java.lang.NoSuchMethodException with Constructor newInstance

问题

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.

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);

This results in a

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)

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.

UserController:

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();
    }
}

回答1:

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);

回答2:

Generally its not recommended to use other then default constructor, as web.xml or struts-config.xml uses to instantiate the servlet using reflection.

If you want to get instance of your class other than default constructor,

Class userController = Class.forName("<packages.>UserController");
Constructor userControllerConstructor = userController.class.getConstructor(HttpServletRequest.class, HttpServletResponse.class, String[].class);
UserController userControllerObj = (UserController)constructor.newInstance(request,response, action);

来源：https://stackoverflow.com/questions/12404963/java-lang-nosuchmethodexception-with-constructor-newinstance

标签

java

web

front-controller