Jsp编译原理

Tomcat编译jsp

在说jsp的编译原理之前，先简单说下tomcat编译后的jsp。在启动tomcat的时候，tomcat是不会立刻将jsp编译为java和class文件的。只有第一次访问jsp页面的时候tomcat才会编译。Tomca编译后的jsp对应的java文件和class文件存放在tomcat/work/Engine名称/Host名称/Context名称目录下（tomcat/work/Catalina/localhost/项目名称）。

Jsp页面

这里使用2个jsp页面，一个包含另一个，而且尽量使用了比较多的特性来展示。

Header.jsp

<html>
<head>
</head>
<body>
    <h2>Hello</h2>
</body>
</html>

Index.jsp

<%@page language="java" contentType="text/html; ISO-8859-1" pageEncoding="ISO-8859-1" %>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" %>
<%@page import="java.lang.String" %>
<%@page import="java.util.Date" %>
<html>
<head>
    <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1" >
</head>
<body>
    <jsp:include page="header.jsp"></jsp:include>
    <%@include file="header.jsp"%>
    <fmt:formatDate value="<%= new Date()%>" type="date" />
    <% for (int i = 0; i < 10; i++) { %>
        <%= new String().valueOf(i) %>
    <% } %>
</body>
</html>

编译后的java 文件（index.jsp）

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.57
 * Generated at: 2018-05-16 16:14:26 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.lang.String;
import java.util.Date;

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;

  static {
    _jspx_dependants = new java.util.HashMap<java.lang.String,java.lang.Long>(1);
    _jspx_dependants.put("/header.jsp", Long.valueOf(1526487042803L));
  }

  private org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005ffmt_005fformatDate_0026_005fvalue_005ftype_005fnobody;

  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() {
    _005fjspx_005ftagPool_005ffmt_005fformatDate_0026_005fvalue_005ftype_005fnobody = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
    _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
    _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
  }

  public void _jspDestroy() {
    _005fjspx_005ftagPool_005ffmt_005fformatDate_0026_005fvalue_005ftype_005fnobody.release();
  }

  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; ISO-8859-1;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("\n");
      out.write("\n");
      out.write("\n");
      out.write("\n");
      out.write("\n");
      out.write("<html>\n");
      out.write("<head>\n");
      out.write("    <meta http-equiv=\"content-type\" content=\"text/html; charset=ISO-8859-1\" >\n");
      out.write("</head>\n");
      out.write("<body>\n");
      out.write("    ");
      org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "header.jsp", out, false);
      out.write("\n");
      out.write("    ");
      out.write("<html>\r\n");
      out.write("<head>\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");
      out.write("    <h2>Hello</h2>\r\n");
      out.write("</body>\r\n");
      out.write("</html>");
      out.write("\n");
      out.write("    ");
      //  fmt:formatDate
      org.apache.taglibs.standard.tag.rt.fmt.FormatDateTag _jspx_th_fmt_005fformatDate_005f0 = (org.apache.taglibs.standard.tag.rt.fmt.FormatDateTag) _005fjspx_005ftagPool_005ffmt_005fformatDate_0026_005fvalue_005ftype_005fnobody.get(org.apache.taglibs.standard.tag.rt.fmt.FormatDateTag.class);
      _jspx_th_fmt_005fformatDate_005f0.setPageContext(_jspx_page_context);
      _jspx_th_fmt_005fformatDate_005f0.setParent(null);
      // /index.jsp(13,4) name = value type = null reqTime = true required = true fragment = false deferredValue = false expectedTypeName = null deferredMethod = false methodSignature = null
      _jspx_th_fmt_005fformatDate_005f0.setValue( new Date());
      // /index.jsp(13,4) name = type type = null reqTime = true required = false fragment = false deferredValue = false expectedTypeName = null deferredMethod = false methodSignature = null
      _jspx_th_fmt_005fformatDate_005f0.setType("date");
      int _jspx_eval_fmt_005fformatDate_005f0 = _jspx_th_fmt_005fformatDate_005f0.doStartTag();
      if (_jspx_th_fmt_005fformatDate_005f0.doEndTag() == javax.servlet.jsp.tagext.Tag.SKIP_PAGE) {
        _005fjspx_005ftagPool_005ffmt_005fformatDate_0026_005fvalue_005ftype_005fnobody.reuse(_jspx_th_fmt_005fformatDate_005f0);
        return;
      }
      _005fjspx_005ftagPool_005ffmt_005fformatDate_0026_005fvalue_005ftype_005fnobody.reuse(_jspx_th_fmt_005fformatDate_005f0);
      out.write("\n");
      out.write("    ");
 for (int i = 0; i < 10; i++) { 
      out.write("\n");
      out.write("        ");
      out.print( new String().valueOf(i) );
      out.write("\n");
      out.write("    ");
 } 
      out.write("\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 {
            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);
    }
  }
}

分析总结编译过程

1） index.jsp文件编译为Java文件后的类名为index_jsp，并且继承了org.apache.jasper.runtime.HttpJspBase类。

2）通过_jspx_dependants保存了当前JSP页面依赖的资源，包括引入的外部jsp页面，导入的标签和jar包等。

3）通过属性_jspx_import_packages存放导入的Java包（这里没有导入）。

4）通过属性_jspx_imports_class存放导入的类。

5）请求处理由_jspService方法完成。

6） _jspService方法中的：pageContext/session/application/config/out/page等参数可以在页面中使用。

7）指定文档类型的指令page被response.setContentType调用。

8）对于html内容由out.write输出。

9）通过<jsp：include/>引入的jsp页面，org.apache.jasper.runtime.JspRuntimeLibrary.include将其包含。

10）通过<%@include%>引入的外部jsp页面，通过out.write输出其内容。

11）对于<%%>中的java代码，直接转换为Servlet中。要输出的内容通过out.write输出。

tomcat-jsp编译原理

Jsp编译原理

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