Tomcat 部署常见问题

基于Tomcat服务器进行web应用的开发，初学者经常会犯一些错误，一下罗列了常见的错误及解决方法.

一. 应用下面(WEB-INF/lib)导入了与Tomcat 自带的jar(%CATALINA_HOME%/lib) 想同的包，如：el-api.jar, jsp-api.jar 时，启动服务器，然后发送请求会报如下异常

   1. jsp-api.jar 会引起NullPointer 异常，发生于jsp所对应java 类(%CATALINA_HOME%/work/Catalina/localhost/xxx)的方法中

     public void _jspInit() {
            _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
            _jsp_annotationprocessor = (org.apache.AnnotationProcessor) getServletConfig().getServletContext().getAttribute(org.apache.AnnotationProcessor.class.getName());
      }

    2. el-api.jar 会引起

    javax.servlet.ServletException: java.lang.LinkageError: loader constraint violation: when resolving interface method "javax.servlet.jsp.JspApplicationContext.getExpressionFactory()Ljavax/el/ExpressionFactory;" the class loader (instance of org/apache/jasper/servlet/JasperLoader) of the current class, org/apache/jsp/WEB_002dINF/jsp/hello_jsp, and the class loader (instance of org/apache/catalina/loader/StandardClassLoader) for resolved class, javax/servlet/jsp/JspApplicationContext, have different Class objects for the type javax/el/ExpressionFactory used in the signature

所以当出现这两种情况时，只需要将应用中的这两个jar包删除就行了

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