Constructor.newInstance() without knowing parameter sequence?

Take this code for example :

public class User {
  private String username;
  private String password;

  protected User()
  {}

  public User(String username , String password)
  {...}

  //getters & setters
}

We can use User.class.getConstructors() and find there are 2 constructors , and with constructor.getParameterTypes() , we can identify there's one constructor with two String parameters. We can also use reflection to find two properties : username and password. But , at run time , we don't know the proper sequence of the two parameters being used to call constructor.newInstance(?,?).

constructor.newInstance(username , password) , and constructor.newInstance(password , username) are both legal but with totally different result.

I cannot use User.class.newInstance() and set property value because the no-arg constructor is protected.

I encounter this problem because I am trying to write a generic JPA CRUD tool. List/Read/Update/Delete are OK. But now I face the problem that I cannot online create an object. The entity classes are all with a protected no-arg constructor (so I cannot use class.newInstance()) , and one a-lot-of-parameter public constructor (but the parameter names are erased at runtime).

How to do this ? Does javassist or other proxy-techniques he开发者_JAVA百科lp ? how ? Thanks a lot !

You can use:

Constructor<?> c = class.getDeclaredConstructor();
c.setAccessible(true);
c.newInstance();

That's what JPA will do anyway, because it instantiates objects via their no-arg constructor.

That might be just a workaround, but couldn't you create the User using (I omit reflection for the purpose of readability)

User u = new User(null, null);

and then set the properties in your tool?

u.setUsername(...);
u.setPassword(...);

You could generate code that is more or less atomic, so it wouldn't matter to you

The order will be the same at runtime though, so why not just figure out the correct order, and treat it as such.