HttpMessageNotWritableException: No converter found for return value of type，可能出现的原因及解决方法

首先，演示一下出现的情况，创建一个实体类，代码如下：

public class User2 {
   
   

    private Long id;

    private String username;

    public User2() {
   
   
    }

    public User2(Long id, String username) {
   
   
        this.id = id;
        this.username = username;
    }