先上实体类代码的前半部分：
这是出现问题时的：

package com.cheng.pojo;public class People {private int id;private String name;private int age;private String address;public People(int id, String name, int age, String address) {this.id = id;this.name = name;this.age = age;this.address = address;}//getter , setter and toString 略
}

检查了JSP页面也没问题

<jsp:useBean id="people" class="com.cheng.pojo.People" scope="page"/>
<%--等价于People people = new People();--%>

后来发现是没有显式地定义无参构造。因为

定义有参构造后，如果想使用无参构造，必须显式地定义一个无参构造

而且上面JSP代码等价于new People();已经调用了无参构造，但是我在实体类没有显式写出无参构造。

所以实体类加上无参构造，问题解决

package com.cheng.pojo;public class People {private int id;private String name;private int age;private String address;public People(int id, String name, int age, String address) {this.id = id;this.name = name;this.age = age;this.address = address;}//此情况必须显式定义无参构造！！！！！！！public People() {}//getter , setter and toString 略
}