What is the difference between getDeclaredConstructors and getConstructors in the Class API?

Question

I notice that in the Java Reflection API there are two different methods for invoking constructors: the getDeclaredConstructors/getConstructors method. Although the Java docs are slightly different (getDeclaredConstructors seems to imply that it returns ALL constructors, rather than public ones), its not clear why the API explicitly supports these two different methods.

More importantly, I'm wondering : when would one method be preferable to another if we are invoking classes dynamically ? For example, what is the purpose of accessing a private constructor?

Solution 1

getDeclaredConstructors (when you want all the constructors)

Returns an array of Constructor objects reflecting all the constructors declared by the class represented by this Class object. These are public, protected, default (package) access, and private constructors.

getConstructors (when you want only public constructors)

Returns an array containing Constructor objects reflecting all the public constructors of the class represented by this Class object.

So, looking at the docs for both of them, I think a difference is that getConstructors returns only public constructors while getDeclaredConstructors returns all the constructors (public, protected, default (package) access, and private)

So, it's easy if you need only the public constructors then use getConstructors. Otherwise, if you need all the constructors (disregarding the access-modifier of the constructor) then use getDeclaredConstructors.

Solution 2

The getDeclaredXX() methods exist for the manipulation of classes in ways that weren't necessarily intended by the maker of those classes. Note that there is a getDeclaredMethod() method that allows you to invoke private methods, and getDeclaredField() method that allows you to get/set private fields.

I'm not completely sure about "legitimate" use cases, but these are obviously useful for doing certain things. Also, this family of methods only returns things specifically declared in the class, not things that exist in the class because of the superclass.

Accessing a private constructor could be useful for the same reasons, I suppose.

Comments

[MeBigFatGuy]

and by using the constructor.setAccessible(true), you can reflectively call constructors that are private.

[Mike Nakis]

This is wrong. The getConstructors() methods do _not_ behave similarly to the getMethods() methods. They both fetch constructors of the given class only, ignoring constructors of superclasses. (Their difference is in the visibility of the constructors they fetch.) That's in accordance to the fact that when instantiating a class you must use one of its constructors, you cannot bypass them and directly invoke one of the constructors of its superclass. You can write a couple of test classes to test this. (I did.) Please delete this misleading answer.

[Paŭlo Ebermann]

Constructors are not inherited in Java.

[neXus]

While this answer is correct, there is a little caveat. The compiler automatically provides a default (no-argument) public constructor for any class that does not explicitly define any constructors. getConstructors will not return this default constructor but getDeclaredConstructors does.

[Marco Servetto]

Hi neXus, are you sure? it looks very creazy that getDeclaredConstructor returns a constructor that was NOT DECLARED. Is it possible that is just an issue with the visibility of the "autodeclared" constructor?

[Per Lundberg]

My testing right now: getDeclaredConstructor() and getConstructor() both returns compiler-created (default) constructors. So I think your guess might be correct @MarcoServetto.