本文共 8888 字，大约阅读时间需要 29 分钟。
今天遇到一个问题，首先是在tomcat中间件上跑的web项目，一个jsp文件，因为代码行数实在是太多了，更新了几个版本之后编译报错了，页面打开都是报500的错误，500的报错，知道http协议返回码的都知道，这是服务端的报错。
jsp编译过程是先编译为servlet，然后再通过类加载器编译为.class文件，再执行为Servlet实例。这就是jsp的编译过程。所以jsp报500错误也可以理解，属于服务端的报错没什么好怀疑的。
服务端报错，肯定就是去console拿日志了。从CONSOLE拿到日志关键信息：
The code of method _jspService(HttpServletRequest, HttpServletResponse) is exceeding the 65535 bytes limit
这个报错意思大概是超过字节限制。通过网上资料搜索，很多地方都是给出了一个解决方法，不过大部分都没说明为什么。
网上一大堆差不多的博客，都是这样说的，在tomcat的conf文件夹里，找到web.xml，然后在JspServlet的servlet配置里，加上mappedfile参数 修改后的代码jsp org.apache.jasper.servlet.JspServlet fork false xpoweredBy false mappedfile false 3
其实也就是加上
mappedfile false
大部分博客并没有给出原因。不过还是可以解决问题的。不过网上所说的这种方法并不是很好的方法，只能说是暂缓之策。
首先要从jsp的编译说起，jsp经过tomcat编译后，文件会保存在哪里？
下面介绍一下，一般路径都会在${TOMCAT_HOME}\work\Catalina\localhost\项目名称\org\apache\jsp文件夹下面。 假如新建了一个index.jsp，经过编译之后，都会在该路径下面生成index_jsp.java文件和index_jsp.class文件，index_jsp.java文件是什么？其实可以理解为tomcat编译生成的servlet类，index_jsp.class呢？当然就是servlet类编译之后生成的.class文件了。 随便找个index_jsp.java文件，拿代码来看看：/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/7.0.32 * Generated at: 2016-11-19 03:26:12 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.util.*;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 _jspx_dependants; private javax.el.ExpressionFactory _el_expressionfactory; private org.apache.tomcat.InstanceManager _jsp_instancemanager; public java.util.Map 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=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'); out.write('\n'); if (true) { _jspx_page_context.forward("/login_toLogin"); return; } out.write('\r'); out.write('\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); } }} 从代码可以看出，类继承于HttpJspBase类实现JspSourceDependent接口，先看一下HttpJspBase类，这个类从哪来的呢？HttpJspBase是tomcat库提供的，所以拿tomcat库的源码来看看，在${TOMCAT_HOME}/lib里找到价包jasper.jar,反编译代码，找到HttpJspBase类
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; 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;} 代码并不是说多复杂，HttpJspBase类继承HttpServlet类，实现HttpJspPage接口，也就是说HttpJspBase重写了HttpServlet的service(),init()等等方法，HttpServlet，我们就很熟悉了。HttpJspPage又是什么?看它的包名，马上知道它是jdk提供的接口，马上找到它的代码：
/* * The contents of this file are subject to the terms * of the Common Development and Distribution License * (the "License"). You may not use this file except * in compliance with the License. * * You can obtain a copy of the license at * glassfish/bootstrap/legal/CDDLv1.0.txt or * https://glassfish.dev.java.net/public/CDDLv1.0.html. * See the License for the specific language governing * permissions and limitations under the License. * * When distributing Covered Code, include this CDDL * HEADER in each file and include the License file at * glassfish/bootstrap/legal/CDDLv1.0.txt. If applicable, * add the following below this CDDL HEADER, with the * fields enclosed by brackets "[]" replaced with your * own identifying information: Portions Copyright [yyyy] * [name of copyright owner] * * Copyright 2005 Sun Microsystems, Inc. All rights reserved. * * Portions Copyright Apache Software Foundation. */ package javax.servlet.jsp;import javax.servlet.*;import javax.servlet.http.*;import java.io.IOException;/** * The HttpJspPage interface describes the interaction that a JSP Page * Implementation Class must satisfy when using the HTTP protocol. * ** The behaviour is identical to that of the JspPage, except for the signature * of the _jspService method, which is now expressible in the Java type * system and included explicitly in the interface. * * @see JspPage */public interface HttpJspPage extends JspPage { /** The _jspService()method corresponds to the body of the JSP page. This * method is defined automatically by the JSP container and should never * be defined by the JSP page author. *
* If a superclass is specified using the extends attribute, that * superclass may choose to perform some actions in its service() method * before or after calling the _jspService() method. See using the extends * attribute in the JSP_Engine chapter of the JSP specification. * * @param request Provides client request information to the JSP. * @param response Assists the JSP in sending a response to the client. * @throws ServletException Thrown if an error occurred during the * processing of the JSP and that the container should take * appropriate action to clean up the request. * @throws IOException Thrown if an error occurred while writing the * response for this page. */ public void _jspService(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException;}
很关键的方法名：_jspService，不就是刚才CONSOLE报错提示的方法名？ 也就是说jdk提供接口，然后tomcat对接口进行实现，我们知道Java内存模型(JMM)规定了一个方法的大小只能是64k，所以，从刚才的报错，我们简单从源码分析了一下，报错的原因其实就是jsp反编译为Servlet之后，代码要经过_jspService这个方法，这个方法超过了64k，导致报错。
查看一下tomcat7官方给出的文档：http://tomcat.apache.org/tomcat-7.0-doc/jasper-howto.html#Configuration
找到mappedfile属性的意思mappedfile - 我们是否应该为每个输入行生成一个print语句的静态内容，以便于调试？ true或者false，默认true。
现在分析一下具体原因。代码报错的原因就是因为jsp编译为Servlet之后，经过_jspService这个方法，方法超过64k导致报错。然后通过设置mappedfile参数的原因是尽量减少print代码，暂时使代码不超过，也就是说只是一种暂缓的方法。网上资料说通过jsp:include方法或许可以，我并没有实践过，所以不讨论。
转载地址：http://yycof.baihongyu.com/