javax.el.PropertyNotFoundException求解

JSP报错javax.el.PropertyNotFoundException: Property 'a' not found on type com.cn.action.system.FlowController$Flow
有这个属性，有getset方法。。拼写没错，，

类结构

 class Flow{
        private String a;
        private String b;
        private Integer c;

        public String getA() {
            return a;
        }

        public void setA(String a) {
            this.a = a;
        }

        public String getB() {
            return b;
        }

        public void setB(String b) {
            this.b = b;
        }

        public Integer getC() {
            return c;
        }

        public void setC(Integer c) {
            this.c = c;
        }

        public Flow(String a, String b, Integer c) {
            this.a = a;
            this.b = b;
            this.c = c;
        }

    }

少了el表达式的jar包，，加进去就好了，，