE html>\n");
out.write("<html>\n");
out.write("<head>\n");
out.write(" <meta charset=\"UTF-8\">\n");
out.write(" <title>在线书籍</title>\n");
out.write("</head>\n");
out.write("<body>\n");
out.write("<table style=\"text-align:left; width:100%;\" border=\"0\">\n");
out.write(" <tbody>\n");
out.write(" <tr>\n");
out.write(" <td style=\"background-color: rgb(51, 255, 255);\">书名</td>\n");
out.write(" <td style=\"background-color: rgb(51, 255, 255);\">分类</td>\n");
out.write(" </tr>\n");
out.write(" ");
Map<String, String> bookMap = new HashMap<String, String>();
bookMap.put("三国演义", "小说");
bookMap.put("资治通鉴", "历史");
bookMap.put("窦娥冤", "戏剧");
Set<Map.Entry<String, String>> entries = bookMap.entrySet();
for (Map.Entry<String, String> entry : entries) {
out.write("\n");
out.write(" <tr>\n");
out.write(" <td>");
out.print( entry.getKey() );
out.write("</td>\n");
out.write(" <td>");
out.print( entry.getValue());
out.write("</td>\n");
out.write(" </tr>\n");
out.write(" ");
}
out.write("\n");
out.write(" </tbody>\n");
out.write("</table>\n");
out.write("</body>\n");
out.write("</html>\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);
}
}
}
JSP响应内容、字符编码设置
jsp源文件： <%@page contentType="text/html; charset=GBK" pageEncoding="UTF-8" %>
index_jsp.java： response.setContentType("text/html; charset=GBK");
jsp源文件： <%@page contentType="text/html" pageEncoding="UTF-8" %>
index_jsp.java： response.setContentType("text/html;charset=UTF-8")
jsp源文件： <%@page contentType="text/html;" pageEncoding="UTF-8" %>
index_jsp.java： response.setContentType("text/html;;charset=UTF-8")
前两种方法都可以，第三种方法的index_jsp.java里多个一个分号 response.setContentType("text/html;;charset=UTF-8")
pageEncoding属性用于告诉容器使用UTF-8读取.jsp转译为.java，然后编译时也使用UTF-8，并且如果在contentType属性里没有指定字符编码的情况下设置内容类型附加的charset也为UTF-8。
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