java.lang.NoSuchMethodException 与构造函数 newInstance [英] java.lang.NoSuchMethodException with Constructor newInstance

问题描述

我目前正在用 Java 开发一些 web 开发项目，我已经实现了一个前端控制器，它的工作是根据路径实例化新的控制器.

I am currently working on some web dev project in Java, i have implemented a frontcontroller, which job is to instantiate new controllers, depending on the path.

所以当用户运行时?q=user/login ex.前端控制器应该设置 UserController，我正在尝试使用这段代码.

So when the user is running ?q=user/login ex. the front controller should instatiate the UserController, that i am trying to do with this piece of code.

    String q = request.getParameter("q");

    try {
        String[] page = q.split("/");
        // Make first char upper, to match class name conventions.
        page[0] = (page[0].substring(0, 1).toUpperCase() + page[0].substring(1).toLowerCase()).trim();

      Class contDes = Class.forName("dk.elvar.rocks." + page[0]+ "Controller");
      Constructor co = contDes.getConstructor();
      co.newInstance(request, response, page);

结果是

java.lang.NoSuchMethodException: dk.elvar.rocks.UserController.<init>()
at java.lang.Class.getConstructor0(Class.java:2706)
at java.lang.Class.getConstructor(Class.java:1657)
at dk.elvar.rocks.FrontController.doGet(FrontController.java:35)

我试图在谷歌上查找它，并且错误，因为在加载的对象中声明一个构造函数，使类公开，已经存在.

I've tryed to look it up at google, and bugs as, declaring a constructor in loaded object, make the class public, is already there.

用户控制器:

public class UserController extends HttpServlet  {

private final String USERNAME = "Martin";
private final String PASSWORD = "David";

    private static final long serialVersionUID = 1L;
HttpServletRequest request;  
HttpServletResponse response;

public UserController(HttpServletRequest request, HttpServletResponse response, String[] action)  {
    this.request = request;
    this.response = response;

    if(action[1].equalsIgnoreCase("login")) {
        this.renderLoginAction();
    }
    if(action[1].equalsIgnoreCase("val-login")) {
        this.validateLoginAction();
    }
}

推荐答案

您可能会收到此异常，因为该类没有默认构造函数.您可以通过将参数传递给 getConstructor 方法来获取带参数的构造函数:

You probably get this exception because that class does not have a default constructor. You can get a constructor with parameters by passing them to the getConstructor method:

Constructor co = contDes.getConstructor(
                HttpServletRequest.class, 
                HttpServletResponse.class, 
                String[].class);
co.newInstance(request, response, page);

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