security - why `java.lang.SecurityException: Prohibited package name: java` is required?

Question

I created a class "String" and placed that in package "java" [ actually i wanted to create java.lang to see which class is loaded by classLoader as

Once a class is loaded into a JVM, the same class (I repeat, the same class) will not be loaded again

quoted from oreilly ] . But that thing later, why on running this class i am getting
java.lang.SecurityException: Prohibited package name: java

For which security reason java is not allowing me to have a class in java package? What one could do if there will not be no such check?

Answer 1

User code is never allowed to put classes into one of the standard Java packages. That way, user code cannot access any package-private classes/methods/fields in the Java implementation. Some of those package-private objects allow access to JVM internals. (I'm thinking of SharedSecrets in particular.)