溫故知新之從原始碼探析jsp執行原理

程式設計師生態圈發表於2017-11-27
原始碼JS

最近翻出大學時縮印版的jsp與servlet教材，紙質泛黃，如古董一般！粗略翻閱幾章，大感快慰，感覺不再像學校時那般晦澀難懂，看來自己果然是成長了，哈哈……書籍真是每看一遍都會有不同的收穫與感悟！溫故知新，注重基礎，天天向上！

借書上這幅圖闡述jsp執行原理，感覺流程分明、結構清晰、步驟詳實。

通過上圖，可以說對jsp執行原理是一目瞭然，這裡主要研究一下jsp檔案轉換為java檔案後執行的詳細情況。

我們都知道jsp其本質就是servlet，jsp首次請求時會被jsp引擎（如Tomcat）轉換為與其相應的java檔案，即servlet，故其執行過程與servlet的執行一樣。因此，我們不難推斷，它具有servlet的基本的屬性與方法，也就是說轉換後的java類是直接或間接地繼承自HttpServlt。這裡以原始碼為證，瞭解其本質,見下面程式碼片test1.jsp和test1_jsp.java。

我們先看看類test1_jsp.java的宣告,可知其繼承自HttpJspBase

public final class test1_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent {
我們跟進一下可發現HttpJspBase繼承HttpServlet，在這裡我們可看到_jspInit(),_jspDestroy()和_jspService的宣告。當jsp檔案對應的servlet被例項化後，_jspInit()方法就會執行，_jspInit()在servlet的生命週期中只會被執行一次。_jspService()方法被呼叫來處理客戶端的請求。對每一個請求，JSP引擎建立一個新的執行緒來處理該請求。如果有多個客戶端同時請求該JSP檔案，則JSP引擎會建立多個執行緒。每個客戶端請求對應一個執行緒。以多執行緒方式執行可以大大降低對系統的資源需求，提高系統的併發量及響應時間。但不過也應該注意多執行緒的程式設計限制，由於該Servlet始終駐於記憶體，所以響應是非常快的。

public abstract class HttpJspBase extends HttpServlet
implements HttpJspPage
{
public final void init(ServletConfig config)
throws ServletException
{
super.init(config);
jspInit();
_jspInit();
}
public String getServletInfo() {
return Localizer.getMessage("jsp.engine.info");
}

public final void destroy() {
jspDestroy();
_jspDestroy();
}
public final void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
_jspService(request, response);
}
public void jspInit()
{
}
public void _jspInit()
{
}
public void jspDestroy()
{
}
protected void _jspDestroy()
{
}
public abstract void _jspService(HttpServletRequest paramHttpServletRequest, HttpServletResponse paramHttpServletResponse)
throws ServletException, IOException;
}

在_jspService中我們可以看到這樣一段程式碼

PageContext pageContext = null;
HttpSession session = null;
ServletContext application = null;
ServletConfig config = null;
JspWriter out = null;
Object page = this;
JspWriter _jspx_out = null;
PageContext _jspx_page_context = null;


try {
  response.setContentType("text/html;charset=ISO-8859-1");
  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;複製程式碼

這讓我不難聯想到jsp的9個內建物件，我們繼續探究pageContext可發現jsp的內建物件都可從pageContext中獲得，因此，在jsp頁面上我們不用宣告便可直接使用request、response、session、spplication等，這也是它們為什麼叫jsp內建物件的原因所在。
jsp檔案：test1.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">




My JSP 'test1.jsp' starting page







This is my JSP page.



轉換後的servlet檔案：test1_jsp.java
package org.apache.jsp;

import javax.servlet.;
import javax.servlet.http.;
import javax.servlet.jsp.;
import java.util.;

public final class test1_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent {

private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory();

private static java.util.List _jspx_dependants;

private javax.el.ExpressionFactory _el_expressionfactory;
private org.apache.AnnotationProcessor _jsp_annotationprocessor;

public Object getDependants() {
return _jspx_dependants;
}

public void _jspInit() {
_el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
_jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName());
}

public void _jspDestroy() {
}

public void _jspService(HttpServletRequest request, HttpServletResponse response)
throws java.io.IOException, ServletException {

PageContext pageContext = null;
HttpSession session = null;
ServletContext application = null;
ServletConfig config = null;
JspWriter out = null;
Object page = this;
JspWriter _jspx_out = null;
PageContext _jspx_page_context = null;




try {
  response.setContentType("text/html;charset=ISO-8859-1");
  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');
  out.write('\n');複製程式碼

String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";

  out.write('\r');
  out.write('\n');

  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 'test1.jsp' starting page</title>\r\n");
  out.write("    \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("\t<link rel=\"stylesheet\" type=\"text/css\" href=\"styles.css\">\r\n");
  out.write("\t-->\r\n");
  out.write("\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("  </body>\r\n");
  out.write("</html>\r\n");
} catch (Throwable t) {
  if (!(t instanceof 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);
  }
} finally {
  _jspxFactory.releasePageContext(_jspx_page_context);
}複製程式碼

}
}

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