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

<servlet>
        <servlet-name>jsp</servlet-name>
        <servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
        <init-param>
            <param-name>fork</param-name>
            <param-value>false</param-value>
        </init-param>
        <init-param>
            <param-name>xpoweredBy</param-name>
            <param-value>false</param-value>
        </init-param>
        <init-param>  
            <param-name>mappedfile</param-name>  
            <param-value>false</param-value>  
        </init-param> 
        <load-on-startup>3</load-on-startup>
    </servlet>

其实也就是加上

<init-param>  
           <param-name>mappedfile</param-name>  
           <param-value>false</param-value>  
 </init-param>

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<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=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.
 *
 * <p>
 * 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.
     * <p>
     * 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。