Are Strings really immutable?

From java Basic String Class is immutable.But actually speaking String is not immutable.This is due to lack of security in String Class.Therefore you can change it using Reflection Concepts.
e.g:
Normal :

String lc = "lowercase";
lc.toUpperCase();
System.out.println(lc);

Now it may look immutable.But you can modify it with the following snippet:

Field stringValue = String.class.getDeclaredField("value");
stringValue.setAccessible(true);
stringValue.set(orig, orig.toUpperCase().toCharArray());

This is because of SecurityManager is not turned on.Though
you have an immutable class it is mutable unless the security manager
is turned on.