embedded tomcat运行java web，Unable to compile class for JSP
环境
- eclipse：4.5.2
- jre：1.8
- java project compiler：1.8
- embedded tomcat：7.0.32
可以正常启动，但是访问时，会报错。
HTTP Status 500 - Unable to compile class for JSP:
type Exception report
message Unable to compile class for JSP:
description The server encountered an internal error that prevented it from fulfilling this request.
exception
org.apache.jasper.JasperException: Unable to compile class for JSP:
An error occurred at line: 1 in the generated java file
The type java.util.Map$Entry cannot be resolved. It is indirectly referenced from required .class files
An error occurred at line: 1 in the generated java file
The type java.io.ObjectInputStream cannot be resolved. It is indirectly referenced from required .class files
Stacktrace:
org.apache.jasper.compiler.DefaultErrorHandler.javacError(DefaultErrorHandler.java:102)
org.apache.jasper.compiler.ErrorDispatcher.javacError(ErrorDispatcher.java:331)
org.apache.jasper.compiler.JDTCompiler.generateClass(JDTCompiler.java:469)
org.apache.jasper.compiler.Compiler.compile(Compiler.java:378)
org.apache.jasper.compiler.Compiler.compile(Compiler.java:353)
org.apache.jasper.compiler.Compiler.compile(Compiler.java:340)
org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:646)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:357)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:123)
note The full stack trace of the root cause is available in the Apache Tomcat/7.0.32 logs.
Apache Tomcat/7.0.32
内容摘要就是jsp转换成java文件，java文件编译成class文件时报错。根据标红部分指示的信息，开始以为是未引用类造成的，检查了jsp文件没有任何问题。
通过搜索引擎，发现出现jsp无法编译，有各种各样的原因。比如servlet-api在应用中有一份，tomcat中有一份，二者不是一个版本，报错了；或者jsp文件本身就有语法错误。
最后调试发现，具体错误和上面标红的字体有所不同。
在jsp翻译的java文件进行编译时，CompilerSourceVM值为1.6，CompilerTargetVM也是1.6，即将jsp翻译的java文件，以符合1.6 java语法约束的java，编译为可在1.6 jre中运行的class。
而jsp翻译成java的文件内容如下所示，注意
_jspx_imports_packages = new java.util.HashSet<>();
此种方式的泛型使用，在版本号小于1.7的jdk中是错误的，小于1.7版本的java约束中，要求必须将泛型类型写上的，不能为空，因此文件约束就过不了，自然无法compile成1.6版本的class文件，自然就报错了。
/* * Generated by the Jasper component of Apache Tomcat * Version: Apache Tomcat/8.5.9 * Generated at: 2017-03-01 08:22:23 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 default_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("/browse/part/cache-control-header.jsp", Long.valueOf(1488337979237L)); } private static final java.util.Set<java.lang.String> _jspx_imports_packages; private static final java.util.Set<java.lang.String> _jspx_imports_classes; static { _jspx_imports_packages = new java.util.HashSet<>(); _jspx_imports_packages.add("javax.servlet"); _jspx_imports_packages.add("java.util"); _jspx_imports_packages.add("javax.servlet.http"); _jspx_imports_packages.add("javax.servlet.jsp"); _jspx_imports_classes = null; } private volatile javax.el.ExpressionFactory _el_expressionfactory; private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager; public java.util.Map<java.lang.String, java.lang.Long> getDependants() { return _jspx_dependants; } public java.util.Set<java.lang.String> getPackageImports() { return _jspx_imports_packages; } public java.util.Set<java.lang.String> getClassImports() { return _jspx_imports_classes; } public javax.el.ExpressionFactory _jsp_getExpressionFactory() { if (_el_expressionfactory == null) { synchronized (this) { if (_el_expressionfactory == null) { _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory(); } } } return _el_expressionfactory; } public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() { if (_jsp_instancemanager == null) { synchronized (this) { if (_jsp_instancemanager == null) { _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig()); } } } return _jsp_instancemanager; } public void _jspInit() { } 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 java.lang.String _jspx_method = request.getMethod(); if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) { response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD"); return; } 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, "errorReport.jsp", true, 8192, true); _jspx_page_context = pageContext; application = pageContext.getServletContext(); config = pageContext.getServletConfig(); session = pageContext.getSession(); out = pageContext.getOut(); _jspx_out = out; out.write("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01//EN\" \"http://www.w3.org/TR/html4/strict.dtd\">\r\n"); out.write("\r\n"); out.write("\r\n"); out.write("\r\n"); response.setHeader("Pragma", "No-cache"); response.setHeader("Cache-Control", "no-cache"); response.setDateHeader("Expires", 0); out.write('\r'); out.write('\n'); String errMsg = ""; if (request.getAttribute("errMsg") != null) { errMsg = request.getAttribute("errMsg").toString(); } String RndData = ""; int nRndLen; char Upper = '9'; char Lower = '0'; Random r = new Random(); for (int i = 0; i < 15; i++) { int tempval = (int) ((int) Lower + (r.nextFloat() * ((int) (Upper - Lower)))); RndData += new Character((char) tempval).toString(); } session.setAttribute("RandomData", RndData); nRndLen = RndData.length(); //输出html内容，为了省略篇幅，删去 } 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); } } }
解决方法
1、通过更换embedded tomcat的版本到8.5.9（未测试最低版本是多少），此时CompilerSourceVM和CompilerTargetVM的值全部升到了1.7，再次运行，jsp文件顺利编译为class，程序运行不报错。
2、项目组其他人员验证，未本人验证。使用ide的server功能，server版本是tomcat 6即可。
额外发现
在embedded tomcat中，jasper会将将要访问的jsp文件，转换为java文件，然后再编译为class文件，最后再运行。
一直以为jsp文件是动态解析的，有一个专门的jsp解析模板引擎。
