java---无参构造实现反射

首先来看报错信息：

java.sql.SQLException: Cannot create com.eshop.entity.SUser: com.eshop.entity.SUser Query: select * from s_user Parameters: []
	at org.apache.commons.dbutils.AbstractQueryRunner.rethrow(AbstractQueryRunner.java:392)
	at org.apache.commons.dbutils.QueryRunner.query(QueryRunner.java:351)
	at org.apache.commons.dbutils.QueryRunner.query(QueryRunner.java:289)
	at com.eshop.util.JdbcUtil.queryList(JdbcUtil.java:49)
	at com.eshop.dao.impl.UserDaoImpl.select(UserDaoImpl.java:12)
	at com.eshop.service.impl.UserServiceImpl.findAll(UserServiceImpl.java:14)
	at com.eshop.controller.UserServlet.findAll(UserServlet.java:17)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.eshop.controller.BaseServlet.service(BaseServlet.java:26)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:544)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
	at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:616)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:831)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1634)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:748)

第一步：分析报错

这里可以看到因为at com.eshop.util.JdbcUtil.queryList(JdbcUtil.java:49)，导致Cannot create com.eshop.entity.SUser

那我们首先看JdbcUtil中是否有代码的错误，但是这里不是代码问题，那我们就回到实体类中
常常使用的是默认空构造函数，但若需要一个有参构造函数，且这个参数并不直接对应某个属性时，例如：

这里的有参构造不直接对应某个属性，当添加迁移的时候，就会报错：
Cannot create com.eshop.entity.SUser: com.eshop.entity.SUser Query: select * from s_user Parameters: []
解决方法很简答，手动添加一个无参构造即可。
可事实上，并不希望使用这个空的构造函数，因为它会缺少一些空值检测等判定。
微软的eShopOnWeb示例项目中找到了如下写法：

public class Order : BaseEntity, IAggregateRoot
{
    
    
    // 注意这里是private
    private Order()
    {
    
    
        // required by EF
    }
 
    // 含参构造函数包括了空值检测
    public Order(string buyerId, Address shipToAddress, List<OrderItem> items)
    {
    
    
        Guard.Against.NullOrEmpty(buyerId, nameof(buyerId));
        Guard.Against.Null(shipToAddress, nameof(shipToAddress));
        Guard.Against.Null(items, nameof(items));
 
        BuyerId = buyerId;
        ShipToAddress = shipToAddress;
        _orderItems = items;
    }
    public string BuyerId {
    
     get; private set; }
 
    public DateTimeOffset OrderDate {
    
     get; private set; } = DateTimeOffset.Now;
    public Address ShipToAddress {
    
     get; private set; }
 
    private readonly List<OrderItem> _orderItems = new List<OrderItem>();
    public IReadOnlyCollection<OrderItem> OrderItems => _orderItems.AsReadOnly();
 
    // ...
}

EF Core在创建实例时，会首先去调用无参构造函数，且无论该构造函数是何访问类型，所以，我们只需添加私有的无参构造函数即可！