当开发JBPM的相关应用时，会出现如下异常：
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/index_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
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:275)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
仔细观察上边异常提示，不难发现是由于解析jsp文件的类有两个，java不知道使用哪个解析jsp文件造成的。
解决方法：
1. 删除你项目中WEB-INF/lib目录下的三个jar包（juel.jar, juel-engine.jar, juel-impl.jar ）。
2. 将三个jar包（juel.jar, juel-engine.jar, juel-impl.jar ）拷贝到${tomcate 1.6}/lib目录下。