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。

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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.

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Apache Tomcat/7.0.32

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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">
");
            out.write("
");
            out.write("
");
            out.write("
");

            response.setHeader("Pragma", "No-cache");
            response.setHeader("Cache-Control", "no-cache");
            response.setDateHeader("Expires", 0);

            out.write('
');
            out.write('
');

            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。