我写这段代码只是为了简单地将新数据保存到mysql db，创建了类、存储库和主控制器，它实际上只适用于一个类，但不适用于这个类，请欣赏一些指针，因为我不太明白为什么会使用空指针：
代码：主控制器：

@GetMapping(path = "/addExchange") 
public @ResponseBody String addNewExchange (@RequestParam String exchange)
{
    //check1
    System.out.println("creating Exchange constructor");
    Exchange e = new Exchange(); 
    //check 2
    System.out.println("created exchange constructor, " + exchange);
    e.setE_name(exchange);
    //check 3
    System.out.println("Exhange name is, " + e.getE_name());
    //check 4
    System.out.println(e);
    exchangeRepository.save(e); 
    return "Saved Exchange";
}

eclipse控制台的输出错误：下面的检查显示对象是从Map中提取的：-创建exchange构造函数-创建的exchange构造函数，orange-exhange name is，orange-exchange[ideexchange=0，e\u name=orange]
2018-05-21 15:59:43.528错误5118---[nio-8080-exec-4]o.a.c.c.[/].[dispatcherservlet]：路径为[]的上下文中servlet[dispatcherservlet]的servlet.service（）引发异常[请求处理失败；嵌套异常是java.lang.nullpointerexception]，其根本原因是
java.lang.nullpointerexception:com.instantmoney.moneyservices.maincontroller.addnewexchange（maincontroller）处为空。java:47)~[类/：na]
exchange存储库：

public interface ExchangeRepository extends CrudRepository<Exchange, Long> {

}
交换班

@Entity 
public class Exchange {

@Id
@GeneratedValue (strategy = GenerationType.AUTO)
private int idexchange; 

private String e_name;

public Exchange() {}

//接受者和接受者
curl

http://localhost:8080/moneyservice/addExchange?exchange=orange