Why is Class.newInstance() "evil"?

Question

Ryan Delucchi asked here in comment #3 to Tom Hawtin's answer:

why is Class.newInstance() "evil"?

this in response to the code sample:

// Avoid Class.newInstance, for it is evil.
Constructor<? extends Runnable> ctor = runClass.getConstructor();
Runnable doRun = ctor.newInstance();

so, why is it Evil?

Answer 1

The Java API documentation explains why (http://java.sun.com/javase/6/docs/api/java/lang/Class.html#newInstance()):

Note that this method propagates any exception thrown by the nullary constructor, including a checked exception. Use of this method effectively bypasses the compile-time exception checking that would otherwise be performed by the compiler. The Constructor.newInstance method avoids this problem by wrapping any exception thrown by the constructor in a (checked) InvocationTargetException.

In other words, it can defeat the checked exceptions system.

Answer 2

One more reason:

Modern IDEs allow you to find class usages - it helps during refactoring, if you and your IDE know what code is using class that you plan to change.

When you don't do an explicit usage of the constructor, but use Class.newInstance() instead, you risk not to find that usage during refactoring and this problem will not manifest itself when you compile.