tomcat系列之编译超过64k大小的jsp文件报错原因

「爱情、让人受尽委屈。」 2021-09-17 09:26 165阅读 0赞

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。